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The felsic plutonic core of the western Talkeetna island arc crustal section, Alaska: Its formation and implications for crustal growth along continental margins 8 24 4 Al2O3 22 K 2O 20 3 med-K 18 16 14 10 MgO 8 4 4 0 The Talkeetna island arc is an exhumed and tilted section where deep levels of arc lower crust as well as middle and upper crust arc lithologies are now exposed at the surface. 4 CaO TiO2 0 45 50 55 60 65 70 75 45 50 55 SiO2 (wt %) 100 60 65 70 75 Whole-rock major element variation diagrams for western arc rocks with published data from the eastern arc (Clift et al., 2005; Greene et al., 2006). RARE EARTH ELEMENTS Eastern Arc Cs Rb Ba Th U Nb Ta La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy 100 Volcanic and volcaniclastic rocks 5 Petrographic Characteristics Volcanic and volcaniclastic rocks; Talkeetna Formation (53.6 - 72.7 wt % SiO2) Samples include fine-grained massive volcanic rocks and rounded volcanic clasts within volcaniclastic sediment. Predominantly porphyritic, samples range from basaltic-andesite to rhyolite. Phenocrysts are mostly subhedral plagioclase and cpx (when present). Plagioclase is commonly concentrically zoned and intergrown, many contain seive textures. Amphibole phenocrysts are largely broken up and surrounded by reaction rims of Fe-Ti oxides + pyroxene. Trace minerals: quartz. Alteration minerals: chlorite. Intermediate to felsic plutonic rocks (54.3 - 76.1 wt % SiO2) Subhedral to anhedral granular texture. Samples include diorite, quartz diorite, granodiorite, tonalite, and trondhjemite. Subhedral to anhedral plagioclase, commonly corroded and concentrically zoned. Poikilitic anhedral to subhedral amphibole containing plagioclase and/or biotite. Consertal and interstitial anhedral quartz. Some Mg-rich samples from the suite contain trace amounts of clinopyroxene and/or orthopyroxene (Mg-no. 61-65). Trace minerals: K-felspar, apatite, titanite, and zircon. Alteration minerals: chlorite, epidote, calcite, and pyrite. Anhedral granular texture dominant. Samples are predominately gabbroic. Anhedral to subhedral plagioclase, faint concentric zoning and corroded cores common. Anhedral poikilitic amphibole containing plagioclase. Clinopyroxene and orthopyroxene commonly cored in amphibole. Trace minerals: apatite and titanite. Alteration minerals: chlorite. 87.0 % 4,860 km2 0.4 % 24 km2 1 100 1 100 10 10 Int-Felsic Plutonic avg. [La/Yb]N = 4.0 Plutonic and volcanic exposures in the western Talkeetna island arc and the locations of rock samples used in this study. Western arc exposures are from Detterman & Reed (1980), Riehle et al. (1993), and Reed et al. (1982). 58.2 ~20-30% xtal Step 1 1.1 0.9 0.7 0.5 cpx, opx, plag, hbl, mag ~15-25% xtal Step 2 La Ce plag, hbl, mag, ilm Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu 1 1 100 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Mafic Plutonic < 54 wt % SiO2 10 Compositional Group II (blue) avg. [La/Yb]N = 2.1 1 Cs Rb Ba Th U Nb Ta La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu 1 REE normalized to C1 chondrite (Sun & McDonough, 1989) 60.4 wt % SiO2 Intermediate end-member (5721J03) 10 10 90 71.2 wt % SiO2 1 La Ce Pr % of felsic end-member Felsic end-member (5711J01) plutonic samples (observed) volcanic samples (observed) modeled REE (mixing) interm. end-member (5721J03) felsic end-member (5711J01) Nd Sm Eu Gd Tb Dy Ho Er Yb Lu 20% 5721J03 80% 5711J01 compared 65.8 wt to 5712J07 % SiO2 1.1 0.9 0.7 0.5 melting assemblage cpx, opx, plag, hbl, mag Pr 44.8 wt % SiO2 Gabbroic composition (5710J10B) 10 Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Several gabbroic samples collected from the western arc can be used to produce felsic liquids. On average, ~15-20% La melting of gabbroic compositions can Ce produce the observed felsic, HREEPr Nd from group II. depleted samples Sm Eu Gd Tb Dy Felsic end-member (5711J01) plutonic samples (observed) 100.0 samples (observed) volcanic modeled REE (melting) gabbroic rock (5710J10B) 10.0end-member (5711J01) felsic 100 Compositional Group I REE patterns are relatively flat like the eastern arc, while 10 Compositional Group II samples exhibit trends of concave-up HREE depletion. La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd 100 0.1 100 71.2 wt % SiO2 1 La Ce 1.0 Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu 1.1 0.9 0.1 0.7 Tb Dy Ho Er Yb Y Lu 0.5 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu 20% partial melt of 5710J10B compared to 5711J01 How does western arc chemistry, petrologic modeling, and age correlation relate to the development of the arc crustal section? La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu 7 Cpx Mineral Modes 40-70% Matrix (plag dominated) Phenocrysts: 50-95% Plagioclase 0-20% Cpx (Mg-no. 73-84) 0-5% Opx (Mg-no. 65-67) 0-5% Amphibole (Mg-no. 67-73) 0-5% Fe-Ti oxide 40-70% Plagioclase (An 51-75) 10-30% Amphibole (Mg-no. 46-70) 5-20% Quartz 5-15% Biotite 1-5% Fe-Ti oxide Zoned cpx phenocryst (volcanic sample 5713J05) Typical magnesio-hornblende (intermediate-felsic sample 2727M01) Cpx 45-65% Plagioclase (An 73-92) 25-45% Amphibole (Mg-no. 60-69) 0-15% Cpx (Mg-no. 68-82) 0-10% Opx (Mg-no. 54-69) 0-5% Biotite 2-5% Fe-Ti oxide Cpx cores in amphibole (mafic sample 2729M10A) TWO COMPOSITIONAL GROUPS The arc crustal section studied in the east comprises a calc-alkaline suite where all lithologies display consistent major and trace element trends. In contrast, two compositionally distinct groups can be defined in the western arc using geochemistry of plutonic and volcanic rocks. 100 Sr/Y 100 Early Stages (compositional group I) • Arc volcanism (starting ~198 Ma) 60 Compositional Group I (75 samples) Compositionally similar to rocks from the eastern arc: relatively low K at a given SiO2 and flat REE patterns ([La/Yb]N < 5; average 2.6) where REE abundances increase with increasing SiO2. Rocks cover a wide range of compositions (43.0 – 76.1 wt % SiO2), including mafic and intermediate-felsic plutonic as well as volcanic samples. 20 • Older plutons (~185-170 Ma) emplaced in relatively thin, immature arc crust 10 • Fractional crystallization and accumulation dominant Arc Maturation (compositional group II) • Younger plutons (~170-160 Ma) intrude into mature, thicker arc crust 1 0 • Conductive heating of gabbroic lower crust produces felsic partial melts 100 Y 15 [La/Yb]N 10 • Mixing of felsic and intermediate magmas produces the observed range of compositions 10 Compositional Group II (21 samples) Compositional Group I rocks show trends similar to eastern arc samples. Compositional Group II samples show an increase in [La/Yb]N and Sr/Y with increasing SiO2. [La/Yb]N 80 40 Show an entirely different trend of LREE enrichment and HREE depletion ([La/Yb]N > 5; average 7.3) where REE abundances decrease with increasing SiO2. Samples exhibit higher Al2O3, Sr, and Ba, and lower TiO2, FeO, and Y compared to group I at the same silica contents. Define a more restricted range of compositions (56.1 – 73.6 wt % SiO2), including intermediatefelsic plutonic and volcanic samples. 9 ARC SECTION DEVELOPMENT Age Correlation 120 Fractional crystallization, partial melting, and magma mixing may all have contributed to the formation of the western Talkeetna island arc crustal section. 5 1 200 0 190 180 170 160 Age (Ma) 45 50 55 60 65 SiO2 (wt %) 70 75 Group I rocks are older (~185 – 175 Ma) Group II rocks are younger (~170 – 160 Ma) 150 Pr Nd Eu Gd Partial Melting of Mafic Compositions avg. [La/Yb]N = 1.7 > 54 wt % SiO2 Compositional Group I (green) Magma mingling of intermediate and felsic compositions, north shore Lake Grosvenor (5710J05) 61.8 1.0 concentration / C1 chondrite Int-felsic avg. [La/Yb]N = 2.0 Ce Tb Dy Ho 1.1 0.9 0.7 0.5 La Ce Cc/Cm 700 km2 Total = 5,584 km2 A mp h Mafic plutonic rocks (43.0 - 53.7 wt % SiO2) 12.6 % Mafic MINERALOGY AND PETROGRAPHY Lithology Volcanic Several intermediate and felsic endmember pairs from the group II suite seem to predict the observed range Ho Yb Y Lusamples. Presumably, of Er observed 100.0 the intermediate liquid formed via fractional crystallization, but how ppm The Talkeetna magmatic arc was created by northward dipping subduction (present-day coordinates) of the oceanic Farallon plate beneath the Peninsular terrane (part of the WCT) (Plafker et al., 1994; Clift et al., 2005). Exposed Area 10 Volcanic avg. [La/Yb]N = 4.6 concentration / C1 chondrite Mafic plutonic rocks Unit Lower Cook Inlet region study area and other Talkeetna island arc plutonic and volcanic exposures. Western arc exposures from Detterman & Reed (1980) and Riehle et al. (1993). Terranes from Barker et al. (1994), Siberling et al. (1994), and Wilson et al. (1998). 10 concentration / C1 chondrite Uppermid crust Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu La Sm Mixing Intermediate and Felsic End-member Magmas did the felsic10.0liquid form? ~7 km Intermediatefelsic plutonic rocks Cc/Cm = 1 is a perfect match Simple fractional crystallization (FC) and assimilation fractional crystallization (ECAFC developed by Bohrson & Spera, 2001) do not explain REE trends for compositional group II rocks, specifically decreasing REE abundances with increasing SiO2 (fractionation) and HREE depletion. So what processes formed these rocks? 4 Western Arc Upper crust Measured (observed) concentration (Cm) Compositional Group II 8 0.0 Schematic section showing rocks exposed in the western arc (Lower Cook Inlet region) Calculated concentration (Cc) Increasing REE abundances with increasing SiO2 (proxy for fractionation) and REE pattern shape are matched well by modeling fractional crystallization from andesitic to dacitic compositions. 12 0.5 The western arc section is composed predominantly of intermediate-felsic plutonic rocks (diorites and tonalites), minor amounts of gabbroic rock, and a significant component of overlying Talkeetna Formation volcanic and volcaniclastic rocks. The western arc is a small sub-section of the entire arc crustal section exposed in the east. La Ce Step 1 Daughter (5710J02) Parent (81AR193) 64.5 0 1.5 EXPOSED ARC CRUSTAL SECTION 10 concentration / C1 chondrite 3 0 Fractional Crystallization 8 6 1.0 GEOLOGIC SETTING Compositional Group I 68 12 FeO 2 Continental Growth: what implications do the chemical trends and magmatic formation of western arc rocks have on models for the growth of continental crust at convergent margins? 1 low-K 12 Processes of magma differentiation: what magmatic processes are responsible for the formation of the exposed intermediatefelsic plutonic and volcanic rocks in this region? 48 (5721J04) Cc/Cm Along strike compositional variation: is there significant geochemical variability along arc strike (compared to the eastern arc)? 2 concentration / C1 chondrite MAJOR ELEMENTS Research presented here addresses three main questions yet to be resolved for the intermediate-felsic core of the western Talkeetna arc: MAGMATIC PROCESSES What magmatic processes are responsible for the formation of intermediate-felsic plutonic and volcanic rocks exposed in the western arc? Were these rocks formed by simple fractional crystallization or did assimilation of existing crust and/or magma mixing play an important role? Greene et al. (2006) modeled fractional crystallization from basalt to andesite compositions in the 100 step 2 daughter (observed) eastern arc. The research presented here Le Maitre et al. (1989) step 2 daughter (calculated) attempts to model the formation of more step 1 daughter (observed) step 1 daughter (calculated) wt % SiO2 48 compositions starting with an felsic parent liquid (81AR193) 68 andesitic composition. Step 2 Daughter weight % The accretion of island arc crust is believed to be a major contributor to the growth of continents. A particularly important question in arc evolution is the origin of felsic plutonic rocks in island arcs. Felsic rocks represent the nucleus of continents, yet there is no clear consensus on how these rocks originate. The Jurassic Talkeetna island arc in south-central Alaska is an exhumed and titled arc section where middle and upper crust lithologies are now exposed at the surface, providing us with the rare opportunity to directly model processes responsible for the formation of the felsic core of an island arc. Studies in the eastern Talkeetna arc (Talkeetna and Chugach Mountains) indicate that the arc crustal section comprises a calc-alkaline suite where all lithologies display consistent major and trace element trends, each group of rocks forming by processes of fractional crystallization and accumulation. In contrast, two chemically distinct groups can be defined in the western Talkeetna arc (Lower Cook Inlet region). Compositional group I (n = 75; 43.0-76.1 wt % SiO2) shares many of the same trends as rocks from the eastern arc: relatively low K at a given SiO2 and flat REE patterns ([La/Yb]N < 5; average 2.6) where REE abundances increase with increasing SiO2. Compositional group II (n = 21; 56.1-73.6 wt % SiO2) shows trends of LREE enrichment and HREE depletion ([La/Yb]N > 5; average 7.5), where REE abundances decrease with increasing SiO2. The most silicic members of this group exhibit concave-up patterns of HREE depletion. Of particular interest is the formation of the voluminous felsic core of the western arc, exposed over more than 4800 km2 of the region. Major element and REE modeling indicate the majority of these rocks formed through fractional crystallization and accumulation (compositional group I). In addition, modeling results for compositional group II rocks indicate that the observed range of intermediate-felsic compositions can be produced by variable mixing of an andesitic parent liquid (presumably formed by fractional crystallization) with a felsic end-member magma (formed by ~15-25% partial melting of mafic crust). This study provides important insights into the range of processes responsible for the formation of the felsic core of island arcs and potentially the nucleus of continental crust. 6 WHOLE-ROCK CHEMISTRY Michael Johnsen, Department of Geology, WWU, Bellingham, WA Susan DeBari, Department of Geology, WWU, Bellingham, WA Matthew Rioux, Department of Earth Science, UCSB, Santa Barbara, CA Cc/Cm 2 RESEARCH QUESTIONS weight % 1 ABSTRACT Paper No. 30 - 8 Both compositional groups are components of the buoyant intermediate-felsic nucleus of the western arc that accreted to the continental margin. Er