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Western Washington University
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WWU Masters Thesis Collection
WWU Graduate and Undergraduate Scholarship
Summer 2015
High-Sr Volcanic Domes from the Lassen Volcanic
Region, Southernmost Cascade Arc, Northern
California: Implications for Andesite and Dacite
Magma Generation
Christina M. Stout
Western Washington University, [email protected]
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Stout, Christina M., "High-Sr Volcanic Domes from the Lassen Volcanic Region, Southernmost Cascade Arc, Northern California:
Implications for Andesite and Dacite Magma Generation" (2015). WWU Masters Thesis Collection. 429.
http://cedar.wwu.edu/wwuet/429
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HIGH‐SR VOLCANIC DOMES FROM THE LASSEN VOLCANIC REGION, SOUTHERNMOST CASCADE ARC, NORTHERN CALIFORNIA: IMPLICATIONS FOR ANDESITE AND DACITE MAGMA GENERATION By Christina M. Stout Accepted in Partial Completion Of the Requirements for the Degree Master of Science Kathleen L. Kitto, Dean of the Graduate School ADVISORY COMMITTEE Chair, Dr. Susan DeBari Dr. Scott Linneman Dr. Michael Clynne MASTER’S THESIS In presenting this thesis in partial fulfillment of the requirements for a master’s degree at Western Washington University, I grant to Western Washington University the non‐exclusive royalty‐free to archive, reproduce, distribute and display the thesis in any and all forms, including electronic format, via any digital library mechanisms maintained by WWU. I represent and warrant this is my original work and does not infringe or violate any rights of others. I warrant that I have obtained written permissions from the owner of any third party copyrighted material included in these files. I acknowledge that I retain ownership rights to the copyright of this work, including but not limited to the right to use all or part of this work in future works, such as articles or books. Library users are granted permission for individual, research and non‐commercial reproduction of this work for educational purposes only. Any further digital posting of this document requires specific permission from the author. Any copying or publication of this thesis for commercial purposes, or for financial gain, is not allowed without my written permission. Christina M. Stout July 24, 2015 HIGH‐SR VOLCANIC DOMES FROM THE LASSEN VOLCANIC REGION, SOUTHERNMOST CASCADE ARC, NORTHERN CALIFORNIA: IMPLICATIONS FOR ANDESITE AND DACITE MAGMA GENERATION A Thesis Presented to The Faculty of Western Washington University In Partial Fulfillment Of the Requirements for the Degree Masters of Science By Christina M. Stout July 2015 Abstract The Onion Butte (OB) and Barkley Mountain (BM) groups are two lineaments of volcanic domes and lava flows located with the Lassen segment of the southernmost Cascades, northern California. The OB group (~2.5 Ma) consists of 13 domes that are dominantly dacitic, but span the range from andesite to dacite. The BM group (~1.5 Ma) comprises 21 domes that range from basaltic andesite to andesite, but are mostly andesitic. The lavas of both groups are petrographically similar, but differ geochemically. The lavas are fine‐grained, sparsely phyric containing needle‐like hornblende phenocrysts, but lack the large plagioclase phenocrysts so characteristic of typical andesitic to dacitic suites. While the groups share some geochemical characteristics, their differences are significant. The lavas of the OB group have high Sr concentrations (> 1,000 ppm), low abundances of Rb and Pb, high (Sr/P)N, and have isotopic compositions that are similar to the high (Sr/P)N primitive magmas in the Lassen region. This group has the highest Sr concentrations and lowest 87Sr/86Sr compositions of any silicic lavas in the Lassen segment. The BM group is defined by higher Pb abundances, lower Th concentrations, low (Sr/P)N, and isotopic compositions similar to the low (Sr/P)N primitive magmas of the Lassen segment. The andesites of the BM group are subdivided into two subgroups based on HREE concentrations: subgroup 1 has no variation in HREE concentrations and subgroup 2 has variable HREE concentrations. Regional calc‐alkaline mafic volcanism in the Lassen segment forms a compositional continuum that has been generally divided into two geochemical groups: high (Sr/P)N (> 3.3) with MORB‐like isotopic compositions and low (Sr/P)N (< 3.3) with OIB‐like compositions. The basaltic andesites from the Barkley Mountain group are geochemically and isotopically similar to the low iv (Sr/P)N mafic magmas. It is likely the basaltic andesites were generated by fractionation from those parental mafic magmas. The OB group are geochemically similar to the high (Sr/P)N magmas which are likely sources for the dacites of the Onion Butte group. The Barkley Mountain andesites can be produced by simple fractional crystallization from Barkley Mountain basaltic andesites. Fractional crystallization models require moderate amounts of fractionation (20‐40%) with hornblende ± garnet in the fractionating assemblage to recreate observed REE patterns and HREE concentrations. In contrast, the origin of the single sample of the Onion Butte andesite proved more difficult to constrain. While major element data suggests simple mixing between the OB basaltic andesite and dacites to produce intermediate andesites, the Ni, Sr, and Nb concentrations of the andesites belies such a simplistic explanation. Addition or subtraction models of mafic material (e.g. olivine) to the dacites to reproduce andesite compositions were unsuccessful. The relationship between the Onion Butte andesite and the dacites is unclear, but both share the same geochemical and physical characteristics of the group. Partial melting of the lower continental crust with a source composition similar to the high (Sr/P)N mafic magmas results in geochemical characteristics of the Onion Butte dacites. Models were successful at reproducing observed OB dacite concentrations after 20% melting with hornblende ± garnet in the melting assemblage. Partial melting is consistent with published thermal calculations that demonstrate emplaced basaltic magmas in the lower continental crust as the heat source to induce melting. Although some geochemical characteristics of the Onion Butte dacites (high Sr, high Sr/Y, and low 87Sr/86Sr isotopic compositions) are shared with proposed slab derived melts, partial melting v modeling using an eclogite mineralogy fails to reproduce observed Sr/Y concentrations. Low La/Yb ratios in the OB dacites also preclude this slab melt origin. A petrogenetic model is offered that demonstrates that: 1) BM basaltic andesites are related to regional low (Sr/P)N magmas, 2) BM andesites can be generated through fractional crystallization of BM basaltic andesites; 3) OB dacites are thought to be generated by partial melting of amphibolitic lower continental crust similar in composition to the high (Sr/P)N primitive magmas; 4) the distinctive geochemical characteristics (high Sr, high Sr/Y, low 87Sr/86Sr isotopic compositions) of the Onion Butte group likely reflects the rarity of the high (Sr/P)N primitive magmas in the Lassen segment; and 5) Sr concentrations can be used as a discriminator of magma sources in the Lassen region; and 6) both groups equilibrate with a mineral assemblage that requires hornblende ± garnet and minor plagioclase. vi Acknowledgments I would like to recognize the several individuals and organizations whose time, help, and support made this project a successful completion. First and foremost, I want to extend my deepest thanks and gratefulness to my advisor Sue DeBari for your unwavering patience, encouragement, numerous discussions, and support. Your guidance and knowledge has helped me become a better scientist, writer, and critical thinker. I am deeply fortunate to have had the opportunity to work with you. I would also like to thank my committee members, Scott Linneman and Michael Clynne, for their careful review and constructive comments that aided and improved the arguments presented. A sincere thanks for the organizations whose financial assistance made this project possible: the Graduate Student Research Grant from the Geological Society of America, Western Washington University RSP Fund for the Enhancement of Graduate Research, and the Western Washington University Department of Geology Advance for Fieldwork. I would like to extend another heartfelt thanks to Michael Clynne for providing rock samples, rock powders, and numerous maps of the study area. To Bruce Nelson and Scott Kuehner from University of Washington, for your incredible patience and assistance you provided in the Isotope Geochemistry Lab. To my field assistant Janet Stout: thank you for hauling rocks and battling bears with me. To my father, David Stout, for financial support without I would not have been able to finish. To my friends (particularly Courtney Sisney, Jamie Karlsson, Taryn Dunkin, Amy Wagner, and Susie Lindner) who have been a constant source of encouragement and support. And to Lisa Hammersley whose counsel at a very crucial time helped me stay the course. vii Table of Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ix List of Figure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .x Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Tectonic and Geologic Setting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Analytical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Whole Rock Major and Trace Element Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Isotope Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Petrography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Onion Butte Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Barkley Mountain Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Whole Rock Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Major Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Trace Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Isotope Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17 Comparison to Magmas from the Lassen Segment and Potential Petrogenetic Hypotheses. 18 Basaltic Andesites and Andesites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18 Onion Butte Dacites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Models for the Generation of the Barkley Mountain and Onion Butte Groups . . . . . . . . . . . .23 Generation of Intermediate Lavas from the Barkley Mountain Group. . . . . . . . . . . . 23 Barkley Mountain Andesites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24 Fractional Crystallization Efforts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Generation of Subgroup 1 Andesites . . . . . . . . . . . . . . . . . . . . . . . . . 28 Generation of Subgroup 2 Andesites . . . . . . . . . . . . . . . . . . . . . . . . . 29 Generation of Dome 7 Andesite from the Onion Butte Group. . . . . 29 Generation of Intermediate to Silicic lavas from the Onion Butte Group. . . . . . . . . . 31 Onion Butte Andesites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 Onion Butte Dacites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32 Lower Crustal Melting Models for the Onion Butte Dacites. . . . . . . . . . . . . . 36 Petrogenetic Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Further Work on the Onion Butte and Barkley Mountain Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . .41 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43 Appendix I: Sr/Rb/Nd/Sm Column Chemistry Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Appendix II: Major Element Modeling Utilizing MELTS to test for Fractional Crystallization . . . . .98 Appendix III: Olivine Addition and Subtraction Modeling Efforts . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Appendix IV: Table of Sample Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 viii List of Tables Table 1. Whole Rock Major and Trace Element Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50 Table 2. Sr and Nd Isotopic Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Table 3. Summary of Petrography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57 Table 4. Partition Coefficients and Starting Composition (Co) used in Fractionation Models . . . . . . 58 Table 5. REE fractionation results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Table 6. REE fractionation results for dome 7 andesite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60 Table 7. Partition Coefficients and Starting Composition (Co) used in Partial Melting Models . . . . . 61 Table 8. REE and incompatible trace elements (Rb, Ba, K, and Sr) partial melting results . . . . . . . . .62 ix List of Figures Figure 1. Map of the Cascade Volcanic Arc and Lassen Volcanic Region study area . . . . . . . . . . . . . .63 Figure 2 Simplified geologic map of the Lassen segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Figure 3 Geologic map of study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65 Figure 4 Representative photomicrographs of the Onion Butte group . . . . . . . . . . . . . . . . . . . . . . . . 66 Figure 5 Representative photomicrographs of the Barkley Mountain group . . . . . . . . . . . . . . . . . . . 67 Figure 6 AFM diagram and K2O versus SiO2 variation diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Figure 7 Whole rock major element variation diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Figure 8 Trace element variation diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70 Figure 9 Primitive‐mantle‐normalized trace element diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Figure 10 Primitive‐mantle‐normalized trace element diagram highlighting differences in the Onion Butte group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Figure 11 Primitive‐mantle‐normalized trace element diagram highlighting differences in the Barkley Mountain basaltic andesites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73 Figure 12 Primitive‐mantle‐normalized trace element diagram highlighting differences in the Barkley Mountain andesites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Figure 13 Chondrite‐normalized rare earth element diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Figure 14 Chondrite‐normalized rare earth element diagram highlighting differences in the Onion Butte group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Figure 15 Chondrite‐normalized rare earth element diagram highlighting differences in the Barkley Mountain group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77 Figure 16 Isotopic variation diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Figure 17 Select major and trace element variation diagrams comparing the Onion Butte and Barkley Mountain groups to Lassen segment lavas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Figure 18 (Sr/P)N versus SiO2 variation diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Figure 19 Comparison of Onion Butte dacites to Lassen silicic lavas. . . . . . . . . . . . . . . . . . . . . . . . . . .81 Figure 20 Isotopic variation diagrams of Onion Butte dacites to Lassen silicic lavas. . . . . . . . . . . . . .82 Figure 21 Primitive‐mantle‐normalized trace element diagram highlighting differences of Onion Butte andesites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Figure 22 Isotope variation diagram of Onion Butte dome 1 andesite compared to Lassen segment lavas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Figure 23 Sr/Y versus Y variation diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Figure 24 Chondrite‐normalized rare earth element diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .86 x Figure 25 Dy/Yb versus SiO2 variation diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87 Figure 26 Partial melt results: dacite of the Onion Butte group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Figure 27 Partial melt results: dacite of the Onion Butte group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Figure 28 Raleigh fractionation results: subgroup 1 andesites of the Barkley Mountain group. . . . .90 Figure 29 Raleigh fractionation results: subgroup 1 andesites from dome 28 of the Barkley Mountain group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .91 Figure 30 Raleigh fractionation results: subgroup 2 andesites of the Barkley Mountain group. . . . .92 Figure 31 Raleigh fractionation results: dome 7 andesites of the Onion Butte group. . . . . . . . . . . . .93 Figure 32 Schematic diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 xi Introduction The petrogenesis of intermediate to silicic magmas within volcanic arcs is a focus of many studies (Borg and Clynne, 1998; Conrey et al. 2001; Gerlach and Grove, 1982; Hidalgo et al., 2007; Hildreth and Moorbath, 1988). These studies aim to understand how differentiated compositions are generated from a more mafic, mantle‐derived parent and how they evolve from their mafic counterparts. Common explanations include partial melting of a basaltic lower crust, fractional crystallization, assimilation, magma mixing, or some combination of these processes. Studying intermediate to silicic magmas within a magmatic system is important because they provide examples for testing of magma differentiation. The most silicic (evolved) magmas are often mixing end‐members with basalt in magma mixing processes (Gerlach and Grove, 1982; Borg and Clynne, 1998; Clynne, 1999) and this mixing process results in the generation of intermediate composition magmas (Sakuyama, 1979; Sparks and Marshall, 1986; Gardner et al., 1995; Dreher et al., 2005; Kent et al., 2010; Ruprecht et al., 2012). In addition, the production of intermediate and silicic magmas is integrally tied to the formation and growth of continental crust. Studying differentiated magmas, ultimately, leads to a better understanding of volcanic systems and processes which can lead to better volcanic hazard preparedness. Within the Cascade arc, multiple processes have been attributed to the generation of intermediate to silicic magmas. In the northern part of the arc, at Mount Baker, andesite generation has been attributed to magma mixing and other open system processes (Baggerman and DeBari, 2011). Dacite generation at Mount Baker is also complex, involving multiple processes, including fractional crystallization and magma mixing (Gross, 2012). Mount Saint Helens dacites have been interpreted to be generated by partial melting of the subducting Juan de Fuca plate (Defant and Drummond, 1993). This interpretation has been disputed by Dawes (1994) and Green (1994) who favor partial melting of a basaltic lower crust that is supported by past studies (Smith and Leeman, 1987). Andesite generation at Mount Hood is attributed to magma mixing between mafic and felsic magmas (Kent et al., 2010). At Mount Jefferson, two stage partial melting has been credited as an origin for andesites and dacites (Conrey et al. 2001). Mount Shasta andesites and dacites are thought to have been generated by fractional crystallization of hydrous primitive mantle melts and subsequent magma mixing of water‐rich primitive magnesian andesites (Grove et al. 2005). At the Lassen volcanic center (Brokeoff Volcano and Lassen Peak), intermediate lavas are attributed to magma mixing processes and silicic lavas are thought to be generated by partial melting of amphibolite lower arc crust (Clynne, 1990; Bullen and Clynne, 1990; Tepley et al.,1999; Clynne, 1999). The focus of this study is the origin of a compositionally unique suite of mafic to felsic domes from the Lassen volcanic region (but not the Lassen volcanic center) in the southernmost (Lassen) segment of the Cascade volcanic arc (Figure 1). The Lassen segment displays both regional scale volcanism (short‐lived cinder cones to larger lava cones and shields) as well as a few long‐lived, large intermediate to silicic volcanic centers (Clynne, 1990). Silicic volcanic rocks are volumetrically important in the Lassen volcanic center, but are very sparse within the regional scale volcanism (Clynne, 1990; Hildreth, 2007). Consequently, all previous studies of regional volcanism have focused on primitive magmas (Clynne and Borg, 1997; Borg et al. 1997; Borg et al. 2000; and Borg et al. 2002). There have been no previous studies that focus on the petrogenesis of intermediate to silicic volcanism in these regional scale volcanics. This study presents detailed petrologic and geochemical analyses of intermediate to silicic volcanism within the Barkley Mountain dacite chain (southwest of Lassen National Park), which includes the Onion Butte and Barkley Mountain groups. The two groups were initially distinguished by dominant rock type; Onion Butte is generally dacitic in composition and the Barkley Mountain group is mostly andesitic in composition. These rocks are significant because preliminary studies 2
revealed the highest Sr and the lowest 87Sr/86Sr isotopic concentrations for their silica compositions in the entire Lassen segment (M. Clynne, written communication). Petrographically, the Onion Butte and Barkley Mountain group are sparsely phyric, containing hornblende and lacking plagioclase phenocrysts, which is uncharacteristic of the Lassen segment volcanic rocks. Tectonically, both groups trend ~60˚from the arc axis while most volcanic vents within the Lassen volcanic region (LVR) align north south due to tectonism imposed by impingement of the Basin and Range (Figure 2); as well as, align approximately to the edge of the subducting Juan de Fuca plate system and the slab window (Clynne, written communication). As such, these suites of rocks provide a strong contrast to the Lassen region in both physical and chemical attributes. The factors responsible for this contrast may provide a counterpoint to the generally accepted understanding of magma generation and evolution in the Lassen segment. The main objective of this study is to determine the origin and evolution of the Onion Butte and Barkley Mountain intermediate to silicic lavas and determine the processes that generated these lavas. In so doing, questions regarding why these lavas have the highest Sr and lowest 86Sr/87Sr isotopic concentrations will be addressed. This study will primarily focus on detailed petrographic and geochemical analyses and will rely on comparison of the Onion Butte and Barkley Mountain groups to other intermediate to silicic lavas in the Lassen segment. These observations will be used to constrain possible mechanisms for their generation. Geochemical modeling, of both major and trace elements, will be used to determine the best fit model for the generation of the Onion Butte and Barkley Mountain groups. Tectonic and Geologic Setting The Cascade volcanic arc extends from Lassen Peak in the south (Northern California) to Mount Meager in the north (southern British Columbia) (Figure 1). Cascade arc volcanism is 3
generated by subduction of the Juan de Fuca plate system beneath the North American Plate. The Cascade arc has been divided into five segments based on the distribution of volcanic vents and the structure of the Juan de Fuca plate (Figure 1; Guffanti and Weaver, 1988). More recently, Schmidt et al. (2008) have divided the arc into four similar segments based on isotopic compositions of primitive magmas. The Lassen segment is the southernmost arc segment and is 80 km wide (for volcanism < 3 Ma) and 80 km long (Figure 2; Clynne, 2010). This wide volcanic region is generated by oblique subduction of the young and relatively warm Gorda Plate, part of the Juan de Fuca plate system, beneath the North American Plate (Guffanti et al., 1990). Convergence of the Gorda plate beneath the North American plate has been estimated at 29 mm/yr (Riddihough, 1984). Recent seismic studies have shown the slab depth is 90 km beneath Lassen Peak (McCrory et al., 2012). Slab depth in the study area gets deeper to the east, with slab depth 80 km beneath the Onion Butte group to 95 km beneath the Barkley Mountain group (McCrory et al., 2012). Geophysical studies have estimated the crustal thickness beneath the Lassen segment to be 38 ± 4 km (Mooney and Weaver, 1989) with a crustal basement composed of igneous and metamorphic rocks of the Klamath Mountain province and igneous and metamorphic rocks of the Sierra Nevada province (Guffanti et al., 1990; Borg et al., 1997). Volcanism in the Lassen segment at its present location began approximately 3‐4 Ma and has been described on two scales: 1) a regional scale that built a volcanic platform, and 2) a focused scale of large volcanic centers (Clynne, 1990). On the regional scale, volcanism is dominantly mafic to intermediate in composition, erupting from hundreds of small volume short‐lived vents. Silicic volcanism (>63% SiO2) on the regional scale is sparse (Hildreth, 2007; Clynne and Muffler, 2010). The focused long‐lived volcanic centers have erupted basaltic andesite to rhyolitic compositions and are surrounded by and built up on the regional volcanics. Five volcanic centers have been identified in 4
Lassen region: Latour (3.5‐3.0 Ma), Yana (3.4‐2.4 Ma), Dittmar (2.4‐1.3 Ma), Maidu (2.4‐1.2 Ma), and Lassen (825‐0 ka). These centers each have (or had) an intermediate composite cone, flanking silicic domes and flows, and a hydrothermal system (Clynne, 1990). The youngest volcanic center, the Lassen volcanic center (LVC), is the best preserved center and its general evolution is well understood (Clynne, 1990). Volcanism at the LVC began approximately 825 ka with the Rockland caldera complex a series of dacitic to rhyolitic domes. This sequence ended with caldera producing eruption that is recognized by the Rockland tephra (Clynne and Muffler, 2010). After the caldera eruption, renewed volcanism was focused at Brokeoff volcano. This period was marked by composite cone building and filling in of the caldera. Eruptions from Brokeoff volcano ranged in composition from basaltic andesite to dacitic lavas with subordinate pyroclastic flows. Volcanism ceased from Brokeoff volcano approximately 385 ka and shifted to silicic flank volcanism erupting hybrid andesites, and dacitic to rhyolitic lavas (Clynne and Muffler, 2010). Flank volcanism is associated with an active hydrothermal system. The LVC has an active hydrothermal system with the most recent silicic eruptions occurring at Chaos Crags (1.1 ka) and Lassen Peak (1914‐1917) (Clynne and Muffler, 2010; Tepley, 1999). The Latour, Yana, Dittmar, and Maidu volcanic centers had similar composite cone building, silicic domefield volcanism, and hydrothermal systems which are now extinct (Clynne and Muffler, 2010). Although mafic to intermediate volcanism is widespread on the regional scale, silicic volcanism is not. The Barkley Mountain dacite chain is a group of domes identified through mapping and geochemical data to be intermediate to silicic in composition and in a location where silicic volcanism is sparse (Hildreth, 2007). Approximately 34 domes were identified that form two distinct chains based on dominant rock type: a northern chain composed of the Onion Butte group (dacitic) and a southern chain composed of the Barkley Mountain group (andesitic). These domes are located 40 km southwest of the Lassen Volcanic Center (Figure 2). The domes were emplaced into and on 5
top of debris flows, lahars, and pyroclastic flows derived from Yana volcanic centers, called the Tuscan Formation. The Onion Butte and Barkley Mountain domes are short lived, erupting from single vents that are more characteristic of regional scale volcanism, than the longer lived silicic volcanism associated with the large volcanic centers and hydrothermal systems. An overview of each group is described below and is organized by the K‐Ar age of the groups. The Onion Butte group is the northern chain of domes (Figure 2 and 3). This group is composed of 13 domes that erupted ~2.5 Ma along a linear trend striking 290˚. The Onion Butte chain is primarily dacitic (64.0‐65.2 wt. % SiO2, Table 1) with subordinate andesites (57.4‐58.4 wt. % SiO2, Table 1). This group also has anomalously high Sr concentrations (1075‐1634 ppm; Table 1), leading to some speculation about this group being possible adakites (M. Clynne, written communication). The Barkley Mountain group is the southern chain of domes (Figure 2 and 3). These 21 domes were emplaced ~1.5 Ma along a trend that strikes 310˚. The Barkley Mountain group is intermediate in composition and consists of basaltic andesites (53.6‐56.7 wt % SiO2, Table 1) and andesites (57.9‐62.3 wt. % SiO2, Table 1). Analytical Methods One hundred and two samples from seventeen domes were collected (Figure 3). Michael Clynne, from the USGS, supplied 35 additional samples, 17 from the Onion Butte group and 18 from the Barkley Mountain group. Outcrops were selected for sampling in order to produce adequate spacial representation of the dome (5‐10 samples/dome). Outcrops with visible weathering and alteration were avoided to make sure geochemical data would be reliable. In order to ensure a complete data set, samples supplied by Michael Clynne were used for the domes were not visited. 6
A total of 26 samples were analyzed for whole rock geochemistry. For smaller domes, one sample per dome was chosen to represent the geochemistry of that dome. For larger domes (9, 13, and 24) multiple samples were chosen to adequately represent the geochemistry. Samples that showed weathering or visible alteration were discarded. The samples were prepared for whole rock analysis at Western Washington University (WWU). Large samples were shattered by hand and each fragment selected was checked for hammer marks and alteration. A total of 50 grams of rock fragments were selected for each sample. Rock fragments were crushed by tungsten carbide plates in a Sephor Chipmunk Jaw Crusher. Rock chips were used to prepare beads and powders for wavelength dispersive X‐ray fluorescence (WD‐XRF) and inductively coupled plasma mass spectrometry (ICP‐MS) at Washington State University (WSU). Whole Rock Major and Trace Element Analysis Major element oxides and 19 trace elements were analyzed for 23 samples by WD‐XRF at the WSU GeoAnalytical Laboratory. XRF beads were made at WWU following the method of Johnson et al. (1999). Rock powders were prepared at WWU by grinding rock chips in a tungsten carbide SPEX Shatterbox grinding mill. Powders were fused into beads by combining a 2:1 blend of dry dilithium tetraborate to dry rock powder in a 1,000˚C furnace for 10 minutes. The beads were reground and refused by above method to make sure samples were homogenous. The beads were sent to WSU and analyzed using a ThermoARL Advant’XP+ sequential XRF spectrometer. A detailed explanation of analytical procedure, precision, and accuracy is detailed in Johnson et al. (1999). Whole rock data (and six trace elements: Ni, Cr, V, Ga, Cu, and Zn) from WD‐XRF analyses are reported in Table 1 All 14 naturally occurring rare earth elements (REE) and 13 trace elements (Ba, Rb, Y, Nb, Cs, Hf, Ta, Pb, Th, U, Sr, Sc, and Zr) were analyzed for 25 samples by ICP‐MS at the WSU GeoAnalytical 7
Laboratory. Rock powders were prepared at WWU by grinding rock chips in an alumina ceramic SPEX Shatterbox grinding mill. Powders were fused into beads by combining a 1:1 blend of dry dilithium tetraborate to dry rock powder in a 1,000˚C furnace for 10 minutes. The beads were reground to make sure samples were homogenous. The resulting powders were sent to WSU and analyzed using an Agilent 4500+ ICP‐MS. A detailed explanation of analytical procedure, precision, and accuracy is detailed by Knaack et al. (2004). ICP‐MS data is reported with XRF data in Table 1. Isotope Analysis Eight samples were chosen for isotope analyses (Sr and Nd) with a focus on adequately representing the broadest range of compositions. The most mafic, intermediate, and most silicic of each group were selected. For the Onion Butte group, dome 13 was chosen to represent the most mafic composition, dome 1 and 7 for the intermediate, and dome 4 and 9 for most silicic. Two intermediate samples for Onion Butte group were analyzed because of the great difference in Sr concentration between them (dome 1: 1447 ppm Sr; dome 7: 530 ppm Sr). Two silicic samples were analyzed for the Onion Butte group because dome 9 has the highest Sr concentration (1634 ppm) and dome 4 has the lowest Sr concentration (1075 ppm). For the Barkley Mountain group only three samples were analyzed because of their compositional uniformity. Dome 14 was chosen to represent the most mafic, dome 20 for the intermediate, and dome 26 for the most silicic of the group. Rock powders for the eight samples were prepared using the method described above using an alumina ceramic SPEX Shatterbox grinding mill. Powders were sent to the University of Washington Isotope Geochemistry Lab and analyzed using a Nu Instruments multiple collector inductively coupled plasma mass spectrometer (MC‐ICP‐MS). To measure Sr and Nd isotope ratios, samples were dissolved in a 10:1 mixture of HF‐HNO3. Samples were dried and the resulting fluoride 8
salts are dissolved with HClO4. Once the mixture is dry, samples are prepared for column chemistry by adding 6M HCl to each sample. Column chemistry and Nd isotope analysis was done under the direction of Dr. Bruce Nelson and Dr. Scott Kuehner and is described in Appendix I. Sr isotope analysis was done by Dr. Bruce Nelson. Sr and Nd isotope ratios are reported in Table 2. Sr isotopic values were normalized to the standard NBS 987 which yielded an average isotopic ratio of 87Sr/86Sr=0.710240. Nd isotope values were normalized to the La Jolla value of 143Nd/144Nd=0.511843. Error on the Sr and Nd analyses are ± 30 ppm (2σ) or ±0.3 epsilon units. The 145Nd/144Nd was monitored as a measure of analytical quality and yielded a value of 0.348413. Petrography In the following section, the petrography of the Onion Butte and Barkley Mountain groups is described. This section outlines the similarities and differences between the two groups. Descriptions include minerals present, modes, sizes and textures. All modal abundances were estimated visually. Petrographic characteristics are summarized in Table 3. In the following paragraphs, the following textural terms are used to describe the textures of the rocks and the phases observed in both groups. Porphyritic is applied to samples with visible phenocryst phases in hand sample. Aphyric is used for samples with no visible phenocrysts. For porphyritic samples, microporphyritic is used to describe some groundmasses. It is also applied to samples that are aphyric. For samples that are not microporphyritic, microcrystalline is used to describe the groundmass or overall nature of aphyric samples. The Onion Butte and Barkley Mountain groups are petrographically similar, but with minor characteristic differences. Both groups are sparsely porphyritic to nearly aphyric. The groundmass is 9
microporphyritic to microcrystalline with up to 20% glass. Minerals in both groups do not show any visible sign of weathering in the samples. Characteristic differences between the two groups (at similar SiO2 concentrations) include differences in microphenocryst type and abundance, and groundmass mineralogy (Table 3). For example, andesites from the Onion Butte group contain up 15% microphenocrysts, while the Barkley Mountain andesites generally have a higher abundance, with up to 30% microphenocrysts. Hornblende abundance in the Onion Butte group is higher than the Barkley Mountain group. The following sections will detail the petrography of each group. Onion Butte Group Andesite compositions are sparsely porphyritic with phenocrysts of hornblende, in a hypocrystalline microporphyritic groundmass with 5‐17% microphenocrysts of hornblende (3‐5%), clinopyroxene (1‐2%), and plagioclase (0‐10%) (Figure 4a‐b). Hornblende crystals are pseudomorphed by fine‐grained black aggregates of Fe‐Ti oxides, plagioclase, and pyroxene. Hornblende psuedomorphs occur as 0.25‐1.5 mm (most are 0.25‐0.5 mm) subhedral acicular crystals. Clinopyroxene occur as 0.5‐0.75 mm subhedral crystals. Euhedral to subhedral plagioclase occurs as un‐zoned 0.25 to 0.5 mm laths. The groundmass is microcrystalline with 60‐70% microlitic plagioclase (<0.1 mm), 10‐15% Fe‐Ti oxides (<0.1 mm), and interstitial glass. Dacitic compositions are sparsely porphyritic with phenocrysts of hornblende, in a hypocrystalline microporphyritic to microcrystalline groundmass with 3‐20% microphenocrysts of plagioclase (0‐20%), hornblende (3‐5%), and clinopyroxene (0‐2%) (Figure 4 c‐d). Plagioclase occurs as 0.25‐0.5 mm euhedral to subhedral un‐zoned laths. Sparse (< 1%) sieve texture is evident in the cores of plagioclase crystals. Hornblende appears as 0.5 mm to 2 mm (mostly 0.5 mm) acicular crystals. Hornblende textures are varied and include intact cores with thick opacite rims, hollow 10
cores, and some crystals are pseudomorphed by fine‐grained black aggregates of Fe‐Ti oxides, plagioclase, and pyroxene. Subhedral clinopyroxene appears as 0.5‐0.75 mm grains. The groundmass is microcrystalline with 50‐55% microlitic plagioclase (<0.1 mm), 10‐15% oxides (<0.1 mm), and interstitial glass. Barkley Mountain Group Basaltic andesite compositions of the Barkley Mountain group are sparsely porphyritic with sparse phenocrysts of olivine and clinopyroxene to aphyric in a microcrystalline groundmass with 25‐30% microphenocrysts of plagioclase (15‐20%), clinopyroxene (1‐5%), olivine (3‐5%), and hornblende (0‐1%) (Figure 5 a‐b). Plagioclase occurs as 0.25‐0.5 mm unzoned to weakly zoned subhedral laths. Subhedral clinopyroxene are 0.5 mm grains. Anhedral olivine occurs as 0.1‐0.5 mm grains with most displaying iddingsite rims. Acicular hornblende occurs as 0.25‐0.5 mm psuedomorphs of fine‐grained black aggregates of Fe‐Ti oxides, plagioclase, and pyroxene. The groundmass is microcrystalline with 50‐55% microlitic plagioclase (<0.1 mm), 10‐15% Fe‐Ti oxides (<0.1 mm), and minor interstitial glass. Andesite compositions are sparsely phyric with sparse phenocrysts of hornblende in a hypocrystalline microporphyritic groundmass with 2‐30% microphenocrysts of hornblende (2‐5%), plagioclase (0‐20%), clinopyroxene (0‐3%), olivine (0‐3%), and orthopyroxene (0‐1%) (Figure 5 c‐d). Hornblende occurs as 0.25‐1 mm (mostly 0.25‐0.5 mm) subhedral acicular crystals. Textures are varied and include intact cores with thick opacite rims, hollow cores, grains that are completely pseudomorphed by black Fe‐Ti oxides, plagioclase, and pyroxenes. Some hornblende grains are completely resorbed and only acicular dark areas are left. Subhedral laths of plagioclase are 0.25‐0.5 mm grains. No compositional zoning was observed. Clinopyroxene appears as 0.25‐0.5 mm subhedral grains. Subhedral equant olivine occurs as 0.25‐0.5 mm grains with thick pyroxene 11
reaction rims. Orthopyroxene occurs as subhedral 0.25‐0.5 mm grains. The groundmass is microcrystalline with 30‐50% microlitic plagioclase (<0.1 mm), 10‐15% Fe‐Ti oxides (<0.1 mm), 5‐7% pyroxene (<0.1 mm), and interstitial glass. Whole Rock Chemistry Major Elements Whole rock data are reported in Table 1 and Harker variation diagrams are presented in Figures 6 and 7. Both the Onion Butte and Barkley Mountain groups are subalkaline according to the TAS (total alkalis versus silica) diagram (not shown). According to the classification of Irvine and Baragar (1971), the groups are calc‐alkaline in nature (Figure 6a) and are classified as Medium‐K (Figure 6b) by using the classification of Le Maitre et al. (1989). The Onion Butte and Barkley Mountain groups are compositionally similar as seen on Harker variation diagrams in Figure 7 with the exception of TiO2. The Onion Butte group is almost exclusively dacitic (64.0‐65.2 wt. % SiO2), but subordinate andesite (57.4‐58.4 wt. % SiO2) are shown in Figure 7. The Onion Butte group form near linear trends, but have compositional gaps with clusters of samples showing small ranges in SiO2 (Figure 7). Conversely, the Barkley Mountain group shows less compositional variation with only basaltic andesite to andesite (53.6‐62.3 wt. % SiO2). The Barkley Mountain group shows significant scatter in elemental variation with SiO2, especially for MgO and Al2O3. The only major element that does not show a scattered trend in the Barkley Mountain group is FeO*. 12
Some elemental trends are similar for both groups, but there are some important distinctions. Concentrations of TiO2, FeO*, MgO, and CaO all decrease with increasing SiO2. Although both groups show the same compatible behavior for TiO2, the Onion Butte group displays less TiO2 depletion with increasing SiO2 than the Barkley Mountain group (Figure 7). Concentrations of Na2O and K2O increase with increasing SiO2. For intermediate compositions, the Onion Butte rocks have slightly higher average abundances of CaO wt. % (Figure 7). The Onion Butte group has small variation in Al2O3 (16.9‐18.2 wt. %) with increasing SiO2; conversely, the Barkley Mountain group displays wider variation in Al2O3 (16.1‐19.8 wt. %) with no correlation with SiO2 (Figure 7). Magnesium number (Mg #), a measure of Mg enrichment relative to Fe (Mg# = 100(Mg/Mg+FeOt), shows a wide range within the Onion Butte group. Conversely, the Barkley Mountain group shows a scattered correlation with increasing SiO2 and varies from 45 to 64 (Figure 7). Trace Elements Trace element abundances generally show significant scatter with increasing SiO2, but some trends are discernible (Figure 8). The trace element trends of the Barkley Mountain group, like the major element trends, are scattered and have wide variations for the same SiO2 concentrations. The Onion Butte group tends to cluster with like SiO2 concentrations. The most significant compositional distinction is high Sr (>1,000 ppm Sr) in the Onion Butte group with the exception of dome 7 andesite (530 ppm Sr, Figure 8). This distinction in Sr is also true for Nb (high Sr samples are distinctly lower in Nb) and (Sr/P)N. The primitive‐mantle‐normalized trace element diagram (Figure 9a) shows patterns distinctive of most calc‐alkaline arc lavas. Both groups are enriched in the large ion lithophile elements (LILE), and have the characteristic depletions in Nb and Ta, and lower abundances in other high field strength elements (HFSE) of arc magmas. The Onion Butte and Barkley Mountain groups 13
have similar shaped patterns (with some exceptions), but vary significantly in overall abundances. The distinctions between the groups include significantly higher Sr and (Sr/P)N, and lower Rb, Pb, and HREE in the Onion Butte as compared to Barkley Mountain (Figure 9a). Interestingly, the dacites of Onion Butte that have significantly higher Sr also have significantly lower HREE abundances than any other rock type or group. The andesites of the Barkley Mountain group tend to have slightly higher U than the Onion Butte group (Figure 9a). An important characteristic of the Onion Butte group are geochemical differences between samples that have similar SiO2 (Figure 10). For example, Onion Butte andesites (Domes 1 and 7) show significantly different trends from each other. Dome 1 (58.4 wt. % SiO2 with 1447 ppm Sr) has nearly the same pattern (and concentrations) as the dacites (Figure 10a). However, Dome 7 with similar SiO2 (57.4 %) has 530 ppm Sr and a pattern more characteristic of a Barkley Mountain andesite with higher abundances of Rb, U, and Pb, and lower abundances of Th, Sr, and has lower abundances in HREE’s (Figure 10b). The Onion Butte dacites have the same overall trend with a notable difference in dome 5. It has the same LILE trend as the other dacite domes (including high Sr), but has a HREE trend that is remarkably like the Barkley Mountain group 1 andesites (described below) that have a strong depletion in Ti, and higher abundances of Tb to Lu (Figure 10a). Like the Onion Butte group, the Barkley Mountain group has geochemical differences for samples having similar SiO2 (Figures 11 and 12). The Barkley Mountain basaltic andesites have the widest variation in the HFSE (Figure 11). The basaltic andesites (Domes 15, 18, and 25) have Ti depletions and a slight positive Eu anomaly. Dome 24 has the lowest abundances in the HREE while it is the most silicic of the basaltic andesite group (Figure 11). Dome 15 has low Th and Pb, and higher Sr than any other basaltic andesites from the Barkley Mountain group (Figure 8 and 11). 14
The Barkley Mountain andesites have slightly different trends for similar SiO2 that can be divided into two subgroups (Figure 12). Barkley Mountain subgroup 1 andesites include domes 17, 24, 20, and 28. This subgroup has lower Zr and Hf abundances, pronounced Ti depletions, pronounced depletions in Ti, and tightly clustered Yb and Lu (Figure 12a). Barkley Mountain subgroup 2 andesites include domes 23, 26, 31, 33, 34, and 35. This subgroup is characterized by higher Zr and Hf abundances, no pronounced Ti depletion, and wider variations in Yb and Lu (Figure 12b). Dome 26 has a slight Ti depletion, but shares all described geochemical characteristics from subgroup 2 (Figure 11b). On the chondrite‐normalized rare earth element (REE) diagram (Figure 13a and b), the Onion Butte and Barkley Mountain groups are very similar. Both have moderately steep REE patterns with (La/Sm)N of 1.9‐3.6, no overall correlation of LREE with SiO2, and no Eu anomaly (Figure 13c). Both groups show a negative correlation of HREE with SiO2 (Figure 13c). The Onion Butte group generally has similar REE concentrations and patterns with a few exceptions. The dacite from dome 5 has higher REE concentrations, but similar Sr concentrations (1104 ppm) and (La/Sm)N as other Onion Butte dacites (Figure 14a). Dome 9‐02 dacite has the same overall pattern but has the highest LREE concentrations and Sr (1634 ppm), but has the same HREE concentrations of the dacites (Figure 14a). Dome 1 andesite has the same concentrations and patterns as the dacites, but dome 7 does not follow that trend. Dome 7 andesite has the lowest LREE and highest HREE concentrations of the Onion Butte group (Figure 14b). Within the Barkley Mountain group, the REE of the andesites highlight the distinctions in the two subgroups described above. In subgroup 1 andesites, the HREE abundances decrease with increasing SiO2 and tightly cluster together (Figure 15a). The HREE abundances in subgroup 2 andesites decrease with increasing SiO2, but are more variable in concentration and do not cluster 15
like subgroup 1 andesites (Figure 15b). Both subgroup andesites share similar LREE abundances as the basaltic andesites and do not show a clear correlation with SiO2 (Figure 13c and Figure 15). To summarize, the Onion Butte group is characterized by significantly higher Sr abundances and (Sr/P)N, and low Rb and Pb abundances. Within this group the dacites and andesites from domes 1 and 9 have lower HREE abundances and moderately steep REE patterns with (La/Sm)N of 2.6 – 3.6. The Barkley Mountain group is characterized by higher abundances in P and low (Sr/P)N. Within this group the andesites have lower HREE concentrations, but separate in two subgroups based on the abundances of HREE. Subgroup 1 andesites tightly cluster in HREE concentrations while subgroup 2 is more variable. Both subgroups have moderately steep REE patterns with (La/Sm)N of 2.1 – 3.4. Isotope Composition Isotopic compositions for Sr and Nd were analyzed for the most mafic, the intermediate, and most silicic of each group and are reported in Table 2. The samples fall within or just outside the mantle array on an εNd versus 86Sr/87Sr plot (Figure 16a). The Onion Butte isotopic ratios tend to cluster together as does the Barkley Mountain group. The only exception is dome 7 (low‐Sr Onion Butte andesite) which plots with the Barkley Mountain group. The distinct isotopic signature of the Onion Butte group compared to the Barkley Mountain group mirrors the isotopic distinction between high (Sr/P) N and low (Sr/P)N primitive magmas found within the regional scale volcanism (Borg and Clynne, 1998). The fact that the lone, low‐Sr Onion Butte sample matches the isotopic characteristics of the low‐Sr Barkley Mountain group is a dramatic indication of the importance of Sr concentrations as a discriminator of magma sources in the Lassen region. Measured 86Sr/87Sr values range from 0.7030 to 0.7044. The 86Sr/87Sr range is wider in the Onion Butte group (0.7030 ‐ 0.7033) than the Barkley Mountain group (0.7042 ‐ 0.7044). The 86
Sr/87Sr does not vary in either group with increasing SiO2 (Figure 16b). In the Onion Butte group, 16
the 86Sr/87Sr is lowest for high Sr concentrations (Figure 16c). The Onion Butte dacites have the lowest 86Sr/87Sr in the LVR compared to other silicic rocks. Measured εNd values show more variation ranging from +3.4 to +6.5. The Barkley Mountain group shows very little variation (+3.6 ‐ +5.0) that slightly decreases with increasing SiO2 (Figure 16b). The Onion Butte dacites show the widest variation in εNd ranging from +3.4 to +6.5. No clear correlation with SiO2 is observed, however, εNd increases with increasing Sr (Figure 16b and c). In summary, the Onion Butte and Barkley Mountain group cluster, for the most part, in separate groups. The Onion Butte group isotopic compositions indicate a depleted source with the lowest 86Sr/87Sr and the highest εNd. Conversely, the Barkley Mountain group has more enriched compositions with higher 86Sr/87Sr and lower εNd ratios. The Onion Butte group includes one exception; the low‐Sr dome 7 andesite. Within the Onion Butte group, both 86Sr/87Sr and εNd show a weak correlation with increasing Sr abundance. There are no clear correlations with increasing SiO2 for any group. Discussion To describe the processes active in producing the magma suites erupted in the Onion Butte and Barkley Mountain groups, this discussion will be divided into two sections: 1) geochemical comparison of the Onion Butte and Barkley Mountain groups to other Lassen Segment units to help constrain processes and potential parents and; 2) hypotheses for generation of the Onion Butte and Barkley Mountain magmas. This second section will start by constraining inputs into petrologic models followed by descriptions of modeling efforts. This will be followed by a summary of the results of modeling efforts, and a description of the resultant acceptable hypotheses for the generation of each group. 17
Comparison to Magmas from the Lassen Segment and Potential Petrogenetic Hypotheses This section aims to elucidate possible petrogenetic hypotheses by comparing the Onion Butte and Barkley Mountain groups to other Lassen segment lavas. There are distinct petrographic characteristics (e.g. lack of plagioclase phenocrysts and mixing textures, hornblende phenocrysts, etc) and geochemical characteristics (e.g. high Sr and low 87Sr/86Sr concentrations in the Onion Butte group and decreasing HREE with increasing SiO2 for both groups) that are distinct from other Lassen segment lavas. Any differences and possible similarities may provide insight into the generation of the Onion Butte and Barkley Mountain group and provide constraints on inputs for petrologic modeling. Basaltic Andesites and Andesites The most primitive compositions analyzed in this study, the basaltic andesites from Barkley Mountain group, are geochemically and isotopically similar to the basaltic andesites from the Lassen segment (Figure 16 and 17). These regional basalts and basaltic andesites are thought to have been derived by fractional crystallization of regional primitive calc‐alkaline basalts (Bullen and Clynne, 1990; Clynne, 1990). Two compositionally distinct primitive magmas are identified in the Lassen segment: low‐
potassium olivine tholeiites (LKOTs; also called high alumina olivine tholeiites) and calc‐alkaline basalts. The calc alkaline primitive basalts form a continuum of isotopic and compositional diversity. On one end of the spectrum are primitive lavas with low concentrations incompatible elements, high ratios of LILE to HFSE abundances (i.e. Sr/P), higher LREE/HREE, and Sr and Nd isotopic compositions similar to those of MORB. On the other end of the spectrum, are primitive lavas with high abundances of incompatible elements, lower LILE/HFSE, lower LREE/HREE, and Sr and Nd isotopic compositions like those of ocean island basalts (OIB) (Borg and Clynne, 1997 and Borg et al, 18
2000). To divide the continuum of compositions, Borg et al. (1998 and 2000) used (Sr/P)N = 3.3 as to distinguish high (Sr/P)N from low (Sr/P)N primitive lavas. The same division is used in this discussion. Isotopically the Onion Butte group plots with the high (Sr/P)N primitive lavas while the Barkley Mountain group plots with the low (Sr/P)N primitive magmas (Figure 16). These data clearly indicates that the Onion Butte group is derived from high (Sr/P)N sources and the Barkley Mountain group is derived from low (Sr/P)N sources. Moreover, the Barkley Mountain basaltic andesites have low (Sr/P)N similar to the low (Sr/P)N primitive magmas as well as the entire Lassen segment (Figure 18). The Onion Butte is slightly lower than the (Sr/P)N = 3.3 division, but generally has higher (Sr/P)N than similar SiO2 concentrations in the Lassen segment (Figure 18) Intermediate volcanism is found at both scales (regional and large volcanic centers) in the Lassen segment. Intermediate volcanism in the regional scale occurs at <60 wt. % SiO2 while the intermediate volcanism at the volcanic centers has continuous silica concentrations from 57‐63 wt. % SiO2. The andesites from both Barkley Mountain and Onion Butte generally plot with the Lassen intermediate lavas (Figures 17 and 18). Both groups share similar MgO, K2O (wt. %), Sr, Y (ppm), and (Sr/P)N concentrations. Dome 1 andesite Sr concentration is shown in Figure 17 as the highest Sr concentration; however, it should be noted other andesites in the regional scale with high Sr concentrations occur (Clynne, 1993). These andesites are exceedingly rare. The andesites of the Barkley Mountain group share geochemical characteristics with the Lassen volcanic center andesites, potentially indicating similar processes for their generation. While the Onion Butte group share similarities in major elements to the Lassen andesites, their high Sr concentrations are distinct and may indicate a different process or source. Closed system fractional crystallization models for regional scale andesites and Brokeoff andesites (from the Brokeoff Volcano, a composite cone at the Lassen volcanic center) successfully 19
reproduced major elements, but failed to reproduce K2O and trace element trends (especially incompatible elements in the regional scale andesites) (Bullen and Clynne, 1990; Clynne, 1990; Clynne, 1993). Successful models for Brokeoff andesite generation included fractional crystallization from regional basalts with understated, but significant, magma mixing with a trace element poor high SiO2 magma (Bullen and Clynne, 1990; Clynne, 1990). Disequilibrium textures observed in all Brokeoff andesites supported their interpretation (Bullen and Clynne, 1990; Clynne, 1990). For regional scale andesite generation, successful models indicate fractional crystallization of a medium‐
K calc‐alkaline basalt combined with magma mixing of a high‐silica rhyolite (Clynne, 1993). Evidence for magma mixing in the regional scale andesites is sparse compared to the Lassen volcanic center. Evidence is subtle to cryptic indicating magma mixing occurred between compositionally similar magmas or enough time passed after mixing episodes for textural disequilibrium to be erased (Clynne, 1993). Magma mixing is an important process in the Lassen segment. Magma mixing requires two distinct magma types or texturally different magmas that either mix to different degrees to form various compositionally hybrid magmas, or mixes incompletely to form quenched magmatic inclusions (Sparks and Marshall, 1986; Clynne 1999). This type of mixing is very common at Brokeoff Volcano from Lassen volcanic center. In the regional scale, magma mixing is less overt and textural evidence is rare (Clynne, 1993). Regardless of the type of magma mixing, it is an important process that contributes to chemical (whole rock and mineral) and textural diversity (Sparks and Marshall, 1986; Bacon, 1986; Stimac and Pearce, 1992; Davidson and Tepley, 1997; Clynne, 1999; Tepley et al., 1999). In considering an origin by magma mixing for the Barkley Mountain andesites, at least two possibilities exist: 1) mixing between the silicic magmas similar to Onion Butte dacites and a basaltic andesite from either group, or 2) mixing between silicic magmas from the Lassen segment and a 20
basaltic andesite from either group. While Figure 7 shows the existence of some linear trends between the Onion Butte and Barkley Mountain group, the trends of many elements preclude a simple mixing possibility with both end members having constant concentrations (i.e. Rb, Ni, Pb, Sr, Ba, Nb; Figures 8, 9, and 13). The second possibility (mixing between a basaltic andesite from the Barkley Mountain group and a silicic magma from the Lassen segment) is permissible given Harker variation diagrams (not shown) that display straight line patterns characteristic of magma mixing trends. Despite permissive geochemical evidence for magma mixing, there is not sufficient physical evidence (magmatic inclusions, fragments of magmatic inclusions, and phenocryst disequilibrium textures; e.g. Bacon, 1986; Sparks and Marshall, 1986; Clynne 1999). While regional scale mixing episodes do not have obvious physical evidence for magma mixing, mineral chemistry should provide evidence for any magma mixing process (either compositionally distinct magmas or compositionally similar). This project does not have mineral chemistry to give as evidence for possible magma mixing processes. Given the lack of physical evidence and mineral chemistry for magma mixing, this process is not further considered in the generation of the Barkley Mountain andesites. A magma mixing only origin for the Onion Butte andesite from dome 1 is also not reasonable. The andesite from the Onion Butte group lacks disequilibrium textures in microphenocryst phases (including hornblende phenocrysts), and disaggregated inclusions that would provide evidence for a mixing origin. Furthermore, the andesite from dome 1 has the same trace (and REE) concentrations (Figures 8, 9b, and 13) as the dacites which cannot be explained by magma mixing processes. Thus, mixing will not be considered for the Onion Butte andesite. 21
Dacites Silicic volcanism is more abundant at the long lived, large volcanic centers and as a result, petrogenetic studies have been focused at these volcanic centers (Borg and Clynne, 1998). Borg and Clynne (1998) divided these silicic lavas into two groups with group 1 characterized by lower silica, high (Dy/Yb)N, low (La/Yb)N, and flatter REE patterns due to MREE being near equal to HREE. Group 2 has lower (Dy/Yb)N, higher (La/Yb)N, Ce anomalies, and strong MREE depletions creating a strongly concave upward REE pattern. Both groups are characterized by enriched LREE, no variation with silica in LREE or Nb, and indistinguishable isotopic ratios (Borg and Clynne, 1998). Based on the above geochemical characteristics and REE modeling, Borg and Clynne (1998) determined that these silicic magmas were produced by partial melting of a basaltic lower continental crust under variable f(H2O) and temperature conditions. Low f(H2O) and high temperature conditions during melting leaves an amphibole‐poor source characteristic of the group 1 felsic lavas. Partial melting under high f(H2O) and low temperature conditions leaves an amphibole‐rich source characteristic of the group 2 felsic lavas (Borg and Clynne, 1998). The Onion Butte group does not display many geochemical similarities with either of these groups of silicic lavas from the large volcanic centers (Figure 19). These lavas have higher abundances of trace elements such as K, Rb, and Ba (Figure 19a), but are much lower in Sr and (Sr/P)N (Figure 19a and b). In addition the REE patterns of the group 2 lavas have strong MREE depletions that are not observed in the Onion Butte group (Figure 19a). While the LREE concentrations of the group 1 silicic lavas from the large volcanic centers are comparable to the Onion Butte dacites, the HREE concentrations of the Onion Butte dacites are slightly lower and the (Dy/Yb)N ratios slightly higher, than the felsic group 1 lavas (Figure 19b). The silicic center lavas have slight negative Eu anomalies while the Onion Butte group has no Eu anomaly (Figure 19a). 22
As might be expected from the lower Sr abundances and lower (Sr/P)N, the silicic center lavas are isotopically distinct from the Onion Butte suite, and display isotopic characteristics of low (Sr/P)N primitive lavas (Figure 20). As described earlier, the Onion Butte group is isotopically similar to high (Sr/P)N primitive lavas (Figures 16 and 20). The differences between the Onion Butte dacites and the large volcanic center silicic lavas suggest that if partial melting of lower crust is responsible for their generation, the crustal source must be both chemically and isotopically distinct from the source that generated the large volcanic center silicic lavas. The lower crustal source would need to be low in incompatible elements, with high Sr concentrations, and similar isotopic compositions to the high (Sr/P)N primitive lavas. Models for Generation of the Barkley Mountain and Onion Butte Groups The primary argument in the following discussion uses the assumption described above that basaltic andesites from Barkley Mountain are related to the distinctive low (Sr/P)N primitive magma suites. It is the more differentiated compositions (andesites and dacites) whose origins are more complicated. Because there are distinct geochemical differences between the Onion Butte and Barkley Mountain group intermediate and silicic lavas, their generation will be analyzed and modeled separately. The following sections will provide an evaluation of potential processes for modeling efforts, potential parents, and mineral control. The last section will describe a petrogenetic model describing how each group was generated. Generation of Intermediate Lavas from the Barkley Mountain Group In this section, I employ major and trace element modeling to help elucidate the processes that generated the Barkley Mountain group. Major element modeling is addressed first to test whether fractional crystallization of Barkley Mountain basaltic andesite to andesite is a viable 23
hypothesis, followed by fractional crystallization modeling of rare earth elements. Because the andesites were previously divided into two subgroups based on Zr, Hf, Ti, and Yb (Figure 11), modeling efforts will be discussed separately. Barkley Mountain Andesites Andesite generation within the Cascade arc is varied and most models are attributed to open system processes. Andesite generation models include simple closed system crystal fractionation (Bacon and Druitt, 1988), magma mixing between mafic and felsic end members (Kent et al., 2010), possible assimilation of mafic to ultramafic crystal debris in silicic magmas (Streck et al., 2007), and melting of a MORB‐like granulite to generate Sr‐rich andesite (Conrey et al., 2001). Within the Lassen segment, fractional crystallization from regional basalts with understated, but significant, magma mixing with a trace element poor high SiO2 magma (Bullen and Clynne, 1990; Clynne, 1990; Clynne, 1993). In the following sections, I test whether closed‐system fractional crystallization is viable alternative to open system processes for the Barkley Mountain andesites. In addition to the smoothly continuous increasing or decreasing trends with silica, Barkley Mountain andesites have moderately steep REE patterns and HREE’s that decrease with increasing silica. These geochemical characteristics suggest fractional crystallization that involves hornblende ± garnet. To test whether fractional crystallization is viable, major element modeling was first considered. I used the thermodynamic model MELTS, a computer program (Ghiorso et al. 1993), to determine whether fractional crystallization of Barkley Mountain basaltic andesites could reproduce observed major element trends in the Barkley Mountain andesites. There are several intensive variables that are input into the model before it is run. The following is a list of the variables and the values selected: parental magma composition (BM‐BA‐14), water concentrations (1.5 and 3.0 wt. %), 24
oxygen fugacity f(O2) (Ni‐NiO and Q‐F‐M +2), pressure (6 and 12 kbars), and temperature. The starting temperature for each run was set to the liquidus of the system which is calculated by MELTS for the variables input into the program. In each run garnet, hornblende, or some combination thereof was fractionating. Plagioclase was not readily calculated as a fractionating phase. For a complete discussion on MELTS modeling see Appendix III. MELTS modeling had mixed results at the parameters set for the Barkley Mountain group (Figure A1). All runs successfully reproduced SiO2 (wt. %) concentrations observed in the Barkley Mountain andesites. For most runs, modeling underestimated or overestimated major element oxide concentrations while reproducing observed patterns (increasing or decreasing with increasing SiO2). Some runs were able to reproduce MgO, FeO*, Al2O3, Na2O, K2O, and P2O5 wt. % observed trends and concentrations. TiO2, CaO, and MnO wt. % failed to reproduce observed trends and concentrations for all runs (Figure A1). Fractional crystallization may still be considered to be a viable process despite results. Reproduced trends (increasing or decreasing with SiO2 wt. %) with overestimated or underestimated concentrations likely indicate the parameters chosen (e.g. fO2, pressure, water content, etc) were not well enough constrained for this magmatic system, not a failure of fractional crystallization. Results may also indicate multiple parents (e.g. using different basaltic andesites) are needed to fully reproduce andesite variability. The fractionating assemblage calculated by MELTS supports hornblende and garnet (to a lesser extent) to be an important fractionating phases. When hornblende was a dominant crystallizing phase models were close in predicting observed trends (Figure A1). This is supported by hornblende phenocrysts (and microphenocrysts) observed in Barkley Mountain andesites. Garnet is considered as a minor phase because it produced decreasing Al2O3 wt % with increasing SiO2 when it was the main phase crystallizing as predicted by MELTS (Figure A1). A decreasing Al2O3 wt % with 25
increasing SiO2 is not observed in the Barkley Mountain andesites. To further test fractional crystallization hypothesis, REE modeling was considered. For REE element modeling the parent composition, as used for major element modeling, is the Barkley Mountain basaltic andesites. As observed in major, trace element concentrations, REE element compositions of the andesites are variable (Figures 13b and 15). Multiple parents (e.g. using different basaltic andesites) or multiple mineral assemblages will be needed to fully account for the wide variability in the Barkley Mountain andesites (Figures 8, 9, and 13). Minerals have a large influence on the REE behavior, observed REE patterns of the Barkley Mountain andesites provide insight into potential fractionating minerals. Significant geochemical characteristics of the Barkley Mountain andesites include no Eu anomaly, decreasing HREE’s and MREE’s with increasing SiO2, and moderately steep and straight REE patterns. Arc magmas typically display a negative Eu anomaly that is attributed to the fractionation of plagioclase. The Barkley Mountain andesites lack a characteristic Eu anomaly which indicates little to no plagioclase is in the fractionating assemblage. This is supported by a lack of plagioclase phenocrysts in the Barkley Mountain andesites and suppression of plagioclase as predicted by MELTS modeling. Other geochemical characteristics of the Barkley Mountain andesites (i.e. decreasing HREEs (and MREEs) with increasing SiO2, moderately steep, and straight REE patterns) can be produced by fractionating hornblende and/ or garnet. A good indication of residual garnet or hornblende is Dy/Yb ratios (Davidson et al., 2007). Dy/Yb increases with SiO2 with garnet fractionation while Dy/Yb decreases with SiO2 with hornblende fractionation. For the Barkley Mountain group, Dy/Yb remains constant with increasing SiO2 (Figure 21). These ratios do not clearly indicate one fractionating phase over the other; it may indicate that garnet and hornblende are both part of the fractionating assemblage. 26
Garnet can be an important mineral in the generation of arc magmas. Evidence for basaltic garnet comes from deep crustal sections such as the Kohistan arc in Pakistan (Burg et al., 1998; Jan and Howie, 1981; Ringuette et al., 1999; Jagoutz et al., 2009; Jagoutz et al., 2010). The ultramafic to mafic rocks in these sections contain garnet that grade into intermediate volcanic rocks. The presence of garnet in situ with intermediate volcanic rocks provides evidence that garnet fractionation can be significant in the generation of arc related magmas (Alonso‐Perez et al, 2009). Fractionation models involving garnet have successfully modeled volcanic rocks that range from andesitic to dacite (Day et al., 1992; Harangi et al., 2001; Macpherson et al., 2006). Experimental studies investigating the role of garnet fractionation in arc magmas found garnet to be stable in a wide variety of conditions and in equilibrium with mafic compositions (Green, 1992; Alonso‐Perez et al., 2009; & Müntener et al., 2001). Müntener et al (2001) examined how water affects primitive arc magmas as crystallization took place at uppermost mantle conditions. Their results indicate that under high water concentrations (>3 wt. %), amphibole and garnet crystallize while plagioclase was suppressed. Alonso‐Perez et al (2009) showed liquids that fractionated garnet became progressively silica rich spanning from andesite to rhyolite. These experiments indicate that garnet fractionation is plausible in the generation of some andesitic magma. Garnet will be considered in the fractionating assemblage in the generation of the Barkley Mountain andesites as a minor phase. Geochemical characteristics, namely decreasing HREE with increasing SiO2 and constant Dy/Yb with increasing SiO2, supports garnet in the fractionating assemblage even though visible (either as a phenocryst, microphenocryst, or groundmass phase) garnet is not observed in the Barkley Mountain group. It must be that as garnet fractionates, it is removed from the melt and isolated so that it does not further react with the remaining melt. 27
Fractional Crystallization Modeling Efforts Fractional crystallization, or Rayleigh fractionation, will be used because it is effective in predicting trace element behavior. This equation assumes that crystals are removed from the melt. The Rayleigh fractionation equation is defined as CL = (FD‐1)Co Where CL is the concentration of a trace element in the residual melt, Co is the initial concentration of the trace element in the parent source, D is the bulk distribution coefficient of the trace element in the melt, and F is the fraction of melt remaining. Co will be the Barkley Mountain basaltic andesites. The wide variation of the andesites probably requires different parent compositions to fully model the andesites variability. Sample BM‐
BA‐14, BM‐BA‐18, BM‐BA‐25, and BM‐BA‐33 were chosen to use in modeling because they were the most mafic or had similar patterns to the target andesite compositions. Partition coefficients for REE in orthopyroxene, clinopyroxene, hornblende, plagioclase, garnet, and magnetite in andesitic liquids were compiled from several sources. These were used with assumed mineralogies crystallizing from basaltic andesite melts and were used to calculate bulk D for each REE. Partition coefficients and Co compositions are reported in Table 4 and successful modeled compositions are reported in Table 5 and 6. Generation of Subgroup 1 Andesites For modeling subgroup 1, sample BM‐A1‐24 was chosen as the target composition. This sample has the same HREE abundances, but has the lowest LREE abundances of this subgroup. It is assumed that if this model is successful, then progressive crystallization will reproduce samples that have higher abundances of LREE. 28
Modeling fractional crystallization with a Co as BM‐BA‐18 successfully reproduces observed trends after 20% crystallization (80% melt remaining) with a fractionating assemblage of cpx:hbl:grt = 31:68:1 (Figure 22a). All andesite samples (BM‐A1‐17 and BM‐A1‐20) can be reproduced with this fractionating assemblage (Figure 22b). Only BM‐A1‐28 cannot be reproduced (Figure 22b). Barkley Mountain sample BM‐A1‐28 is successfully modeled using BM‐BA‐25 as the parent basaltic andesite (Figure 23). Observed trends are reproduced for BM‐A1‐28 after 40% crystallization (60% melt remaining) with a fractionating assemblage of cpx:hbl:grt:mt = 66:25:1.5:7.5. Generation of Subgroup 2 Andesites To fully encompass the REE variability of subgroup 2 andesites, BM‐A2‐26 and BM‐A2‐31 were selected as target compositions. Sample BM‐A2‐26 represents the lowest REE concentrations of subgroup 2 while BM‐A2‐31 represents the highest REE concentrations. To fully encompass the variability of the subgroup 2 andesites, two parents were needed: BM‐BA‐25 for BM‐A2‐31 and BM‐BA‐33 for BM‐A2‐26. Modeling fractional crystallization with a Co as BM‐BA‐33 successfully reproduces observed trends in BM‐A2‐26 after 40% crystallization (60% melt remaining) with a fractionating assemblage of opx:cpx:hbl:plag:grt = 11:13:65:10:1 (Figure 24a). BM‐A2‐31 is successfully reproduced after 40% crystallization (60% melt remaining) with a fractionating assemblage of opx:cpx:hbl:plag:grt = 10:78.5:10:1.5 (Figure 24b). Generation of Dome 7 Andesite from the Onion Butte Group As stated above, dome 7 andesite of the Onion Butte group has the same geochemical and isotopic characteristics as the andesites of the Barkley Mountain group (Figures 16 and 25a). The generation of dome 7 andesite is thought to be similar to the Barkley Mountain andesites with parent compositions from the Barkley Mountain basaltic andesites. The following paragraph details 29
fractional crystallization of Barkley Mountain basaltic andesite (BM‐BA‐14) to Onion Butte andesite (OB‐A‐7). Dome 7 andesite of the Onion Butte group can successfully be modeled by fractional crystallization from BM‐BA‐14 (Figure 26). Observed trends are reproduced after 40% crystallization (60% melt remaining) with a fractionating assemblage of cpx:hbl:plag:grt = 19.5:70:10:0.5. Fractional crystallization of the Barkley Mountain basaltic andesites to andesites can be successfully modeled after 20‐40% crystallization of varying fractionating assemblages of orthopyroxene, clinopyroxene, hornblende, with minor plagioclase and garnet. A fractionating assemblage of hornblende and garnet can reconcile decreasing HREE with increasing SiO2, moderately steep and straight REE’s patterns, and constant Dy/Yb with increasing SiO2. Multiple parents successfully reproduce the wide variability observed in the andesites and can accurately explain the scattered trend observed in the Barkley Mountain andesites. As noted above, magma mixing processes have a subtle, but important role in the generation of regional andesites (Clynne, 1993). Evidence for magma mixing in the regional andesites is usually subtle to cryptic (Clynne, 1993). If magma mixing was an important process the evidence would have to be cryptic in the Barkley Mountain andesites because textural evidence was not observed. As such, magma mixing was not considered in the generation of the Barkley Mountain andesites. This does not discount magma mixing as a potential process, but evidence for magma mixing in the absence of mineral disequilibrium textures may come from mineral chemistry. Mineral chemistry was not analyzed in this project which would help elucidate magma mixing processes. This provides a basis for further work in the Barkley Mountain group. 30
Generation of Intermediate to Silicic Lavas from the Onion Butte Group Onion Butte Andesites The generation of the Onion Butte andesites (57 – 58 wt. % SiO2) is complicated. Of the two andesites observed in the Onion Butte group, only dome 1 is considered. Dome 7 andesite has the same geochemical and isotopic characteristics as the andesites of the Barkley Mountain group (Figure 25a) and was described above. The origin of the Dome 1 andesite is difficult to constrain. Its major element and Ni concentrations are distinct from the dacites (Figure 7 and 8), however; its trace element (i.e. Sr, Nb, Ba, Rb, La, Yb; Figure 25b), REE abundances, and isotopic characteristics (Figure 16) are identical. The generation of the andesite by fractional crystallization or partial melting from a mafic source is permissible, but that interpretation cannot reconcile the almost identical trace element characteristics that the andesite shares with the dacites. Assimilation of upper to middle crustal material is not feasible. Upper to middle crustal compositions are poorly constrained in the Lassen segment, but crustal xenoliths from Cinder Cone volcano have higher Sr and lower Nd isotopic compositions than Lassen segment lavas (Borg and Clynne, 1998). Mesozoic upper crust Sierra Nevada granodiorite has isotopic compositions similar to the low (Sr/P)N primitive lavas (Borg and Clynne, 1998) (Figure 27). Furthermore, Dome 1 andesite has the same isotopic compositions to the high (Sr/P)N primitive magmas (Figure 27). As noted above, simple magma mixing between a mafic magma and the Onion Butte dacites is not a viable option, but another possibility is mixing in of xenocrystic material (e.g. olivine). Addition of olivine into the dacites would alter major element chemistry, but not the trace element chemistry with the exception of Ni. If xenocrystic material (e.g. olivine) was mixed into the Onion Butte dacites the major element and Ni concentrations of the dacitic magma would be driven 31
toward andesitic compositions affecting Ni concentrations without affecting other trace and REE concentrations. While olivine is not observed in the dacites, it is likely any olivine added to a dacite would be in disequilibrium and be resorbed into the magma. Olivine subtraction modeling was done to test this hypothesis. Olivine compositions from (Clynne, 1997) were subtracted from Onion Butte andesite (dome 1) compositions. A complete description of modeling efforts is detailed in Appendix II. Results from the modeling failed to reproduce observed MgO and SiO2 concentrations (Figure A2). The generation of dome 1 andesite and its relationship to the dacites is unclear and cannot be constrained by straightforward processes (i.e. fractional crystallization, assimilation, magma mixing, or mixing of xenocrystic material). If the andesite from dome 1 is related to the dacites, then a complex model of multiple processes may need to be invoked to explain its origin and relationship to the dacites Onion Butte Dacite In the previous section comparing of large volcanic center silicic lavas to the Onion Butte dacites, I suggested that similar processes of lower crustal melting may be a viable option for their origin; however, it does not rule out other possibilities. Within the Cascade arc, dacite generation has been attributed to partial melting of the lower crust (Smith and Leeman, 1987; Venezky and Rutherford, 1997; Borg and Clynne, 1998), partial melting of the subducting slab (Defant and Drummond, 1993), magma mixing (Gross, 2012), and fractional crystallization (Bacon and Druitt, 1988; Hildreth, 2007; and Gross, 2012). I have already ruled out magma mixing given dacite geochemical trends (i.e. Rb, Ba, Pb, Nb, and Sr) and a lack of physical evidence (disequilibrium textures, disaggregated inclusion, etc). Fractional crystallization from Onion Butte andesite to dacite is not a feasible process because trace element concentrations of the andesites are same as the 32
dacites (Figures 8, 9, and 13). In the following section, I test whether partial melting (crustal derived or slab derived), best reproduces observed trends. If Onion Butte dacites are derived by partial melting, the source of melting (subducting slab derived or crustal derived) needs to be evaluated. The high Sr/Y of the Onion Butte dacites (Figures 16 and 28) suggests that melting in the garnet stability field and the low 87Sr/86Sr indicates a possible subducting slab source to produce an “adakite” signature is a permissible hypothesis. This hypothesis is evaluated in the following paragraphs. Adakites, originally termed by Defant and Drummond (1990), were first described by Kay (1978) as high magnesium andesites from Adak Island, Alaska. Kay (1978) proposed that the geochemical characteristics of the high magnesium andesites were derived by partial melting of the subducting oceanic crust. Defant and Drummond (1990) proposed a set of geochemical criteria that compositionally define adakites. These criteria implied an origin by partial melting of the subducting slab or “slab melts”, but the term adakites has evolved to include numerous origins including garnet‐
bearing lower crust derived melts and high pressure differentiation (Garrison and Davidson, 2003; Castillo, and 2006; Macpherson, 2006). Adakites are indicated by high Sr/Y (> 40) and La/Yb > 20. These geochemical characteristics are indicative of residual garnet in the source (Defant and Drummond, 1990; Defant and Drummond, 1993; Defant and Kepezhinskas, 2001). Other notable geochemical criteria include high SiO2 (> 56 wt. %), high Al2O3 (> 15 wt. %), slight to no Eu anomaly, and MORB‐like isotopic signature (87Sr/86Sr < 0.704) (Defant and Drummond, 1990). Petrographically, plagioclase and amphibole are very common phenocryst phases (Defant and Drummond, 1990). When the criteria are applied to the Onion Butte dacites, there are striking similarities (high SiO2, high Al2O3, no Eu anomaly, and MORB‐like isotopic signature); however, an adakitic origin cannot be supported. While the Onion Butte dacites have high Sr/Y (55 – 116) (Figure 28) and 33
elevated La/Yb (11 – 23, all but dome 9‐02 are < 20), the La/Yb ratios are not sufficiently high to be derived from melting of a garnet‐bearing subducting slab. REE patterns of the Onion Butte dacites are not steep enough to fit the criteria of Defant and Drummond (1993). The Onion Butte dacites also do not have plagioclase phenocrysts that are common in adakites. Partial melting models were used to further test if the Onion Butte dacites could be reproduced by partial melting of a garnet‐bearing subducting slab. Sample 17‐10 from Davis and Claude (1987) was chosen as representative of the composition of Gorda Ridge basalts (147 ppm Sr and 29 ppm Y) assumed to be melting to produce the Onion Butte dacites. Partial melting models of the subducting Gorda plate with a melting assemblage of cpx:grt = 50:50 fails to reproduce observed Onion Butte Sr/Y concentrations (Figure 28). The model underestimated Sr/Y concentrations as observed in the Onion Butte group. For this reason, partial melting of the subducting slab is not considered as a feasible process in the generation of the Onion Butte dacites. The partial melting result eliminates the subducting slab as the source for the magma, but Onion Butte geochemical characteristics do not rule out the role of garnet in the source region. The high Sr/Y and elevated La/Yb indicate that hornblende ± garnet plays a significant role in the generation of the Onion Butte dacites. Because melting of the subducting slab failed, the source of melting will be considered to be the lower continental crust. A recurring model proposed in the Cascade arc for melting of the lower crust is emplacement of mantle‐derived basalt that provides a heat source for partial melting (Borg and Clynne, 1998; Conrey et al., 2001; Smith and Leeman, 1987). In the following discussion, I will assume that emplacement of mantle‐derived basalt into the lower continental crust will trigger melting. 34
Source composition in partial melting models is an important input. In the Lassen region, it is believed that the lower crust is amphibole‐rich, derived from underplating of mantle‐derived basalt (Stanley et al., 1990). Trace element modeling by Borg and Clynne (1998) has been successful in generating silicic magmas with this crustal composition. They approximated the composition of the lower crust to be geochemically similar to low (Sr/P)N primitive magmas given the shared geochemical characteristics to their target silicic compositions. Furthermore, the low (Sr/P)N primitive magmas are the most abundant in the Lassen segment and common within the main arc axis where the silicic magmas are concentrated (Borg and Clynne, 1998). However, as described above, low (Sr/P)N primitive basalts are not a reasonable source composition for the Onion Butte dacites given their distinct geochemical and isotopic compositions (Figures 16a and 19b). More reasonably, the Onion Butte dacites may be derived by partial melting of a source with similar geochemical characteristics to the high (Sr/P)N primitive magmas of Borg et al. (1997) (Figure 29). This hypothesis would add to the types of compositions predicted to exist in the lower crust in the Lassen segment (c.f., Borg and Clynne, 1998). This is supported by the higher abundance of high (Sr/P)N primitive mafic magmas in the forearc of the Lassen segment where the Onion Butte resides compared to the low (Sr/P)N primitive mafic magmas that are the most abundant at the arc axis and back arc (Borg et al.,1997; Borg et al., 2000; and Borg et al., 2002). Another important consideration is the mineral mode of the source. Rare earth element concentrations and patterns in the Onion Butte dacites can be used to determine potential mineral phases present in the source during melting. The critical geochemical characteristics of the Onion Butte dacites include high Sr concentrations, lack of a Eu anomaly, decreasing HREE concentrations with SiO2, and moderately steep and straight REE patterns. Europium (Eu) and strontium (Sr) are both strongly influenced by the presence of plagioclase in the source. Negative europium (Eu) anomalies in REE patterns are indicative of 35
plagioclase present in the source because Eu is compatible in plagioclase. If plagioclase is in the source during melting, Eu will preferentially stay in the solid phase generating a negative Eu anomaly. The Onion Butte dacites lack a Eu anomaly suggesting that plagioclase cannot be a major mode in the source. This is supported by the uncharacteristically high Sr concentrations in the Onion Butte dacites. Because Sr is compatible in plagioclase, high Sr concentrations and no Eu anomaly can be used as evidence to indicate little to no plagioclase in the source after melting. The HREE are strongly compatible in garnet and to a lesser extent hornblende. A clear indication of residual garnet in the source is increasing Dy/Yb (and Y) with increasing SiO2 (Davidson et al., 2007). In contrast, the MREE are strongly compatible in hornblende, and a clear indication of hornblende in the source is decreasing Dy/Yb with SiO2. In both cases, however, Yb will decrease with increasing SiO2. The Onion Butte group remains constant with increasing SiO2 (Figure 21) suggesting that both garnet and hornblende may be important phases in the source region. This is supported by the decreasing Yb (and Y) with increasing SiO2 in the Onion Butte group (Figure 13). In dehydration‐melting of amphibolite at 10 kbar from 750 – 1,000˚C, the resulting melts were high in SiO2 and in equilibrium with garnet (Wolf and Wylie, 1994). The reactions consist of hornblende + plagioclase liquid + clinopyroxene + hornblende + garnet + orthopyroxene. Because garnet can be in equilibrium with high SiO2 melts, and given the geochemical evidence (decreasing Yb with increasing silica (Figure 12), moderately steep REE’s, lack of plagioclase phenocrysts, etc), garnet and hornblende are reasonable residual minerals in the source. Lower Crustal Melting Models for the Onion Butte Dacites Equilibrium, or batch modal melting, is used in this model because it is a reliable model for conditions of lower crustal melting. Equilibrium melting requires the partial melt to continuously react and re‐equilibrate with the residual solid, and will segregate in one event. Modal melting 36
assumes that the minerals in the residual solid are contributing in equal proportions as they are found in the solid to reduce the number of variables. The batch modal melting equation is defined as CL = Co/ [D+F(1‐D)] Where CL is the concentration of a trace element in the liquid, Co is the initial concentration of the trace element in the melting solid, D is the bulk distribution coefficient of the trace element in the melt, and F is the fraction of melt produced during partial melting. The lower crust in the Lassen segment is hypothesized to be amphibole‐rich, derived from underplating of mantle‐derived basalt (Stanley et al., 1990). High (Sr/P)N basalt was used to estimate the composition of the lower continental crust. An average composition was calculated after Borg and Clynne (1998) that included all Lassen high (Sr/P)N primitive magma data that had MgO > 8 wt. % and primitive magma normalized (Sr/P)N > 3.3. Partition coefficients for REE and select trace elements (Rb, Ba, K, & Sr) in orthopyroxene, clinopyroxene, hornblende, plagioclase, and garnet in dacitic liquids were compiled from several sources. These were used to calculate bulk D for each REE and select trace elements. Partition coefficients and the composition of Co are reported in Table 7 and successfully modeled compositions are reported in Table 8. Because Onion Butte dacites show significant variation in LREE concentrations, target partial melt compositions (OB‐D‐4 and OB‐D‐13) were chosen that represent that variation. Sample OB‐D‐4 has slightly higher LREE abundances than OB‐D‐13 and has lower HREE abundances than OB‐D‐13. Partial melting models successfully reproduce the overall observed pattern and HREE concentrations, but fail to reproduce trace elements (Rb, Ba, K, and Sr) and LREE concentrations (Figure 30). For sample OB‐D‐4, the model can successfully reproduce HREE trends at 20% melting with a melting assemblage of opx:cpx:hbl:plag:grt = 35:47:12:5:1 (Figure 30a). Sample OB‐D‐13 has 37
the same results as sample OB‐D‐4, but requires a melting assemblage of opx:cpx:hbl:plag:grt = 40:44:11:3:2 (Figure 30b). A higher bulk D or a bulk D≈ 1 (for Sr‐Eu) could produce a successful model; however, a mineral assemblage could not be found to increase the bulk D’s. Because a mineral assemblage could not be found to increase the bulk D’s for the trace elements and REE, a secondary solution is to lower the initial concentrations of the source composition for these elements. Lowering the initial compositions is not unreasonable given the difficulty to constrain lower crust composition (Rudnick and Fountain, 1995). Lowering the initial concentrations of the trace elements (Rb, Ba, K, and Sr) and the LREE the model successfully reproduces observed trends in both samples. Both samples are reproduced after 20% melting, but require different melting proportions. Target composition OB‐D‐4 is reproduced with a melting assemblage of opx:cpx:hbl:plag:grt = 35:47:12:5:1 (Figure 31a). Similarly, target composition OB‐D‐13 is reproduced with a melting assemblage of opx:cpx:hbl:plag:grt = 40:44:11:3:2 (Figure 31b). Modeled melt compositions are reported in Table 8. Despite successful lower crustal partial melting models, very recent studies suggest slab melts play a more important role in the generation of Lassen segment magmas (Walowski et al., 2015 and Walowski, written communication). As noted above, the geochemical characteristics of the Onion Butte dacites do not fit the geochemical characteristics of adakites, and partial melting of the Gorda plate does not reproduce the geochemical characteristics of the Onion Butte dacites, namely Sr/Y. However, Walowski et al. (2015) suggests the slab dehydrates deep in the interior of the subducting plate. These released fluids migrate through the subducting slab and can induce melting at the top of the subducting slab (Walowski et al., 2015). The resultant hydrous slab melts migrate through the mantle wedge, interact with the mantle wedge, and induce melting (Walowski, written communication). These melts generated would likely produce adakite‐like signatures (i.e. high Sr/Y and La/Yb), but with lower concentrations of important adakite characteristics (i.e. Sr/Y 38
and La/Yb) (Walowski, et al., 2015). These melts are important because they may be indirectly related the Onion Butte andesites and dacites. This idea provides a basis for future investigation. Petrogenetic Model The following petrogenetic model for the origin of the Onion Butte and Barkley Mountain groups in the Lassen volcanic region are constrained by mineralogy, geochemistry, and modeling efforts. The origin of the Onion Butte group is envisioned to start with emplacement of basaltic magmas similar in composition to high (Sr/P)N primitive magmas into the lower crust (Figure 32). The emplaced basaltic magmas induce melting of a basaltic lower crust. The amphibolite lower crust is composed of underplated basaltic magmas with compositions similar to the high (Sr/P)N primitive lavas exposed on the surface which adds to the diversity of compositions previously hypothesized to lie in the lower crust (Borg and Clynne, 1998). Partial melting (~20%) of the amphibolitic lower crust generates dacitic magmas of the Onion Butte group (Figure 32). Partial melting assemblages in various proportions of pyroxene, hornblende, minor plagioclase and garnet produce silicic magmas characterized by high Sr, high Sr/Y, decreasing HREE with increasing SiO2, moderately steep and straight REE patterns, and with low 87Sr/86Sr and high 143Nd/144Nd isotopic compositions. Partial melting is consistent with published thermal calculations that demonstrate emplaced basaltic magmas in the lower continental crust provides the heat source to induce melting (Guffanti et al., 1996). The unique geochemical characteristics of the Onion Butte group compared to other Lassen segment lavas are thought to reflect the parent composition of the high (Sr/P)N primitive magmas which make up <10% by volume of the mafic suite in the Lassen segment (Borg and Clynne, 1997) which may explain the Onion Buttes rare geochemical characteristics. While the high (Sr/P)N primitive magmas are rarer compared to the Lassen segment as a whole, these lavas are more 39
abundant in the forearc where the Onion Butte group is located (Borg and Clynne, 1997; Borg et al., 2002). This bolsters a more diverse lower crust composition beneath the Lassen segment. One lone Onion Butte andesite (dome 1) composition was not successfully modeled. It is unclear what the relationship is of the Onion Butte andesite to the dacites, but it does share the same characteristics (both geochemical and physical) of the group. Emplacement of low (Sr/P)N mafic magmas at the base of the crust, which subsequently stall and fractionate, are thought to generate the Barkley Mountain basaltic andesites. The basaltic andesite magmas separate as individual batches and fractionate further producing the andesites of the Barkley Mountain group that erupt at the surface (Figure 32). Moderate amounts of fractionation (20‐60%) of various mineral proportions of pyroxene, hornblende, magnetite with minor plagioclase and garnet produce andesitic magmas characterized by low (Sr/P)N, variable LREE’s, decreasing HREE with increasing SiO2, and isotopic compositions after the low (Sr/P)N primitive magmas. Conclusion The driving question for this study was to determine the origin and evolution of the Onion Butte and Barkley Mountain group. Several conclusions have been reached about their origin based on detailed petrographic descriptions, geochemistry, observations on differences between these two groups to other Lassen segment lavas, and geochemical modeling. First, both groups are likely derived from two distinct types of primitive magmas, high (Sr/P)N and low (Sr/P)N. The Onion Butte group is generated from high (Sr/P)N sources and the Barkley Mountain group is generated from low (Sr/P)N sources. The rarity of the distinctive geochemical characteristics (i.e. high Sr and low 87Sr/86Sr isotopic ratios) reflects the rarity of the high (Sr/P)N primitive magmas in the Lassen region. Modeling of both groups (Onion Butte and Barkley Mountain) required hornblende ± garnet to 40
reproduce all observed trends. Sr concentrations in Lassen segment lavas can be used to differentiate magma sources. The dacites of the Onion Butte group are likely formed by partial melting of the lower continental crust. Melting a basaltic lower crust composition similar to the high (Sr/P)N primitive magmas adds to the types of compositions that could exist in the lower crust in the Lassen segment. Although some geochemical characteristics of the Onion Butte dacites (high Sr, high Sr/Y, and low 87Sr/86Sr isotopic compositions) are shared with proposed slab derived melts, the dacites mostly have La/Yb ratios too low to be derived by partial melting of the subducting slab. Furthermore, partial melting modeling using eclogite mineralogy fails to reproduce observed Sr/Y concentrations. The single Onion Butte andesite appears to be part of the Onion Butte group as it has the same characteristics (both geochemical and physical). But, its relationship to the dacites is unclear and cannot be constrained by straight forward processes (i.e. fractional crystallization, assimilation, magma mixing, or mixing of xenocrystic material). Barkley Mountain andesite generation is likely due to fractional crystallization of Barkley Mountain basaltic andesites which are derived from low (Sr/P)N basaltic parents. The Barkley Mountain andesites require multiple basaltic andesite parents to fully reproduce all observed geochemical trends (i.e. clustered HREE vs. variable HREE concentrations). Further Work on the Onion Butte and Barkley Mountain Groups While this study provides a starting point in the generation of the Onion Butte and Barkley Mountain groups further work will help constrain models put forth in this study. More geochemical analyses from the domes, especially from the Onion Butte andesite, will help explain the role and generation of the Onion Butte andesites. With recent studies indicating slab melts are generated beneath the Lassen segment, slab melting models should be revisited. While pure slab melts could 41
not reproduce the Onion Butte dacite characteristics (i.e. high Sr/Y and La/Yb), indirect interaction with slab melts may be plausible. Mineral data would be highly beneficial for both groups. It is crucial for investigating the potential role of magma mixing in the generation of the Barkley Mountain andesites. While physical evidence (i.e. disequilibrium textures) is not observed in the Barkley Mountain andesites, magma mixing is commonly cryptic in regional scale andesites. Mineral data should indicate if magma mixing processes occurred. 42
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19,582. Leeman , W.P., Tonarini, S., Chan, L.H., and Borg, L.E., 2004, Boron and Lithium isotopic variations in a ot subduction zone – the southern Washington Cascades: Chemical Geology, v. 212, p. 101‐124. Leeman, W.P., Lewis, J.F., Evarts, R.C., Conrey, R.M., and Streck, M.J., 2005, Petrologic constraints on the thermal structure of the Cascade arc: Journal of Volcanology and Geothermal Research, v. 140, p. 67‐105 Macpherson, C.G., Dreher, S.T, and Thirlwall, M.F., 2006, Adakites without slab melting: High pressure differentiation of island arc magma, Mindanao, the Philippines: Earth and Planetary Science Letters, v. 243, p. 581‐593. 47
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Sparks, R.S.J, and Marshall, L.A., 1986, Thermal and mechanical constraints on mixing between mafic and silicic magmas: Journal of Volcanology and Geothermal Research, v. 29, p. 99‐124. Stanley, W.D., Mooney, W.D., and Fuis, G.S., 1990, Deep crustal structure of the Cascade range and surrounding regions from seismic refraction: Journal of Geophysical Research, v. 95, no. B12, p. 19,465‐19,474. Stimac, J.A, and Pearce, T.H., Textural evidence of mafic‐felsic magma interaction in dacite lavas, Clear Lake, California: American Mineralogist, v. 77, p. 795‐809. Streck, M.J., Leeman, W.P, and Chesley, J., 2007, High‐magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive mantle melt: Geology, v. 35, no. 4, p. 351‐354. Sun, S. and McDonough, W.F., 1989, Chemical and isotopic systematic of oceanic basalts: implications for mantle composition and processes: Geological Society, v. 42, p. 313‐345. Tepley, F.J., Davidson, J.P., and Clynne, M.A., 1999, Magmatic interactions as recorded in plagioclase phenocrysts of Chaos Crags, Lassen volcanic center, California: Journal of Petrology, v. 40, no. 5, p. 787‐806. Venezky, D.Y., and Rutherford, M.J., 1997, Pre‐eruption conditions and timing of dacite‐andesite magma mixing in the 2.2 ka eruption at Mount Rainier: Journal of Geophysical Research, v. 102, no. B9, p. 20,069‐20,086. Walowski, K.J., Wallace, P.J., Hauri,E.H., Wada, I., and Clynne, M.A., 2015, Slab melting beneath the Cascade arc driven by dehydration of altered oceanic peridotite: Nature Geoscience, DOI: 10.1038/NGEO2417. Walter, S.R., 1986, Intermediate‐focus earthquakes associated with Gorda plate subduction in northern California: Bulletin of Seismological Society of America, v. 76, no. 2, p. 583‐588. Wolf, M.B., and Wyllie, P.J., 1994, Dehydration‐melting of amphibolite at 10 kbar: the effects of temperature and time: Contributions to Mineralogy and Petrology, v. 115, p. 369‐383. 49
Table 1: Whole Rock Major and Trace Element Data
Group
Onion Butte
Onion Butte
Onion Butte
Onion Butte
Sample
OB ‐ A ‐ 1
OB ‐ D ‐ 4
OB ‐ D ‐ 5
OB ‐ A ‐ 7
Lassen Sample #
Dome
1
4
5
7
Major Element (wt. %)
58.45 65.07 65.23 57.41 SiO2
0.769
0.509
0.510
0.752
TiO2
17.32 17.01 17.35 18.22 Al2O3
6.20 4.05 4.07 6.03 FeO*
0.114
0.079
0.075
0.110
MnO
4.66 2.26 1.50 4.55 MgO
7.62 5.53 5.67 7.76 CaO
3.41 3.77 3.90 3.78 Na2O
1.24 1.54 1.54 1.21 K2O
0.21
0.172
0.17
0.18
P2O5
Total
100.00 100.00 100.00 100.00 Mg#
57
50
40
57
Trace Elements by WD‐XRF (ppm)
Ni
72.92 26.82 26.75 70.77 Cr
117.40 39.48 37.10 87.10 V
161.60 94.06 114.60 184.20 Ga
20.50 20.53 21.10 18.00 Cu
62.80 19.15 16.30 56.50 Zn
70.80 62.87 63.40 68.10 Trace Elements by ICP‐MS (ppm)
La
14.76
15.10
23.12
13.14
Ce
31.35
28.68
30.76
25.48
Pr
4.30
4.00
5.98
3.49
Nd
17.84
16.41
24.59
14.67
Sm
3.69
3.45
5.16
3.37
Eu
1.21
1.06
1.56
1.10
Gd
3.22
3.07
4.54
3.37
Tb
0.48
0.47
0.69
0.55
Dy
2.84
2.70
4.02
3.37
Ho
0.56
0.55
0.80
0.69
Er
1.49
1.49
2.20
1.86
Tm
0.21
0.22
0.32
0.27
Yb
1.34
1.34
2.10
1.75
Lu
0.21
0.23
0.35
0.28
Ba
384
378
493
421
Th
3.55
3.87
4.82
2.97
Nb
2.15
2.38
2.06
3.83
Y
14.76
14.39
20.89
18.20
Hf
2.68
2.87
2.81
2.71
Ta
0.13
0.16
0.13
0.27
U
0.91
1.04
1.06
1.12
Pb
4.32
5.52
5.47
6.46
Rb
13.0
16.5
13.3
24.6
Cs
0.33
0.40
0.22
0.87
Sr
1447
1075
1148
530
Sc
18.5
11.2
12.0
20.7
Zr
90
100
98
101
All analyses performed at WSU GeoAnalytical Lab. Fe is reported as all FeO*. Mg # = (Mg/(Mg+Fet))*100. For details on precision and accuracy, see Analytical Methods section. Italicized samples were supplied by M. Clynne. XRF data for sample BM ‐ BA ‐ 24 & BM ‐ A1 ‐ 24 are unpublished data analyzed by the USGS Analytical Lab (Clynne, written communication). 50
Table 1: Whole Rock Major and Trace Element Data
Group
Onion Butte
Onion Butte
Onion Butte
Onion Butte
Sample
OB ‐ D ‐ 9 ‐ 01
OB ‐ D ‐ 9 ‐ 02
OB ‐ D ‐ 10
OB ‐ D ‐ 13
Lassen Sample #
LM93‐3042
LM93‐3049
LM93‐3513
Dome
9
9
10
13
Major Element (wt. %)
64.27 65.02 64.03 64.54 SiO2
0.649
0.645
0.614
0.601
TiO2
18.23 17.34 17.58 16.98 Al2O3
4.26 3.83 4.23 FeO*
3.81 0.086
0.052
0.080
MnO
0.084
1.46 1.82 1.89 MgO
2.48 5.41 5.56 5.90 CaO
5.76 3.89 3.84 3.93 3.86 Na2O
1.56 1.68 1.56 1.67 K2O
0.20
0.23
0.19
0.216
P2O5
Total
100.00 100.00 100.00 100.00 Mg#
38
46
44
54
Trace Elements by WD‐XRF (ppm)
Ni
29.86 29.14 40.52 31.36 Cr
17.60 32.00 20.60 42.54 V
127.50 95.70 123.60 89.92 Ga
22.50 21.70 20.50 19.74 Cu
13.70 30.00 9.50 20.63 Zn
59.90 58.00 62.90 61.29 Trace Elements by ICP‐MS (ppm)
La
17.81
27.77
15.49
17.62
Ce
32.41
43.10
30.44
35.84
Pr
4.81
7.01
4.26
4.70
Nd
19.34
27.32
17.05
18.72
Sm
3.84
4.97
3.44
3.71
Eu
1.26
1.58
1.13
1.16
Gd
3.39
3.81
2.96
3.12
Tb
0.51
0.53
0.45
0.46
Dy
2.94
2.87
2.58
2.59
Ho
0.59
0.53
0.53
0.50
Er
1.57
1.39
1.40
1.32
Tm
0.21
0.19
0.20
0.19
Yb
1.35
1.20
1.26
1.21
Lu
0.21
0.19
0.20
0.19
Ba
304
562
289
436
Th
2.03
6.23
2.00
3.38
Nb
2.74
2.87
2.75
3.18
Y
15.77
14.12
13.97
13.26
Hf
3.39
3.55
3.29
3.58
Ta
0.17
0.18
0.17
0.20
U
0.54
1.42
0.61
0.88
Pb
4.13
5.78
4.23
4.89
Rb
13.7
17.0
14.1
19.3
Cs
0.14
0.28
0.17
0.23
Sr
1403
1634
1448
1315
Sc
10.9
10.6
9.8
11.3
Zr
121
127
118
128
All analyses performed at WSU GeoAnalytical Lab. Fe is reported as all FeO*. Mg # = (Mg/(Mg+Fet))*100. For details on precision and accuracy, see Analytical Methods section. Italicized samples were supplied by M. Clynne. XRF data for sample BM ‐ BA ‐ 24 & BM ‐ A1 ‐ 24 are unpublished data analyzed by the USGS Analytical Lab (Clynne, written communication). 51
Table 1: Whole Rock Major and Trace Element Data
Group
Barkley Mountain Barkley Mountain Barkley Mountain Barkley Mountain
Sample
BM ‐ BA ‐ 14
BM ‐ BA ‐ 15
BM ‐ A1 ‐ 17
BM ‐ BA ‐ 18
Lassen Sample #
Dome
14
15
17
18
Major Element (wt. %)
53.65 55.21 59.99 55.82 SiO2
0.816
0.766
0.472
0.707
TiO2
17.00 16.96 17.27 18.15 Al2O3
8.42 8.06 5.49 7.26 FeO*
0.154
0.153
0.109
0.127
MnO
6.77 5.64 5.46 5.38 MgO
9.19 9.34 6.55 8.12 CaO
3.20 2.85 3.44 3.46 Na2O
0.66 0.87 1.09 0.83 K2O
0.14
0.15
0.13
0.14
P2O5
Total
100.00 100.00 100.00 100.00 Mg#
59
55
64
57
Trace Elements by WD‐XRF (ppm)
Ni
80.64 32.95 89.10 64.09 Cr
207.76 89.03 385.20 138.10 V
258.10 252.08 139.80 225.10 Ga
18.65 17.08 16.50 17.90 Cu
54.68 74.22 32.10 56.30 Zn
84.59 75.70 62.60 71.00 Trace Elements by ICP‐MS (ppm)
La
9.67
19.56
18.42
13.95
Ce
17.43
28.16
20.93
20.61
Pr
2.75
4.63
4.48
3.89
Nd
12.47
19.49
18.53
16.75
Sm
3.28
4.29
4.17
4.11
Eu
1.11
1.39
1.49
1.36
Gd
3.77
4.83
4.58
4.38
Tb
0.63
0.77
0.73
0.74
Dy
3.98
4.84
4.41
4.58
Ho
0.85
1.05
0.88
0.95
Er
2.32
2.84
2.33
2.58
Tm
0.33
0.41
0.33
0.38
Yb
2.09
2.50
2.01
2.33
Lu
0.34
0.41
0.33
0.38
Ba
278
256
484
286
Th
1.24
2.27
3.05
1.77
Nb
3.48
2.63
3.10
3.06
Y
22.66
29.72
22.60
24.10
Hf
2.05
2.20
2.13
2.31
Ta
0.23
0.17
0.22
0.20
U
0.58
0.72
1.34
0.76
Pb
4.53
3.99
9.58
4.73
Rb
11.3
10.7
30.2
17.5
Cs
0.74
0.55
1.37
1.14
Sr
354
866
389
419
Sc
35.8
32.9
20.9
25.2
Zr
71
73
75
82
All analyses performed at WSU GeoAnalytical Lab. Fe is reported as all FeO*. Mg # = (Mg/(Mg+Fet))*100. For details on precision and accuracy, see Analytical Methods section. Italicized samples were supplied by M. Clynne. XRF data for sample BM ‐ BA ‐ 24 & BM ‐ A1 ‐ 24 are unpublished data analyzed by the USGS Analytical Lab (Clynne, written communication). 52
Table 1: Whole Rock Major and Trace Element Data
Group
Barkley Mountain Barkley Mountain
Barkley Mountain
Barkley Mountain
Sample
BM ‐ A1 ‐ 20
BM ‐ A2 ‐ 23
BM ‐ A1 ‐ 24
BM ‐ BA ‐ 24
Lassen Sample #
LM93‐3340
LM94‐3348
LM94‐3373
Dome
20
23
24
24
Major Element (wt. %)
58.82 57.98 58.02
56.33
SiO2
0.550
0.618
0.63
0.74
TiO2
19.56 16.16 19.14
18.88
Al2O3
6.04 6.21 5.91
6.54
FeO*
0.113
0.129
0.12
0.12
MnO
2.75 6.33 3.61
4.86
MgO
6.52 7.94 6.83
7.36
CaO
4.14 3.38 4.04
3.82
Na2O
1.32 1.10 1.34
1.04
K2O
0.19
0.15
0.22
0.17
P2O5
Total
100.00 100.00 99.87 99.85 Mg#
45
64
52
61
Trace Elements by WD‐XRF (ppm)
Ni
7.33 87.86 Cr
3.16 279.42 V
107.78 157.43 Ga
18.46 17.37 Cu
18.95 39.78 Zn
77.68 72.35 Trace Elements by ICP‐MS (ppm)
La
15.82
12.01
16.72
8.70
Ce
28.79
23.25
27.32
18.38
Pr
4.45
3.02
4.30
2.48
Nd
18.56
12.62
17.75
10.76
Sm
4.20
2.88
4.00
2.76
Eu
1.34
0.94
1.32
0.99
Gd
3.97
2.84
3.98
3.05
Tb
0.63
0.46
0.65
0.52
Dy
3.71
2.83
3.91
3.29
Ho
0.76
0.60
0.83
0.68
Er
2.11
1.62
2.25
1.92
Tm
0.31
0.23
0.33
0.27
Yb
1.98
1.48
2.07
1.74
Lu
0.32
0.24
0.33
0.29
Ba
580
466
530
315
Th
3.14
2.85
3.07
1.74
Nb
3.84
3.17
3.76
2.27
Y
19.84
16.19
22.76
17.68
Hf
2.71
2.20
2.79
2.31
Ta
0.26
0.22
0.26
0.15
U
1.27
1.21
1.55
0.80
Pb
8.63
8.15
9.18
6.35
Rb
32.9
28.6
35.8
20.4
Cs
1.18
1.05
1.03
0.70
Sr
512
471
542
441
Sc
12.0
23.9
14.1
21.0
Zr
98
81
101
81
All analyses performed at WSU GeoAnalytical Lab. Fe is reported as all FeO*. Mg # = (Mg/(Mg+Fet))*100. For details on precision and accuracy, see Analytical Methods section. Italicized samples were supplied by M. Clynne. XRF data for sample BM ‐ BA ‐ 24 & BM ‐ A1 ‐ 24 are unpublished data analyzed by the USGS Analytical Lab (Clynne, written communication). 53
Table 1: Whole Rock Major and Trace Element Data
Group
Barkley Mountain
Barkley Mountain
Barkley Mountain
Barkley Mountain
Sample
BM ‐ BA ‐ 25
BM ‐ A2 ‐ 26
BM ‐ A1 ‐ 28
BM ‐ A2 ‐ 31
Lassen Sample #
LM94‐3374
LM94‐3366
LM94‐3383
Dome
25
26
28
31
Major Element (wt. %)
55.81 62.33 59.53 58.17 SiO2
0.785
0.416
0.682
0.755
TiO2
19.86 16.65 18.98 19.53 Al2O3
7.01 4.79 5.81 6.34 FeO*
0.133
0.110
0.105
0.113
MnO
4.82 4.04 3.07 3.19 MgO
6.56 6.31 5.99 6.19 CaO
3.78 3.75 4.27 4.18 Na2O
1.06 1.47 1.36 1.30 K2O
0.17
0.13
0.21
0.24
P2O5
Total
100.00 100.00 100.00 100.00 Mg#
59
60
53
51
Trace Elements by WD‐XRF (ppm)
Ni
43.53 40.62 13.20 13.23 Cr
28.60 151.50 3.70 3.70 V
184.20 128.50 137.10 147.90 Ga
19.70 17.40 18.50 18.90 Cu
37.10 36.30 21.60 19.40 Zn
76.90 66.60 68.40 78.40 Trace Elements by ICP‐MS (ppm)
La
15.28
14.10
24.70
19.80
Ce
25.48
24.31
36.31
32.90
Pr
3.87
3.27
5.85
4.66
Nd
16.74
12.84
23.62
18.46
Sm
3.86
2.72
4.89
3.75
Eu
1.41
0.88
1.80
1.26
Gd
4.63
2.62
4.95
3.68
Tb
0.77
0.42
0.79
0.59
Dy
4.96
2.53
4.61
3.53
Ho
1.15
0.53
0.93
0.74
Er
3.23
1.44
2.48
1.98
Tm
0.47
0.21
0.35
0.28
Yb
2.83
1.33
2.08
1.77
Lu
0.47
0.21
0.33
0.28
Ba
429
628
471
506
Th
2.17
3.78
2.97
3.17
Nb
3.08
3.90
4.50
4.44
Y
33.43
14.33
25.81
20.60
Hf
2.63
2.01
3.03
2.93
Ta
0.21
0.29
0.28
0.28
U
0.84
1.94
1.04
1.12
Pb
6.75
12.83
8.27
8.70
Rb
15.1
41.6
25.8
28.5
Cs
0.81
1.91
1.01
0.95
Sr
438
438
566
554
Sc
20.8
18.0
14.8
13.5
Zr
95
65
111
107
All analyses performed at WSU GeoAnalytical Lab. Fe is reported as all FeO*. Mg # = (Mg/(Mg+Fet))*100. For details on precision and accuracy, see Analytical Methods section. Italicized samples were supplied by M. Clynne. XRF data for sample BM ‐ BA ‐ 24 & BM ‐ A1 ‐ 24 are unpublished data analyzed by the USGS Analytical Lab (Clynne, written communication). 54
Table 1: Whole Rock Major and Trace Element Data
Group
Barkley Mountain Barkley Mountain Barkley Mountain
Sample
BM ‐ BA ‐ 33
BM ‐ A2 ‐ 34
BM ‐ A2 ‐ 35
Lassen Sample #
LM94‐3390
Dome
33
34
35
Major Element (wt. %)
56.71 58.01 58.99 SiO2
0.673
0.648
0.629
TiO2
18.34 17.73 17.65 Al2O3
6.54 6.00 5.57 FeO*
0.118
0.120
0.119
MnO
5.02 4.59 4.32 MgO
7.66 7.54 7.23 CaO
3.78 3.80 3.80 Na2O
1.03 1.37 1.49 K2O
0.12
0.20
0.21
P2O5
Total
100.00 100.00 100.00 Mg#
58
58
58
Trace Elements by WD‐XRF (ppm)
Ni
54.28 32.02 30.87 Cr
42.20 78.10 66.50 V
183.40 166.30 152.20 Ga
17.50 18.60 18.70 Cu
39.70 33.40 30.40 Zn
68.00 79.30 76.40 Trace Elements by ICP‐MS (ppm)
La
8.68
16.03
16.31
Ce
18.20
41.26
38.90
Pr
2.35
3.93
4.07
Nd
10.31
15.75
16.06
Sm
2.63
3.34
3.46
Eu
0.93
1.04
1.04
Gd
2.96
3.13
3.19
Tb
0.50
0.50
0.51
Dy
3.21
3.02
3.09
Ho
0.68
0.63
0.63
Er
1.89
1.72
1.73
Tm
0.28
0.25
0.25
Yb
1.71
1.60
1.60
Lu
0.27
0.26
0.25
Ba
307
528
550
Th
1.75
3.16
3.22
Nb
2.05
4.00
4.16
Y
18.22
16.22
16.52
Hf
2.21
2.53
2.53
Ta
0.15
0.27
0.28
U
0.82
1.37
1.55
Pb
8.03
10.16
10.03
Rb
21.1
31.2
30.4
Cs
0.64
1.82
2.83
Sr
429
611
610
Sc
21.0
19.9
18.2
Zr
77
91
90
All analyses performed at WSU GeoAnalytical Lab. Fe is reported as all FeO*. Mg # = (Mg/(Mg+Fet))*100. For details on precision and accuracy, see Analytical Methods section. Italicized samples were supplied by M. Clynne. XRF data for sample BM ‐ BA ‐ 24 & BM ‐ A1 ‐ 24 are unpublished data analyzed by the USGS Analytical Lab (Clynne, written communication). 55
Table 2: Sr, and Nd Isotopic Data
87
86
143
144
εNd
SiO2
0.512932
5.7
58.45 ‐16.8
0.512811
3.4
65.07 0.7033
‐17.1
‐
‐
65.07 7
0.7040
‐6.9
0.512859
4.3
57.41 Onion Butte
9
0.7030
‐21.9
0.512970
6.5
65.02 BM‐BA‐14
Barkley Mountain
14
0.7042
‐4.6
0.512896
5.0
53.65 BM‐A1‐20
Barkley Mountain
20
0.7042
‐4.3
0.512824
3.6
58.82 BM‐A2‐26
Barkley Mountain
26
0.7044
‐1.9
0.512825
3.6
62.33 εSr
Sample
Group
Dome
OB ‐A‐ 1
Onion Butte
1
0.7030
‐21.6
OB‐D‐4
Onion Butte
4
0.7033
OB‐D‐4 (repeat)
Onion Butte
4
OB‐A‐7
Onion Butte
OB‐D‐9‐02
Sr/ Sr
Nd/
Nd
Analyses performed at the University of Washington Isotope Geochemistry Laboratory under the direction of Dr. Bruce Nelson and Dr. Scott Kuehner.
Error on Nd and Sr analyses are ±30 ppm (2σ) or ±0.3 epsilon units.
OB‐D‐4 (repeat) is a second analyses of the same solution
Italicized samples were given by M. Clynne from the USGS, written communication
56
Table 3: Summary of Petrography
Group
Composition
Size Range Textures/Notes
(mm)
Psuedomorphs
1 ‐ 1.5
0.5 ‐ 1
Psuedomorphs
0.5
Unzoned
0.25 ‐ 0.5
None observed
<0.1
Psuedomorphs
1‐2mm
Unzoned
0.25 ‐ 0.5
0.5 ‐ 2
Completely reacted away, psuedomorphs, 0.5 ‐ 0.75
<0.1
Mineral Phases
Phenocryst
Hbl: 1‐2%
Hbl: 3 ‐ 5%
Microphenocrysts
Andesite
Cpx: 1 ‐ 2%
Plag: 0 ‐ 10%
Groundmass
Plag + oxides
Onion Butte
Phenocryst
Hbl: 1‐2%
Plag: 10 ‐ 20%
Microphenocrysts
Dacite
Hbl: 5 ‐ 10% Cpx: 0 ‐2%
Groundmass
Plag + oxides
Olivine: < 1% Iddingsite rims
Phenocryst
1 mm
Cpx: < 1%
Unzoned to weakly zoned
Plag: 15 ‐ 20%
0.25 ‐ 0.5
Basaltic Cpx: 5 ‐ 6%%
0.5
Microphenocrysts
Andesite
Iddingsite rims
0.1 ‐ 0.5
Ol: 3 ‐ 5%
Psuedomorphs
Hbl: 0 ‐ 1%
0.25 ‐ 0.5
Groundmass
Plag. + oxides + pyx.
<0.1
Barkley Psuedomorphs
Mountain
Phenocryst
Hbl: 1‐2%
1‐2 mm
Hbl: 2 ‐ 5%
0.25 ‐ 1
Psuedomorphs
Unzoned
Plag: 0 ‐ 20%
0.25 ‐ 0.5
Microphenocrysts
Andesite
Cpx: 0 ‐3%
0.25 ‐ 0.5
Reaction rims of pyroxene
Ol: 0 ‐ 3%
0.25 ‐ 0.5
Opx: 0 ‐ 1%
0.25 ‐ 0.5
Groundmass
Plag. + oxides + pyx.
<0.1
Abbreviations: Cpx ‐ clinopyroxene, Ol ‐ olivine, Hbl ‐ hornblende, Plag ‐ plagioclase, Opx ‐ orthopyroxene, Pyx ‐ pyroxene
57
Table 4: Partition coefficients and starting compositions (C o) used in Rayleigh fractionation model
Co
Opx
Cpx
Hbl
Plag
Garnet
Mt
Subgroup 1
Subgroup 1 (BM‐A1‐28)
Subgroup 2 13.95
15.28
9.67
0.22
La
0.031
0.0468
0.366
0.3017
0.08
20.61
25.48
17.43
0.26
Ce
0.028
0.0838
0.574
0.2214
1
16.75
16.74
12.47
0.3
Nd
0.028
0.1828
1.012
0.1487
1.1
4.11
3.86
3.28
0.35
Sm
0.028
0.3774
1.386
0.1024
1.25
1.36
1.41
1.11
0.26
Eu
0.028
0.48
1.212
1.214
1.52
4.38
4.63
3.77
0.32
Gd
0.039
0.5826
1.49
0.0665
5.2
4.58
4.96
3.98
0.28
Dy
0.076
0.7739
1.611
0.0498
15.45
0.95
1.15
0.85
0.25
Ho
0.114
0.7409
1.554
0.0473
23.8
2.58
3.23
2.32
0.22
Er
0.153
0.7078
1.496
0.0448
38.4
2.33
2.83
2.09
0.18
Yb
0.254
0.6335
1.488
0.041
53
0.38
0.47
0.34
0.18
Lu
0.323
0.6654
1.325
0.0389
57
Mineral abbreviations: opx, orthopyroxene; cpx, clinopyroxene; hbl, hornblende; plag, plagioclase; mt, magnetite.
References
1. Opx: Rollinson
2. Cpx: Conrey et al., 2001
3. Hbl: Conrey et al., 2001
4. Plag: Conrey et al., 2001
5. Garnet: Conrey et al., 2001
58
Onion Butte Dome 7
12.00
22.33
14.27
3.28
1.11
3.77
3.98
0.85
2.32
2.09
0.34
Table 5: REE fractionation results Subgroup 1
0.8
0.6
0.4
24
La
57.3
70.6
94.7
Ce
38.4
45.4
57.5
Nd
38.3
41.7
47.0
Sm
27.0
27.3
27.7
Eu
23.6
23.8
24.2
Gd
20.8
20.3
19.6
Dy
16.9
15.5
13.8
Ho
15.5
14.1
12.2
Er
14.0
12.3
10.2
Yb
13.5
11.4
8.9
Lu
14.9
12.7
10.2
0.2
156.5
86.3
57.7
28.4
24.8
18.4
11.2
9.6
7.5
5.9
6.9
Subgroup 1
28
0.8
0.6
0.4
0.2
78.1
49.4
40.9
27.3
26.3
23.2
18.8
19.2
17.7
14.6
16.3
100.1
61.6
48.4
30.3
29.0
24.2
17.9
17.9
15.6
12.3
13.6
141.8
84.0
61.4
35.0
33.2
25.6
16.8
16.1
13.1
9.6
10.5
257.3
143.0
92.3
44.9
42.0
28.2
14.9
13.5
9.7
6.4
6.7
Subgroup 2
Subgroup 2
0.8
0.6
0.4
0.2
0.8
0.6
0.4
0.2
26
31
La
43.0
53.0
71.0
117.1
75.0
91.1
119.9 191.6
Ce
33.8
40.0
50.6
75.7
46.6
53.9
66.1
93.7
Nd
23.5
25.6
28.8
35.2
37.1
38.8
41.4
46.1
Sm
17.3
17.4
17.6
17.9
24.3
23.3
21.9
19.6
Eu
16.1
16.2
16.2
16.3
23.6
22.7
21.4
19.4
Gd
14.0
13.6
13.0
12.1
21.0
19.2
16.9
13.6
Dy
11.8
10.7
9.5
7.6
17.2
14.5
11.5
7.7
Ho
11.1
10.0
8.6
6.7
17.6
14.5
11.2
7.1
Er
10.2
8.9
7.4
5.3
16.2
12.8
9.1
5.2
Yb
8.8
7.4
5.7
3.8
13.2
9.8
6.5
3.2
Lu
9.7
8.2
6.6
4.5
15.2
11.5
7.8
4.0
Modeled melt compositions by fractional crystallization for Subgroup 1 andesites (BM‐A1‐24 and BM‐A1‐28) from a Barkley Mountain basaltic andesite (BM‐BA‐18 and BM‐BA‐25, respectively)with a fractionating assemblage with variable mineral proportions of cpx:hbl:grt = 31:68:1 and cpx:hbl:grt:mt = 66:25:1.5:7.5. Modeled melt compositions for Subgroup 2 andesites (BM ‐ A2 ‐ 26 and BM ‐ A2 ‐ 31) from basaltic andesite (BM‐BA‐33 and BM‐BA‐24, respectively)source with a fractionating assemblage with variable mineral proportions of opx:cpx:hbl:plag:grt = 11:13:65:10:1 and cpx:hbl:plag:grt = 10:78.5:10:1.5. Mineral abbreviations: opx, orthopyroxene; cpx, clinopyroxene; hbl, hornblende; plag, plagioclase; grt, garnet; mt, magnetite.
59
Table 6: REE fractionation results for dome 7 andesite
Onion Butte
0.8
0.2
0.6
0.4
Dome 7 Andesite
La
47.7
58.5
77.8
126.7
Ce
32.2
37.8
47.4
69.6
Nd
28.1
30.1
33.1
39.0
Sm
21.1
20.8
20.3
19.4
Eu
18.8
18.4
17.9
17.0
Gd
17.5
16.6
15.4
13.5
Dy
14.5
13.0
11.3
8.8
Ho
13.9
12.5
10.8
8.5
Er
12.8
11.5
9.9
7.6
Yb
11.2
9.8
8.3
6.1
Lu
12.7
11.5
10.0
7.8
Modeled melt compositions by fractional crystallization for Onion Butte andesite from dome 7 from a Barkley Mountain basaltic andesite (BM‐BA‐14) source with a fractionating assemblage of cpx:hbl:plag:grt = 19.5:70:10:0.5. Mineral abbreviations: cpx, clinopyroxene; hbl, hornblende; plag, plagioclase; grt, garnet.
60
Table 7: Partition coefficients and starting compositions (C o) used in partial melting model
Opx
Cpx
Hbl
Plag
Garnet
Co 0.003
0.032
0.014
0.048
0.009
Rb
3.4
0.003
0.131
0.044
0.36
0.017
Ba
107.0
0.002
0.037
0.081
0.263
0.2
K
3400.0
0.016
0.12
0.3
0.32
0.04
La
4.6
0.019
0.18
0.45
0.28
0.08
Ce
10.2
0.009
0.12
0.46
2.84
0.013
Sr
455.0
0.03
0.4
1.4
0.22
0.2
Nd
8.4
0.042
0.7
2
0.16
1
Sm
2.5
0.052
0.69
1.75
0.95
0.98
Eu
0.75
0.066
0.88
2.5
0.12
3.8
Gd
2.7
0.12
1.1
3.1
0.09
11
Dy
2.5
0.2
1
3
0.075
16
Er
1.6
0.36
0.9
1.85
0.06
21
Yb
1.4
0.45
0.85
1.5
0.055
21
Lu
0.2
Mineral abbreviations: opx, orthopyroxene; cpx, clinopyroxene; hbl, hornblende; plag, plagioclase.
References
1. Opx: Rollinson; Borg and Clynne, 1998
2. Cpx: Rollinson; Borg and Clynne, 1998
3. Hbl: Borg and Clynne, 1998; Conrey et al., 2001
4. Plag: Rollinson; Borg and Clynne, 1998
5. Garnet: Rollinson; Sen and Dun, 1994
61
Table 8: REE and incompatible trace element (Rb, Ba, K, and Sr) partial melting results
Dome 4
10%
20%
30%
Dome 13‐03
40%
10%
20%
30%
40%
12.40
6.78
4.66
3.56
12.57
6.82
4.68
3.57
Rb
Rb
250.18
165.14
123.24
98.30
265.32
170.86
126.00
99.80
Ba
Ba
44.99
26.62
18.90
14.65
46.52
27.09
19.11
14.76
K
K
95.63
66.58
51.07
41.42
100.91
68.81
52.20
42.05
La
La
68.29
50.81
40.45
33.60
71.94
52.57
41.42
34.17
Ce
Ce
189.23
154.55
130.62
113.10
229.39
177.06
144.17
121.58
Sr
Sr
40.74
35.72
31.80
28.66
43.10
37.31
32.89
29.41
Nd
Nd
25.47
23.98
22.66
21.47
26.67
24.92
23.39
22.03
Sm
Sm
19.93
18.80
17.79
16.88
21.25
19.83
18.59
17.50
Eu
Eu
16.37
15.93
15.52
15.13
16.60
16.13
15.68
15.26
Gd
Gd
9.64
9.69
9.73
9.77
9.24
9.33
9.41
9.49
Dy
Dy
9.31
9.37
9.43
9.48
8.52
8.65
8.79
8.92
Er
Er
8.29
8.28
8.27
8.25
7.12
7.22
7.33
7.44
Yb
Yb
9.21
9.17
9.12
9.07
7.81
7.91
8.01
8.11
Lu
Lu
Modeled compositions of partial melts for Dome 4 and Dome 13 (10%, 20%, 30%, 40%) from an average high Sr/P basalt source with variable residual phases ranging from Opx + Cpx + Hbl + Plag + Grt = 35:47:12:5:1 to Opx+ Cpx + Hbl + Plag + Grt =40:44:11:3:2.
62
125˚
130˚
120˚
50˚
45˚
40˚
Figure 1: The Cascade volcanic arc and study area in the Lassen Volcanic Region denoted by the blue dot. Triangles represent major volcanic centers, abbreviated as follows: MM, Mount Meager; MC, Mount Cayley; MG Mount Garibaldi; MB Mount Baker; GP Glacier Peak; MR Mount Rainier; MSH Mount Saint Helens;
MG, Mount Garibaldi; MB, Mount Baker; GP, Glacier Peak; MR, Mount Rainier; MSH, Mount Saint Helens; MA, Mount Adams; SVF, Simcoe Volcanic Field; MH, Mount Hood; MJ, Mount Jefferson; TS, Three Sisters; NV, Newberry Volcano; CLV, Crater Lake Volcano; MMc, Mount McLoughlin; MLV, Medicine Lake Volcano; MS, Mount Shasta; LVC, Lassen Volcanic Center. Pink shaded areas represent volcanism less than 1.8 Ma. The Cascade Volcanic arc is divided into five segments. The black lines denote the trend of arc segmentation (Guffanti and Weaver, 1988). Modified after Borg et al., 1997.
63
Onion Butte Group
Barkley Mountain Group
N
Figure 2: Simplified geologic map of the Lassen segment. This map outlines the regional scale volcanism and the large volcanic centers. The regional scale volcanism is mapped as the blue green color. The study area is outlined by the blue box. Notice the Onion Butte and Barkley Mountain is mapped as part of the regional scale volcanism Modified after M Clynne
Mountain is mapped as part of the regional scale volcanism. Modified after M. Clynne (written communication).
64
1*
3*
2
40˚ 15
40
15'
4*
Onion Butte Group
5*
6
8
11
9
12
7*
34*
13*
35*
15
17*
14*
22
10*
23
24
16
25*
18*
40˚ 07'' 30''
Barkley Mountain Group
19
27
20*
21
32
26
N
0
0
28
33
3 mi
29
4 km
121˚ 45'
121
45'
30
121˚ 37' 30''
121
37' 30''
31
Figure 3: Geologic map of study area. Black lines indicate trend of the Onion Butte and Barkley Mountain groups. Numbers in boxes refer to domes within each group. Numbers 1 – 13 are domes of the Onion Butte group. Numbers 14 – 35 are domes of the Barkley Mountain group. Red asterisks denote where samples were collected. Map abbreviations are as follows: abk, Basaltic Andesite and Andesite of Barkley Mountain group; bdc, basalt of Deer Creek; Kc, Chico Formation; mtf, basaltic andesite of Transfer; MzPz, Mesozoic and Paleozoic basement terrane; Qto Till older glaciations; Tam, Andesites of the Maidu Volcanic Center –
Qto, Till –
older glaciations; Tam Andesites of the Maidu Volcanic Center Stage 1; Tam2, Stage 1; Tam2
Andesites of the Maidu Volcanic Center – Stage 2; Tat, Tuscan Formation derived from the Yana Volcanic Center; Tdm, Dacites of the Maidu Volcanic Center; Tdo, Dacite and Andesite of the Onion Butte group; Tmt, basaltic andesite of Tamarack Spring; Tt, Tuscan Formation; and Twm, debris flows from the Maidu Volcanic Center. Tmt and Twm are now Qmt and Qwm as the T‐Q boundary is set at 2.8 Ma. Modified after M. Clynne (written communication).
65
B
A
Cpx
p
Hbl
Cpx
Plag
Plag
2 mm
2
2 mm
C
D
Hbl
Hbl
Plag
Plag
2 mm
2 mm
Figure 4: Representative photomicrographs of the Onion Butte group taken in cross polarized light. Cpx –
clinopyroxene; Hbl ‐ hornblende, and Plag. – plagioclase. (A) & (B) – Andesite composition. (A) ‐ from dome 1 and (B) is from dome 7. (C) & (D) – Dacite compositions. Notice most of the hornblende are reacted away or have thick opacite rims.
66
B
A
Cpx
Olivine
Plag
Plag
2 mm
2 mm
C
D
Hbl
Hbl
Plag
Plag
2 mm
2 mm
Figure 5: Representative photomicrographs of the Barkley Mountain group taken in cross polarized light. Mineral abbreviations same as Figure 4. (A) & (B) – Basaltic andesite compositions. Notice a abundance of plagioclase microphenocrysts. (C) & (D) – Andesite compositions. Notice the presence of hornblende phenocrysts and microphenocrysts and the decrease in plagioclase microphenocrysts. Most of the hornblende are reacted away or have thick opacite rims.
67
A
FeO
BM
OB
Tholeiite
Calc alkaline
Calc‐alkaline
MgO
K2O + Na2O
3.5
B
high K (calc‐alkaline)
30
3.0
K2O (wt. %)
2.5
medium K
(calc‐alkaline)
2.0
15
1.5
1.0
low K
(tholeiitic)
0.5
Basaltic Andesite
Basalt
0.0
48
50
52
54
56
Andesite
58
60
Dacite
62
64
66
68
70
72
SiO2 (wt. %)
Figure 6: Major element chemistry for the Barkley Mountain (BM) and Onion Butte (OB) groups. (A) – AFM diagram (Irvine and Barager, 1971) classifying tholeiitic versus calc‐alkaline rocks. (B) – subdivision of subalkaline rocks (LeMaitre et al., 1989). 68
21
BM
OB
0.7
Al2O3 (wt. %)
A
TTi2O (wt. %)
0.9
0.5
0.3
19
17
15
8
80
6
60
4
40
2
20
0
0
52
54
56
58
60
62
64
66
68
52 54 56 58 60 62 64 66 68
10
2.0
8
1.5
K2O (wt. %)
FeO* (wt. %)
52 54 56 58 60 62 64 66 68
Mg #
MgO (wt. %
%)
52 54 56 58 60 62 64 66 68
6
4
2
1.0
0.5
0.0
0
52 54 56 58 60 62 64 66 68
10
5
8
4
Na2O (w
wt. %)
CaO (w
wt. %)
52 54 56 58 60 62 64 66 68
6
4
3
2
1
2
0
0
52 54 56 58 60 62 64 66 68
52
54
56
58
60
62
64
66
68
SiO2 (wt. %)
SiO2 (wt. %)
Figure 7: Harker variation diagrams for whole rock major element composition in weight percent versus SiO2 (wt. %) for the Barkley Mountain group (BM) and the Onion Butte group (OB). 69
160
100
BM
120
OB
Zr (ppm)
Ni (ppm)
80
60
40
80
40
20
0
0
52 54 56 58 60 62 64 66 68
750
1800
600
1500
Sr (ppm)
Ba (ppm)
52 54 56 58 60 62 64 66 68
450
300
150
1200
900
600
300
0
0
52 54 56 58 60 62 64 66 68
52 54 56 58 60 62 64 66 68
14
5
4
10
Nb (ppm)
Pb (ppm)
12
8
6
4
3
2
1
2
0
0
40
4
30
3
(Sr/P)N
Y (ppm
m)
52 54 56 58 60 62 64 66 68
20
52
54
56
58
60
62
64
66
68
52
54
56
58
60
62
64
66
68
2
1
10
0
0
52 54 56 58 60 62 64 66 68
SiO2 ((wt. %))
SiO2 ((wt. %))
Figure 8: Harker variation diagrams for select trace element compositions in ppm versus SiO2 (wt. %) for the Barkley Mountain (BM) and Onion Butte (OB) groups. N refers to normalization to values of primitive mantle as defined by Sun and McDonough (1989). 70
1000
A
OB ‐ D
Sample/ Primitivve Mantle
OB ‐ A
100
Barkley Mountain group
10
1
1000
Rb Ba Th U Nb Ta K La Ce Pb Sr P NdSm Zr Hf Eu Ti Tb Dy Ho Er Yb Lu
BM ‐ A
Sample/ Primitive Mantle
BM ‐ BA
100
Onion Butte group
10
1
50
Rb (p
ppm)
40
Rb Ba Th U Nb Ta K La Ce Pb Sr P NdSm Zr Hf Eu Ti Tb Dy Ho Er Yb Lu
3.0
BM
2.5
OB
(Ba//Th)N
B
30
20
10
2.0
1.5
1.0
0.5
0
0.0
52 54 56 58 60 62 64 66 68
0.0
1.0
2.0
3.0
4.0
SiO2 (wt. %)
(Sr/P)N
Figure 9: (A) – Primitive mantle – normalized trace element diagram for representative samples for the Onion Butte group (OB) and the Barkley Mountain group (BM) Normalizing values taken from Sun and
Onion Butte group (OB) and the Barkley Mountain group (BM). Normalizing values taken from Sun and McDonough, 1989. Sample numbers in legend stand for the group (OB/BM) and the rock type (BA –
basaltic andesite, A – andesite, and D – dacite). Solid circles depict basaltic andesites; solid triangles depict andesites; and solid squares depict dacites. Shaded gray fields highlight the differences between the two groups. (B) – Harker variation diagrams showing trace elements variation with increasing silica compared to the normalized trace element diagram. Symbols as in Figure 5.
71
A
100
Sam
mple/ Primitive Manttle
OB‐D
OB‐A‐1
OB‐D‐5
10
1
Rb Ba Th U Nb Ta K La Ce Pb Sr P NdSm Zr Hf Eu Ti Tb Dy Ho Er Yb Lu
B
100
Sample/ Primitive Mantle
BM‐A2
OB‐A‐7
10
1
Rb Ba Th U Nb Ta K La Ce Pb Sr P NdSm Zr Hf Eu Ti Tb Dy Ho Er Yb Lu
Figure 10: Primitive mantle‐normalized trace element diagram highlighting differences found within the Onion Butte (OB) group. Normalized values from Sun and McDonough (1989). (A) – shows the similarity between dome 1 andesites (triangle) to the Onion Butte dacites and difference between dome 5 (diamond) dacite to other dacites. (B) ‐ shows similarity of dome 7 from the Onion Butte group to the Barkley Mountain andesites subgroup 2. Sample numbers are similar to Figure 6, but indicate which dome the sample came from at the end of the sample number.
72
Sample/ Primitive Mantle
1000
BM ‐ BA ‐ 14
BM ‐ BA BM BA ‐ 15
BM ‐ BA ‐ 18
BM ‐ BA ‐ 24
100
10
1
Figure 11:
Figure
11: Primitive mantle
Primitive mantle‐normalized
normalized trace element diagram highlighting differences found within trace element diagram highlighting differences found within
basaltic andesites of the Barkley Mountain (BM) group. Normalized values from Sun and McDonough (1989). 73
A
Sample/ Primitive M
Mantle
1000
BM ‐ A1 ‐ 17
BM ‐ A1 ‐ 24
BM ‐ A1 ‐ 20
BM ‐ A1 ‐ 28
100
10
1
Rb Ba Th U Nb Ta K La Ce Pb Sr P Nd Zr Hf Sm Eu Ti Tb Dy Ho Er Yb Lu
B
Samplee/ Primitive Mantle
1000
BM ‐ A2 ‐ 23
BM ‐ A2 ‐ 26
BM A2 ‐
BM ‐
A2 31
BM A2 ‐
BM ‐
A2 34
BM ‐ A2 ‐ 35
100
10
1
Rb Ba Th U Nb Ta K La Ce Pb Sr P NdSm Zr Hf Eu Ti Tb Dy Ho Er Yb Lu
Figure 12: Primitive mantle‐normalized trace element diagram highlighting differences found within the andesites of the Barkley Mountain (BM) group. Normalized values from Sun and McDonough (1989). (A)
– shows group 1 andesites of the Barkley Mountain group. (B) – shows the group 2 andesites of the Barkley Mountain group. Sample numbers are the same as Figure 7. A1 and A2 in the sample number refer to the andesite groups (i.e. A1 – andesite group 1).
74
1000
A
OB ‐ A
Sample/ Chon
ndrite
OB ‐ D
100
10
1
B
1000
La
Ce
Pr
Nd Sm Eu
Gd Tb
Dy Ho
Er
Yb
Lu
BM ‐ BA
BM ‐ A1
Sample/ Chond
drite
BM ‐ A2
100
10
1
La
C
Ce
Pr
Nd Sm
Eu
Gd
25
2.5
20
2.0
Yb (p
ppm)
La (pp
pm)
30
Tb Dy
3.0
15
10
Er
Yb
Lu
15
1.5
1.0
5
0.5
0
0.0
52 54 56 58 60 62 64 66 68
SiO2 (wt. %)
Ho
BM
OB
52 54 56 58 60 62 64 66 68
SiO2 (wt. %)
Figure 13:
Fi
13 Chondrite‐normalized REE diagram for the Onion Butte (OB) and Barkley Mountain (BM) groups. Ch d it
li d REE di
f th O i B tt (OB) d B kl M
t i (BM)
Normalized values from Sun and McDonough (1989). (A) – Onion Butte group. (B) – Barkley Mountain group. Sample numbers and symbols are the same as Figure 9. (C) – Select REE (La and Yb) versus SiO2 (wt. %) shown to depict how REEs vary with increasing silica. Symbols in the diagrams are the same as Figure 7.
75
A
1000
OB ‐ D
Sample/ Chondrite
OB ‐ D ‐ 5
OB D ‐
OB ‐
D 9‐02
9 02
100
10
1
La
B
Ce
Pr
Nd Sm
Eu
Gd
Tb
Dy
Ho
Er
Yb
Lu
1000
OB‐D
Sample/ Chondrite
OB‐A‐1
OB‐A‐7
100
10
1
La
Ce
Pr
Nd Sm
Eu
Gd
Tb
Dy
Ho
Er
Yb
Lu
Figure 14: Chondrite‐normalized REE diagram for the Onion Butte (OB) group highlighting differences within the group. Normalized values from Sun and McDonough (1989). (A)
the
group Normalized values from Sun and McDonough (1989) (A) – Differences between the Onion Differences between the Onion
Butte dacites and dome 5 (green square) and 9‐02 (red square) dacites (B ) – Shows similarity between andesite from dome 1 (OB‐A‐1; red triangle) to the OB dacites and the difference between andesite from dome 7 (OB‐A‐7; green triangle) to dome 1 andesite and OB dacites.
76
A
1000
Sample/ Chondritte
BM ‐ BA
BM ‐ A1
100
10
1
La
Ce
Pr Nd Sm Eu Gd Tb Dy Ho
Er
Yb
Lu
1000
B
Sample/ Chondritee
BM ‐ BA
BM ‐ A2
100
10
1
La
Ce
Pr Nd Sm Eu Gd Tb Dy Ho
Er
Yb
Lu
Figure 15: Chondrite‐normalized REE diagram for Barkley Mountain (BM) groups. Normalized values from Sun and McDonough (1989). (A) – Group 1 andesites compared to the BM basaltic andesites. This groups tends to cluster tightly in HREE concentrations. (B) – Group 2 andesites of the BM group. This group has ,
similar concentrations in LREE to the basaltic andesites, but have lower abundances in the HREE’s and are more varied than the group 1 HREE’s concentrations.
77
A
9
Mantle Array
8
High (Sr/P)N
7
Low (Sr/P)N
εNd
6
5
4
3
2
1
0
0.7028
OB‐A‐1
OB‐A‐7
OB‐D‐4
OB‐D‐9‐02
BM‐BA‐14
BM‐A‐20
BM‐A‐26
0.7032
0.7036
0.7040
0.7044
0.7048
87Sr/86Sr
B
10
0.705
0.704
εNdd
87Sr/86
6Sr
8
0.703
6
4
2
0.702
0
52
C
54
56
58 60 62
SiO2 (wt. %)
64
66
52
0.705
54
56
58 60 62
SiO2 (wt. %)
64
66
10
0.704
6
εNd
87Sr/86Sr
8
0.703
4
2
0.702
0
300
600
900
1,200 1,500 1,800
Sr (ppm)
300
600
900
1,200
Sr (ppm)
1,500
1,800
Figure 16: Isotopic compositions for the Onion Butte (OB) and Barkley Mountain (BM) group. Sample numbers and symbols are the same as Figure 7 and 8. (A) – εNd versus 87Sr/86Sr. The orange dotted field and purple dashed field depicts range of compositions for primitive magmas found in the LVR with high (Sr/P)N and low (Sr/P)N respectively. Error bars may be covered by symbols. Data compiled from Borg and Clynne (1997). (B) – 87Sr/86Sr and εNd versus silica to show change in isotopic composition with change in silica concentrations. (C) – 87Sr/86Sr and εNd versus Sr (ppm) to depict correlations in change in isotopic compositions with increasing Sr in the dacites of the Onion Butte groups. 78
3.0
14
Lassen
OB
10
Lassen
2.5
K2O(wt. %)
MgO (wt. %
%)
12
8
6
4
OB
2.0
1.5
1.0
0.5
2
0
0.0
52
54
56
58
60
62
64
1600
52
54
56
58
60
62
64
52
54
56
58
60
62
64
40
35
30
Y (ppm)
SSr (ppm)
1200
800
400
25
20
15
10
5
0
0
52
54
56
58
60
62
64
SiO2 (wt. %)
SiO2 (wt. %)
Figure 17: Select major and trace element data of the Onion Butte (OB) and Barkley Mountain (BM) basaltic andesites to andesites compared to Lassen segment basaltic andesites to andesites. Blue squares are the Onion Butte (OB) group. Red triangles are the Barkley Mountain (BM) group. Orange dashes are Lassen segment lavas. Data compiled from Clynne, 1990; Clynne and Bullen, 1990; and Clynne et al., 2008.
79
7
Low Sr/P
6
High Sr/P
Lassen
(Sr/P)N
5
OB
BM
4
3
2
1
0
47
49
51
53
55
57
59
61
63
65
67
SiO2 (wt. %)
Figure 18:
Fi
18 (Sr/P)
(S /P)N vs. SiO
SiO2 wt. % of the Onion Butte (OB) and Barkley Mountain (BM) groups compared to t % f th O i B tt (OB) d B kl M
t i (BM)
dt
Lassen segment lavas (orange dashes) and the low Sr/P (green plusses) and high Sr/P (purple crosses) are the primitive calc‐alkaline basalts. Normalized values from Sun and McDonough (1989). 80
A
Sample/ Chondrite
1000
FG ‐ 1
FG ‐ 2
OB ‐ D ‐ 4
OB ‐ D ‐ 13
100
10
1
Rb Ba
K
La
Ce
Sr
Nd Sm Eu Gd Tb Dy Ho
2
1.6
(Dy/Yb)N
2.0
(Ba/Th)N
B 2.5
1.5
1
0.5
FG ‐ 1
FG ‐ 2
2
(Sr/P)N
3
Er
Yb
Lu
1.2
0.8
0.4
OB
0
0.0
0
1
4
0
5
10
(La/Yb)N
15
20
Figure 19: Comparison of the dacites from the Onion Butte group (blue squares) to silicic lavas from the large volcanic centers. Red triangles are felsic group 1 (FG – 1) and orange diamonds are felsic group 2 (FG – 2) as defined by Borg and Clynne (1998). Normalized values from Sun and McDonough (1989). (A) –
chondrite‐normalized
chondrite
normalized rare earth element (REE) with select trace elements for the Onion Butte (OB) group rare earth element (REE) with select trace elements for the Onion Butte (OB) group
and the two groups of silicic lavas. (B) – primitive mantle normalized (Ba/Th)N versus (Sr/P)N and (Dy/Yb) versus (La/Yb) for the Onion Butte group and silicic lavas from the large volcanic centers. 81
9.0
A
High (Sr/P)N
8.0
OB ‐ Dome 7 Andesite
7.0
Low (Sr/P)N
εNd
6.0
5.0
4.0
3.0
2.0
1.0
OB
FG ‐ 1
FG 2
FG ‐
LVC
0.0
0.7028
0.7033
0.7038
0.7043
0.7048
87Sr/86Sr
0.7050
B
OB ‐ Dome 7 Andesite
87Sr/86Sr
0.7040
0.7030
OB FG ‐ 1
FG ‐ 2
LVC
0.7020
50
54
58
62
66
70
74
78
SiO2 (wt. %)
Figure 20: Comparison of isotopic compositions of the Onion Butte group to silicic lavas from the large volcanic centers. Blue squares are the Onion Butte dacites, green diamonds are felsic group 1 (FG – 1), green triangles are felsic group 2 (FG – 2), and red circles are silicic lavas from the Lassen Volcanic Center (LVC). Lassen data compiled from Borg and Clynne, 1998 and Clynne et al., 2008. (A) – εNd versus 87Sr/86Sr. The orange dotted field and purple dashed field depicts range of compositions for primitive magmas found in the LVR with high (Sr/P)N and low (Sr/P)N respectively. Data compiled from Borg and Clynne (1997). (B) – 87Sr/86Sr versus SiO
(1997). (B)
Sr versus SiO2 (wt. %) to show differences in Onion Butte dacites and (wt. %) to show differences in Onion Butte dacites and
other silicic lavas from the Lassen segment. The Onion Butte group has lowest 87Sr/86Sr compositions for similar silica concentrations from the Lassen segment. 82
3.0
BM
2.5
OB
Dy/Yb
2.0
1.5
1.0
05
0.5
0.0
52
54
56
58
60
62
64
66
68
SiO2 (wt. %)
Figure 21: Dy/Yb versus SiO2 wt % as indicators of residual garnet or hornblende . Both the Barkley Mountain (BM) and Onion Butte (OB) group show Dy/Yb remaining constant with increasing SiO2. This suggests both garnet and hornblende are residual phases 83
A
1000
Sample/Chondrite
Co = BM‐BA‐18
Co = 1
0.8
0.6
0.4
0.2
100
10
Fractionating Assemblage:
Cpx:Hbl:Grt = 31:68:1:1 1
La
Ce
Nd
Sm
Eu
Gd
Dy
Ho
Er
Yb
Lu
16
B
14
(Yb)N (pp
pm)
12
BM ‐ A1
BM‐A1‐28
Co = 1
F
0.8
10
0.6
8
0.4
6
0.2
4
2
Co = BM‐BA‐18
0
0
50
100
150
(La)N (ppm)
200
250
Figure 22: Rayleigh fractionation model results for Barkley Mountain (BM) group 1 andesites. Fractionation models described in text. Chondrite normalized REE abundances for subgroup 1 andesites and resultant modeled fractions of melts. Normalization values from Sun and McDonough, 1989. (A) –
Subgroup 1 andesites, represented by BM‐A1‐24, can be reproduced after 20% crystallization by Barkley (
),
g
g
p
p ,
Mountain basaltic andesite (BM‐BA‐18), with a fractionating assemblage of Cpx:Hbl:Grt = 31:68:1. Cpx, clinopyroxene; Hbl, hornblende; Grt, garnet. (B) – (Yb)N vs. (La)N from Rayleigh fractionation model showing all subgroup 1 andesites. The line (F) represent s fractional crystallization of Barkley Mountain basaltic andesite (BM‐BA‐18) . Various F (melt remaining) given along curve in 0.2 increments. Notice model can reproduce all subgroup 1 andesites except for sample BM‐A1‐28 indicating either a different Barkley Mountain basaltic andesite parent or a different fractionating mineral assemblage.
84
1000
Co = BM‐BA‐25
Co = 1
0.8
0.6
0.4
0.2
BM ‐ A1 ‐ 28
Sam
mple/Chondrite
100
10
Fractionating Assemblage:
Cpx:Hbl:Grt:Mt = 66:25:1.5:7.5 1
La
Ce
Nd
Sm
Eu
Gd
Dy
Ho
Er
Yb
Lu
Figure 23: Rayleigh fractionation model results for Barkley Mountain (BM) group 1 andesites, represented by BM‐A1‐28. Fractionation models described in text. Chondrite normalized REE abundances for subgroup 1 andesite and resultant modeled fractions of melts. Normalization values from Sun and McDonough, 1989. Subgroup 1 andesite (BM‐A1‐28) can be reproduced after 40% crystallization (60% melt remaining) by Barkley Mountain basaltic andesite, p
y
g
g
p
g
represented by BM‐BA‐25, with a fractionating assemblage of cpx:hbl:grt:mt = 66:25:1.5:7.5. Cpx, clinopyroxene; hbl, hornblende; grt, garnet; mt, magnetite.
85
1000
A
Sample/Chondrite
Co = BM‐BA‐33
Co = 1
0.8
0.6
0.4
0.2
BM ‐ A2 ‐ 26
100
10
Fractionating Assemblage:
Opx:Cpx:Hbl:Plag:Grt = 11:10:65:10:1
1
La
Ce
Nd
Sm
Eu
Gd
Dy
Ho
Er
Yb
Lu
1000
B
Sample/Chondrite
Co = BM‐BA‐25
Co = 1
0.8
06
0.6
04
0.4
0.2
BM ‐ A2 ‐ 31
100
10
Fractionating Assemblage:
Cpx:Hbl:Plag:Grt = 10:78.5:10:1.5
1
La
Ce
Nd
Sm
Eu
Gd
Dy
Ho
Er
Yb
Lu
Figure 24: Rayleigh fractionation model results for Barkley Mountain (BM) group 2 andesites. Fractionation models described in text. Chondrite normalized REE abundances for subgroup 2 andesites and resultant modeled fractions of melts. Normalization values from Sun and McDonough, 1989. (A) – Subgroup 2 andesites, represented by BM – A2 – 26, can be reproduced after 40% crystallization (60% melt remaining) by Barkley Mountain basaltic andesite, represented by BM‐BA‐33, with a fractionating assemblage of opx:cpx:hbl:plag:grt = 11:10:65:10:1. (B) –
Subgroup 2 andesite (BM – A2 – 31) can be reproduced by fractionation Barkley Mountain basaltic andesite, represented by BM – BA – 25 after 40% crystallization with a fractionating assemblage of cpx:hbl:plag:grt:mt = 10:78.5:10:1.5. Mineral abbreviation same as previous figures.
86
A
1000
BM ‐ A2
S
Sample/ Primitive Mantle
e
BM ‐ A1
OB ‐ A ‐ 7
100
10
1
Rb Ba Th U Nb Ta K La Ce Pb Sr P Nd Sm Zr Hf Eu Ti Tb Dy Ho Er Yb Lu
B
100
Sample/ Primitive Mantle
OB ‐ D
OB ‐ A ‐ 1
10
1
Rb Ba Th U Nb Ta K La Ce Pb Sr P Nd Sm Zr Hf Eu Ti Tb Dy Ho Er Yb Lu
Figure 25: Primitive mantle‐normalized trace element diagram highlighting the differences of the Onion Butte andesites. Normalized values from Sun and McDonough (1989). (A) – shows the similarity of dome 7 (OB – A – 7) to the Barkley Mountain group andesites. (B) – shows the similarity between dome 1 (OB – A – 1) to the Onion Butte dacites. Dome 1 is geochemically similar in trace element concentrations to the dacites. 87
1000
Sample/Chondrite
Co = BM‐BA‐14
Co = 1
0.8
0.6
0.4
0.2
OB ‐ A ‐ 7
100
10
Fractionating Assemblage:
Cpx:Hbl:Plag:Grt = 19.5:70:10:0.5
1
La
Ce
Nd
Sm
Eu
Gd
Dy
Ho
Er
Yb
Lu
Figure 26:
Figure
26: Rayleigh fractionation model results for Onion Butte (OB) dome 7 andesite Rayleigh fractionation model results for Onion Butte (OB) dome 7 andesite
(OB‐A‐7). Dome 7 andesite had geochemical characteristics and the same isotopic compositions as the Barkley Mountain andesites. Fractionation models described in text. Chondrite normalized REE abundances for subgroup 2 andesites and resultant modeled fractions of melts. Normalization values from Sun and McDonough, 1989. OB‐A‐7 can be reproduced after 40% crystallization (60% melt remaining) by Barkley Mountain basaltic andesite, represented by BM‐BA‐14, with a fractionating assemblage of cpx:hbl:plag:grt = 19.5:70:10:0.5. Mineral abbreviation same as previous figures.
88
12
εNd
8
XCC
GB
Low Sr/P
High Sr/P
OB ‐ A ‐ 1
4
0
‐4
0.702
0.703
0.704
0.705
0.706
87Sr/
/86Sr
Figure 27: εNd vs. 86Sr/87Sr of OB – A – 1 compared to high Sr/P and low Sr/P primitive magmas, xenoliths from Cinder Cone Volcano, and granodioritic basement rocks. Blue triangle is the Onion Butte andesite (OB – A – 1). Orange triangles are the high Sr/P primitive magmas. Purple triangles are the low Sr/P primitive magmas. Blue crosses are xenoliths from Cinder Cone Volcano. (XCC) Green asterisks are granodioritic basement (GB) rocks from the Sierran Nevada. Notice the andesite from dome 1 plots with the least evolved magmas in the Lassen segment Data compiled from
from dome 1 plots with the least evolved magmas in the Lassen segment. Data compiled from Borg and Clynne (1997) and Borg and Clynne (1998).
89
125
Adakite 0.2
Field
0.2
100
OB‐D
Curve 1 ‐ Melting Subducting Slab
Curve 2 ‐ Melting Lower Crust
Sr/Y 75
Co = 1
50
0.7
05
0.5
25
1
1
0
0
5
10
15
20
Y (ppm)
25
30
35
Figure 28: Sr/Y vs. Y (ppm) from the Onion Butte group. Onion Butte dacites (blue squares) fall just inside the adakite field (orange dashed line) as defined by Defant et al. (1990). The purple curve (curve 1) represents partial melting of the subducted Gorda plate. The red curve (curve 2) represent s partial melting of an average high Sr/P basalt from the Lassen segment. Various F
(amount of melt) given along curve in 0.1 increments. Curve 1: results from partial melting of the Gorda Ridge (purple triangle; Sr = 147 ppm and Y = 29 ppm), sample 17 – 10 (Davis and Clague, 1987), with a melting assemblage of cpx:grt = 50:50. Curve 2: results from partial melting of the lower crust with a composition of average high Sr/P basalts (red triangle; Sr = 452 ppm (sample
lower crust with a composition of average high Sr/P basalts (red triangle; Sr = 452 ppm (sample LC86 – 829) and Y = 25 ppm (sample LB91 – 110); Borg and Clynne, 1997), with a melting assemblage of opx:cpx:hbl:grt = 42:45:12:1. Opx, orthopyroxene; cpx, clinopyroxene; hbl, hornblende; grt, garnet. 90
100
OB ‐ D ‐ 4
Samp
ple/ Primitive Mantle
OB D ‐
OB ‐
D 13
High Sr/P
10
1
Rb Ba Th U Nb Ta K La Ce Pb Sr P Nd Sm Zr Hf Eu Ti Gd Tb Dy Y Er Tm Yb Lu
Figure 29: Primitive mantle‐normalized trace element diagram highlighting the similarities of the Onion Butte (OB) and Barkley Mountain (BM) group to the primitive magmas in the Lassen volcanic region. Normalized values from Sun and McDonough (1989). Symbols as in figure 12 and 24. Data compiled from Borg and Clynne (1997). (A) – compares the Onion Butte dacites to the high Sr/P primitive magmas. The dacites from the Onion Butte group are similar to the high Sr/P primitive magmas. The high Sr/P may be a potential parent to the Onion Butte group in partial melting models. 91
A
1000
Sample/Chond
drite
Melting Assemblage:
Opx:Cpx:Hbl:Plag:Grt = 35:47:12:5:1 100
10
Co 0.1
0.2
0.3
0.4
OB‐D‐4
1
Rb
Ba
K
La
Ce
Sr
Nd Sm
Eu
Gd
Dy
Er
Yb
Lu
1000
B
Sample/Chondriite
Melting Assemblage:
Opx:Cpx:Hbl:Plag:Grt = 40:44:11:3:1 100
10
Co 0.1
0.2
0.3
04
0.4
OB D 13
OB‐D‐13
1
Rb
Ba
K
La
Ce
Sr
Nd Sm Eu
Gd
Dy
Er
Yb
Lu
Figure 30: Equilibrium (or batch modal) melting results for Onion Butte (OB) dacites. Models described in text. Chondrite normalized values from Sun and McDonough, 1989. Trace element compositions are modeled by partial melting of a basaltic lower crust with a source composition of an average high Sr/P basalts from Borg and Clynne (1997). Modeled melt fractions (F) are generated with residual mineral phases for Opx + Cpx + Hbl + Plag + Grt = 35:47:12:5:1 (A)
phases for Opx + Cpx + Hbl + Plag + Grt 35:47:12:5:1 (A) to Opx + Cpx + Hbl + Plag + Grt to Opx + Cpx + Hbl + Plag + Grt = 40:44:11:3:1 (B). Opx, orthopyroxene; Cpx, clinopyroxene; Hbl, hornblende; Plag, plagioclase; Grt, garnet. Notice the Onion Butte dacites, represented by OB‐D‐4 (A) and OB‐D‐13 (B), patterns were reproduced, but trace element (Rb, Ba, K, and Sr) and LREE concentrations could not be reproduced by variable degrees of melting.
92
1000
Sample/Chon
ndrite
A
Co 0.2
0.4
0.1
0.3
OB‐D‐4
100
10
Melting Assemblage :
Opx:Cpx:Hbl:Plag:Grt = 35:47:12:5:1 1
Rb
Ba
K
La
Ce
Sr
1000
Sample/Chondriite
B
Nd Sm
Eu
Gd
Dy
Co 0.2
0.4
Er
Yb
Lu
0.1
0.3
OB‐D‐13
100
10
Melting Assemblage :
Opx:Cpx:Hbl:Plag:Grt = 40:44:11:3:1 Opx:Cpx:Hbl:Plag:Grt 40:44:11:3:1
1
Rb
Ba
K
La
Ce
Sr
Nd Sm Eu
Gd
Dy
Er
Yb
Lu
Figure 31: Equilibrium (or batch modal) melting results for Onion Butte (OB) dacites. Models described in text. Chondrite normalized values from Sun and McDonough, 1989. Trace element compositions are modeled by partial melting of an altered basaltic lower crust with a source composition of an average high Sr/P basalts from Borg and Clynne, (1997). Modeled melt fractions (F) are generated with residual mineral phases for Opx + Cpx + Hbl + Plag + Grt = 35:47:12:5:1 (A)
(F) are generated with residual mineral phases for Opx + Cpx + Hbl + Plag + Grt 35:47:12:5:1 (A)
to Opx + Cpx + Hbl + Plag + Grt = 40:44:11:3:1 (B). Opx, orthopyroxene; Cpx, clinopyroxene; Hbl, hornblende; Plag, plagioclase; Grt, garnet. Notice the Onion Butte dacites, represented by OB‐D‐4 (A) and OB‐D‐13 (B), patterns and compositions were reproduced after 20% melting for both A and B.
93
Barkley Mountain Group
(Arc Axis)
Dacitic partial melts of lower crust
Separated basaltic andesites f ti
fractionate to andesites
t t
d it
Partial melting of amphibolite lower crust
High Sr/P
Emplacement of mafic melts
Low Sr/P basalts forming basaltic andesite
Crustal Thickness: 38 ± 4 kkm
Onion Butte Group
(Forearc)
Low Sr/P
Decreasing slab component Figure 32: Schematic illustration of Onion Butte and Barkley Mountain group generation. Dacite magma (light blue) generation by partial melting of the lower continental crust of a high Sr/P composition. Andesite from the Barkley Mountain (light red) generation by fractional crystallization of Barkley Mountain basaltic andesite (red) residues of low Sr/P magmas.
94
Appendix I: Sr/Rb/Nd/Sm Column Chemistry Procedure Introduction Isotopic values are obtained by separating each elemental fraction before running to avoid potential interferences from other isotopes. Rock powder must be dissolved to release the elements from their crystal structure. Each element is put through ion exchange columns to isolate each element. Samples are dried down to be brought back up to 2% HNO3. Then the sample is analyzed on the multiple collector inductively coupled plasma mass spectrometer (MC‐ICP‐MS). Samples were sent in to University of Washington where samples were prepared and dissolved. This appendix covers the procedure for Sr/Rb – Nd/Sm separation and only covers the separation and analysis procedure. Procedure Sr is separated using a cation exchange resin, AG‐50W‐X8. The resin is preloaded into glass columns and stored dry. Before separation can begin, the dry resin has to be made wet by back filling it. This step was done by Dr. Scott Kuehner. Once this step has been done, the resin is conditioned by adding 5mL of 2.5M HCl into the columns. This is done in two steps. The first 2.5mL of HCL is added to make sure no loft contaminates are in the resin. Backwashing may loft contaminants into the resin. Add the second 2.5mL of the total 5mL of HCl into column. This ensures contaminates are rinsed down the column, the column is flowing properly, and to determine if the column is dripping properly. If drip is slow or not dripping, then backwash again as air can be trapped and will act as a barrier to acid flow within the column. Cover each column with a square of parafilm. Dissolve sample in 3mL of 2.5M HCl. Transfer sample to a 4mL centrifuge tube using a borosilicate pipette. Use one borosilicate pipette per sample. Centrifuge sample for five minutes at 3,000 rpm. Load sample into column using each sample’s borosilicate pipette leaving behind gel and 95
any solid particles. Rinse sample beaker with 3mL of 2.5M HCl. Transfer rinse to centrifuge tube and centrifuge for five minutes at 3,000 rpm. Transfer centrifuged sample into column. Because Rb was not collected, 44mL of 2.5M HCl was added to column. Four milliliters was added first and once resin settled and then the remaining 40mL were added. After the acid fully dripped through, a beaker was clearly labeled was sample number and Sr and placed under the column. To elute Sr, 13mL of 2.5M HCl was added to the column. Once the column completely dripped through, the beaker is placed on a hot plate to dry off the HCl. After sample is completely dried off, place a square of parafilm on beaker. To collect REEs for Nd‐Sm isotope analysis, the column is washed twice with 4M HCl filled to the reservoir of the column and allowed to drip through. After column wash, 16mL of 4M HCl is added to column to collect REE fraction. Place beaker on hot plate to dry off HCl. To elute Nd‐Sm from REE’s, columns are filled with Teflon Beads of 80 – 100 mesh coated in Bis(2‐ethylhexyl) phosphate and are always stored wet. Because these columns were not used previously, the resin was cleaned using 20mL of 2M HCl. After cleaning, 5mL of 0.14M HCl was added to condition the resin. Dissolve sample with 0.3mL of 0.14M HCl. After sample is dissolved, pipette sample into column and allow to completely drip through. Rinse beaker with 0.7mL of 0.14M HCl and add to column once the 0.3mL have passed through the column. Once the rinse has passed through the column completely, add 1mL of 0.14M HCl into column and allow to drip through completely. Before Nd can be eluted, 18mL of 0.14M HCl of waste must pass through the column. As the waste is being passed through the column, clearly label a beaker with sample number and Nd. To elute Nd, add 16mL of 0.14M HCl. After dripping is complete, transfer beaker to a hot plate to dry off the HCl. Clean column by filling column reservoir full of 6M HCl. After sample is completely dried off, place a square of parafilm on beaker. 96
Dr. Bruce Nelson prepared Nd samples to be run through the MC‐ICP‐MS. Under the direction of Dr. Bruce Nelson, the samples were run through the MC‐ICP‐MS in the following sequence: standard‐sample‐sample‐sample‐standard. A standard always starts and ends the set to make data valid. Duplicates of one sample were run to ensure sample integrity. Sr isotope analysis was performed by Dr. Bruce Nelson. 97
Appendix II: Major Element Modeling Utilizing MELTS to test for Fractional Crystallization Introduction MELTS is a program that models major element compositions of melts and solid phases that are in equilibrium in magmatic systems (Mark and Sack, 1995; Asimow and Ghiorso, 1998). It has the ability to compute phases in magmatic systems over temperatures of 500 – 2,000˚C and pressure from 0 – 2GPa. MELTS is used as tool to test for fractional crystallization processes in the evolution of magmatic systems. This appendix discusses the use of MELTS to test the validity of fractional crystallization as a permissible process in the generation of the Barkley Mountain andesites from Barkley Mountain basaltic andesites Methods There are several intensive variables that need to be input before the model can be run and include: parental magma composition, water concentrations, oxygen fugacity f(O2), pressure, and temperature. These intensive variables will be described below. Parent magma composition assumed to be a basaltic andesite from the Barkley Mountain group. Sample BM‐BA‐14 was chosen as a basaltic andesite parent for the Barkley Mountain andesites because it is the most mafic basaltic andesite of the group. Water, in varying amounts, is found in arc settings. Dehydration of the subducting slab releases water and is included into arc magmas. Within the Cascade arc, Leeman and others (1990, 2004, and 2005) have suggested water concentrations in the arc are low to absent. Low to absent water concentrations in the Cascade arc is attributed to the subduction of the young and relatively warm Juan de Fuca plate system. Because it is still hot, it is believed the plate is dehydrated as it reaches the point in which arc magmas are generated. Other workers (Bacon et al, 1997; Borg and Clynne, 1997; Grove et al., 2002) argue against low to absent water owing to slab‐derived signature in arc basalts in the Cascade arc. Hornblende is a ubiquitous phase in the Onion Butte and Barkley 98
Mountain groups, and as such low to absent water is not conducive for crystallization of hornblende. For the MELTS model, water concentrations of 3% and 1.5% will be tested. Oxygen fugacity f(O2) is defined as the availability of oxygen (O2) to react within a system. Oxygen fugacity is considered an intensive variable because it affects whether the system is oxidizing or reducing at equilibrium. Because arc magmas are thought to be more oxidized compared to other magmas, two different f(O2) states were modeled. The higher f(O2) is the Ni‐NiO buffer and a slightly lower f(O2) is the Quartz‐Fayalite‐Magnetite buffer +2. Pressure constraints were set based on estimated crustal thickness and pressures needed for crystallization of hornblende. In the Lassen region crustal thickness is approximately 38±km (Moonrey and Weaver, 1989), so pressure constraints are can be as high as 12 kbars. Hornblende crystallization is favored in high to medium pressure conditions, and as such pressure settings will be constrained between 12 and 6 kbars. This model is investigating fractionation at different depths not multiple fractionation depths. A starting and ending temperature can to be set for each run. Before each run, MELTS has an option to find the liquidus and it will vary with each set of new variables input into the model. The temperature of each run will be set to the liquidus. It is estimated that a temperature range of 150˚ result in liquid compositions that are andesitic. Therefore the stopping temperature will be set to the liquidus and each subsequent run the temperature will be decreased in 50˚ increments until the liquidus minus 150˚ is reached. It is assumed that a larger range in temperature will result in liquids that have higher than observed silica contents. A systematic approach was taken to test each possible combination of variables. As a result, 8 test runs were performed. The results of the combinations tested are reported in Tables A1. In order to test the viability of fractional crystallization, resulting liquid compositions were compared 99
to observed andesite and dacite compositions from the Onion Butte and Barkley Mountain group (Figure A1). Results Each of the modeled results was compared to observed trends with the Barkley Mountain andesites. Comparison to observed trends will indicate if MELTS successfully modeled fractional crystallization. Modeled compositions were able to reproduce SiO2 wt. % compositions while the major oxide compositions had varied results. MgO (wt. %) were able to reproduce observed Barkley Mountain concentrations and trends for all modeled pressures and water concentrations (Figure A1). P2O5 wt. % was successful for all runs, but could only reproduce P2O5 concentrations that were greater than 0.14 wt. % P2O5 (Figure A1). Other major oxides that were able to reproduce observed concentrations and patterns are run specific. For example, test runs #1, 2, 5, and 7 were able to reproduce observed concentrations and trends for FeO* wt. % while the rest of the models produced lower than expected concentrations (Figure A1). These successful runs correlated with pressure settings at 12 kbars. Majority of the modeled major oxide trends were parallel to observed trends, but concentrations were either slightly lower or higher than expected (Figure A1). For example, FeO* wt. % consistently produced compositions that were lower than expected and correlated with mid pressure (6 kbars) runs. K2O wt. % were lower than expected while CaO (Figure A1). Some major oxides produced opposite trends for observed concentrations (Figure A1). TiO2, MnO, and Al2O3 (wt. %) consistently produced concentrations or trends that were opposite of observed trends. TiO2, and MnO (wt. %) increased with increasing SiO2. Al2O3 wt. % generally had opposite trends as observed failing to reproduce observed trends (Figure A1). However, a few runs 100
(#2, 6, 3, and 7) remained constant with increasing SiO2 (wt. %) and reproduced Al2O3 wt. % concentrations of the andesites with approximately 16‐17.5 wt. % Al2O3 (Figure A1). Discussion Models that reproduce major oxide concentrations or trends indicate fractional crystallization as a viable process in the generation of the Barkley Mountain andesites. MELTS modeling was successful at reproducing SiO2 wt. % concentrations for both groups. The combinations of parameters set will reproduce target SiO2 wt. % concentrations (Figures A1). Most major element oxide trends (increasing or decreasing with increasing SiO2 wt. %) were able to be reproduced while concentrations were overestimated or underestimated. This indicates the combination of parameters set were not conducive for reproducing target compositions. This may indicate that fractional crystallization is viable, but the parameters chose were not correct for these magmatic systems. Target MnO wt. % and TiO2 wt. % trends and compositions were never reproduced by MELTS (Figure A1). These trends and compositions failed mainly because mineral phases were not crystallizing that would reproduce decreasing trends as observed in both groups. This also indicates parameters selected were not favorable for reproducing target compositions. Conclusion MELTs was used to test for fractional crystallization processes in the generation of the Barkley Mountain andesites. MELTs was able to reproduce observed SiO2 wt % concentrations and observed patterns (increasing or decreasing with increasing SiO2). It was unsuccessful at reproducing observed concentrations for most runs for all major elements. MgO wt. %, FeO* wt. %, Al2O3, Na2O, K2O, and P2O5 wt. % were able to be reproduce for Barkley Mountain andesites. This suggest reproduced trends (increasing or decreasing with SiO2 wt. %) with overestimated or underestimated concentrations likely indicate the parameters chosen were not favorable for this magmatic system, not a failure of fractional crystallization. 101
102
Ni‐NO
Ni‐NO
Ni‐NO
2
3
4
6
6
12
12
1.5
3.0
1.5
3.0
53.7
63.8
971
57.4
1071
61.0
62.6
956
1021
60.4
1006
57.3
1056
54.2
1170
62.5
61.3
1033
1070
58.2
1082
57.9
53.7
1132
1120
SiO2
T
0.8
0.9
0.8
0.7
0.8
0.8
0.7
1.4
1.1
0.9
1.3
1.0
0.9
TiO2
17.0
17.0
17.0
16.7
16.8
16.8
16.7
12.7
14.1
15.8
13.9
15.2
15.7
Al2O3
1.9
0.9
1.2
1.5
0.9
1.2
1.5
1.4
1.9
2.4
1.4
1.7
2.4
Fe2O3
6.7
1.3
1.8
2.5
1.4
1.9
2.5
2.6
3.9
5.4
2.8
3.8
5.7
FeO
8.4
2.1
2.9
3.9
2.2
3.0
3.9
3.9
5.6
7.6
4.1
5.3
7.8
FeO*
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.3
0.2
0.2
0.3
0.2
0.2
MnO
6.8
2.0
3.4
5.6
1.5
2.7
4.6
2.5
3.9
5.1
1.8
2.8
4.7
MgO
Composition of Modeled Results
9.2
8.0
8.8
9.6
7.6
8.2
9.0
7.7
9.3
9.8
7.4
8.5
9.3
CaO
3.2
3.3
3.3
3.4
3.1
3.2
3.3
5.1
4.4
3.6
4.1
3.6
3.4
Na2O
0.7
1.0
0.9
0.7
1.0
0.9
0.8
1.2
0.9
0.7
1.1
0.9
0.7
K2O
0.1
0.2
0.2
0.2
0.2
0.2
0.2
0.3
0.2
0.2
0.2
0.2
0.2
P2O5
Parameters used in the MELTS program. Test runs use a series of intensive variables as inputs into the program. Results of each test run are reported as oxide wt. %. Composition of parent is Barkley Mountain basaltic andesite (BM‐BA‐14).
Composition of Parent
Ni‐NO
1
Intensive Variables
P Run H2O
f (O2)
(kbars)
Number
Table A1: MELTS test run parameters and results with starting composition for the Barkley Mountain group
103
Q‐F‐M
Q‐F‐M
Q‐F‐M
6
7
8
1.5
3.0
6
6
1.5
3.0
12
12
0.84
0.71
0.73
0.84
0.79
0.72
1.44
1.15
0.91
1.31
1.05
0.84
TiO2
16.96
16.75
16.80
16.84
16.80
16.72
12.69
14.09
15.79
13.85
15.16
15.98
Al2O3
53.72 0.817 17.02
61.70
1008
57.69
1108
58.83
62.62
955
1058
60.50
1005
57.40
1055
54.23
1171
62.45
61.29
1033
1071
58.13
1083
57.94
52.63
1153
1121
SiO2
T
1.87
1.04
1.29
1.67
0.90
1.16
1.50
1.37
1.87
2.37
1.37
1.73
2.39
Fe2O3
6.74
1.02
1.30
1.78
1.41
1.89
2.54
2.65
3.86
5.46
2.85
3.84
5.73
FeO
8.42
1.96
2.46
3.28
2.22
2.94
3.90
3.88
5.54
7.59
4.08
5.39
7.88
FeO*
0.154
0.21
0.18
0.17
0.24
0.22
0.18
0.27
0.22
0.17
0.27
0.22
0.16
MnO
6.78
3.14
5.14
5.55
1.50
2.63
4.55
2.55
3.92
5.10
1.81
2.82
5.66
MgO
Composition of Modeled Results
9.21
9.62
10.25
10.14
7.62
8.20
8.93
7.77
9.32
9.85
7.39
8.55
9.38
CaO
3.2
2.71
2.88
2.85
3.14
3.19
3.25
5.08
4.45
3.56
4.12
3.62
3.30
Na2O
0.66
1.04
0.87
0.85
1.04
0.93
0.79
1.16
0.93
0.73
1.14
0.94
0.68
K2O
0.14
0.20
0.17
0.16
0.22
0.20
0.17
0.25
0.20
0.16
0.25
0.20
0.15
P2O5
Parameters used in the MELTS program. Test runs use a series of intensive variables as inputs into the program. Results of each test run are reported as oxide wt. %. Composition of parent is Barkley Mountain basaltic andesite (BM‐BA‐14).
Composition of Parent
Q‐F‐M
5
Run Number
Intensive Variables
P H2O
f (O2)
(kbars)
Table A1: MELTS test run parameters and results with starting composition for the Barkley Mountain group
1.6
6
1.2
K2O (wt. %)
MgO (wt. %)
M
8
4
0.4
2
BM
Parent
2
7
0.0
0
50
55
60
65
70
10
12
8
10
CaO (wt. %)
FeO* (wt. %)
0.8
6
4
2
50
55
60
65
70
50
55
60
65
70
50
55
60
65
70
8
6
4
2
0
0
50
55
60
65
70
0.3
6
P2O5 (wt. %)
Na2O (wt. %)
5
4
3
2
0.2
0.1
1
0.0
0
50
55
60
65
70
SiO2 (wt. %)
SiO2 (wt. %)
Figure A1: Harker variation diagrams for Barkley Mountain group (BM) and liquid lines of descent Figure A1:
Harker variation diagrams for Barkley Mountain group (BM) and liquid lines of descent
calculated by MELTS (black and blue lines). Major element data in weight percent versus SiO2 (wt. %) . Green triangle is BM‐BA‐14. Runs 2 (black line) and 7 (blue line) were chosen as representative of calculated results. Run #2 parameters are as follows: pressure = 12 kbars; H2O = 1.5 wt. %; f(O2) = Ni‐NO . Run #7 parameters are as follows: pressure = 6 kbars; H2O = 3.0 wt. %; f(O2) = Q‐F‐M +2. 104
22
20
1.2
Al2O3 (wt. %)
Ti2O (wt. %)
1.6
0.8
0.4
18
16
14
12
0.0
10
50
55
60
65
70
55
60
65
SiO2 (wt. %)
0.4
BM
MnO (wt. %)
50
Parent
2
60
65
7
0.3
0.2
0.1
0.0
50
55
70
SiO2 (wt. %)
Figure A1 cont.: Harker variation diagrams for Barkley Mountain group (BM) and liquid lines of descent calculated by MELTS (black and blue lines). Major element data in weight percent versus SiO2 (wt. %) . Green triangle is BM‐BA‐14. Runs 2 (black line) and 7 (blue line) were chosen as representative of calculated results. Run #2 parameters are as follows: pressure = 12 kbars; H2O = 1.5 wt. %; f(O2) = Ni‐NO . Run #7 parameters are as follows: pressure = 6 kbars; H2O = 3.0 wt. %; f(O2) = Q‐F‐M +2. 105
70
Appendix III: Olivine Addition and Subtraction Modeling Efforts Mixing of xenocrystic material (e.g. olivine) with dacite compositions could alter major element chemistry, but not the trace element chemistry. If xenocrystic material (e.g. olivine) was mixed into the OB dacites the major element and Ni concentrations of the dacitic magma would be driven toward andesitic compositions without affecting the trace and REE concentrations. Olivine subtraction modeling was done to determine if the Onion Butte andesite from dome 1 can be generated from the Onion Butte dacites. Olivine compositions from LC88 – 1314 and LC88 – 1312 were chose from Clynne and Borg (1997) to represent the composition of the xenocrystic material. Olivine compositions were subtracted from the andesite compositions in various amounts (F) to try to reproduce the dacite compositions. Olivine subtraction models failed to reproduce observed dacitic compositions (Figure A2). The model was able to reproduce MgO (wt. %) concentrations, but underestimated SiO2 wt. % concentrations (Figure A2). Because the model failed to reproduce observed concentrations, contamination by xenocrystic material is not considered a viable explanation for the generation of the Onion Butte andesite. The generation of the Onion Butte andesite cannot be constrained by simple mixing and/or addition/subtraction models. If these processes are responsible for the origin of these andesites further evolution through fractional crystallization and/or assimilation must be invoked to explain the geochemical characteristics. 106
8
OB ‐ A
OB ‐ D
MgO (wt. %)
6
Line 1
Line 2
4
0.02
0.04
0.06
2
0
56
58
60
62
64
66
SiO2 (wt. %)
Figure A2: MgO (wt. %) vs. SiO2 (wt. %) from the Onion Butte group. Onion Butte andesites: OB – A (bl
(blue triangles), and Onion Butte dacites: OB –
i l ) dO i B
d i
OB D (blue squares). Olivine subtraction models trying to D (bl
) Oli i
b
i
d l
i
reproduce Onion Butte dacites by subtracting olivine compositions from Onion Butte andesitic magmas. Line 1 represent addition olivine compositions from sample LC88 – 1314 (Clynne and Borg, 1997). Line 2 represents addition of olivine compositions from sample LC 88 – 1312.. Various F
(amount of olivine added or subtracted) given along the lines. Olivine addition models are in increments of 0.02. Both models fail to reproduce observed concentrations.
107
Appendix IV: Table of Sample Locations
Group
Onion Butte
Barkley Mountain
Sample #
Dome
Latitude
Longitude
OB ‐ A ‐ 1
1
N40 16.646
W121 50.587
OB ‐ D ‐ 4
4
N40 15.151
W121 45.455
OB ‐ D ‐ 5
5
N40 14.688
W121 44.637
OB ‐ A ‐ 7
7
N40 12.043
W121 35.362
OB ‐ D ‐ 10
10
N40 11.850
W121 33.507
BM ‐ BA ‐ 14
14
N40 9.502
W121 41.223
BM ‐ BA ‐ 15
15
N40 9.719
W121 40.779
BM ‐ A1 ‐ 17
17
N40 9.657
W121 40.470
BM ‐ BA ‐ 18
18
N40 9.087
W121 41.048
BM ‐ A1 ‐ 20
20
N40 8.599
W121 40.053
BM ‐ BA ‐ 25
25
N40 9.066
W121 35.725
BM ‐ A2 ‐ 34
34
N40 11.138
W121 42.496
BM ‐ A2 ‐ 35
35
N40 11.044
W121 42.548
108