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University of Colorado, Boulder CU Scholar Geological Sciences Graduate Theses & Dissertations Geological Sciences Spring 1-1-2011 Geology and Geochemistry of Hadean Zircon Bearing Supracrustals from Quad Creek, Eastern Beartooth Mountains (Montana, USA) Analisa Charlotte Maier University of Colorado at Boulder, [email protected] Follow this and additional works at: http://scholar.colorado.edu/geol_gradetds Part of the Geochemistry Commons, and the Geology Commons Recommended Citation Maier, Analisa Charlotte, "Geology and Geochemistry of Hadean Zircon Bearing Supracrustals from Quad Creek, Eastern Beartooth Mountains (Montana, USA)" (2011). Geological Sciences Graduate Theses & Dissertations. Paper 34. This Thesis is brought to you for free and open access by Geological Sciences at CU Scholar. It has been accepted for inclusion in Geological Sciences Graduate Theses & Dissertations by an authorized administrator of CU Scholar. For more information, please contact [email protected]. GEOLOGY AND GEOCHEMISTRY OF HADEAN ZIRCON-BEARING SUPRACRUSTALS FROM QUAD CREEK, EASTERN BEARTOOTH MOUNTAINS, MONTANA, USA by ANALISA C. MAIER B.A University of Colorado, 2010 A thesis submitted to the Facility of the Graduate School of the University of Colorado in partial fulfillment of the degree requirements for the degree of Master of Science Department of Geological Sciences 2011 This thesis entitled: Geology and Geochemistry of Hadean Zircon Bearing Supracrustals from Quad Creek, Eastern Beartooth Mountains (Montana, USA) written by Analisa Charlotte Maier has been approved for the Department of Geological Sciences Stephen J. Mojzsis, Thesis Advisor Rebecca Flowers, Committee Member Kevin Mahan, Committee Member Date The final copy of this thesis has been examined by the signatories, and we Find that both the content and the form meet acceptable presentation standards Of scholarly work in the above mentioned discipline. Maier, Analisa (M.S. Geology) Geology and Geochemistry of Hadean Zircon Bearing Supracrustals from Quad Creek, Eastern Beartooth Mountains Thesis directed by Stephen J. Mojzsis Quad Creek Paleoarchean quartzites (~3300 Ma) in southern Montana contain >3900 Ma detrital zircons. Although an accessible resource for investigating Hadean (>3850 Ma) crust distinct from the well-known Narryer Gneisses in Australia and Acasta Gneisses in Canada, the outcrop-scale geological and geochemical context of these rocks is not well documented. New mapping reveals a varied suite of isoclinally folded, sheared and variably deformed chromiterich banded and massive quartzites, garnet-bearing siliceous paragneisses, amphibolite, quartzbiotite schists and a suite of inter-bedded quartz+magnetite rocks (banded iron-formation, BIF). Conventional ion microprobe U-Pb zircon ages of different populations from these units show neoform growth or outgrowth rims on inherited detrital igneous cores at ~3350 Ma, ~2900 and later (<2500 Ma) that match those for regional metamorphic events evidenced elsewhere in the Wyoming Craton. Weighted mean 207 Pb/206Pb ages for the youngest concordant igneous zircon cores indicate that the maximum age of deposition for the supracrustals is ~3300 Ma. These data support the notion that the oldest crust tapped by the Quad Creek quartzites is comparable in age to the ca. 3960 Ma Acasta Gneiss Complex. This similarity is suggestive of a linkage between the geology of the Wyoming Craton and the Western Slave Province. An ancient supercraton called Sclavia could provide the mechanism for this linkage. Sediment source(s) for some other Paleoarchean supracrustals were also contaminated by Hadean crust suggests that pre-3900 Ma zircons are present in similarly-aged basins elsewhere in North America and perhaps also remnants of Sclavia. The Slave and Wyoming craton may be the modern components of a ‘Slave clan’ which contains the remnants of Sclavia. iii Acknowledgements I owe special thanks to many people, without whom this thesis would have never been accomplished. First and foremost, I would like to thank my advisor Stephen Mojzsis for his endless help and support in both my undergraduate and graduate careers, without him none of this would have been possible. I am grateful to Elizabeth Frank, and Nicole Cates for field assistance. Nicole Cates also deserves special recognition for teaching me laboratory and computer techniques as well as answering hundreds of miscellaneous questions throughout the last few years. Dustin Trail has been instrumental in sending me his original data from his work in the Beartooth Mountains, discussions, and his comments on earlier versions this study. I would like to thank Rebecca Flowers and Kevin Mahan for agreeing to be part of this committee and their comments on this work. I also thank Michelle Hopkins and Elizabeth Frank, my fellow graduate students and group members for their advice and knowledge of navigating through graduate school. Chris Bermel warrants special thanks for his endless support during these last few years. Furthermore, I am indebted to the University of Colorado, the Colorado Scientific Society and NASA Exobiology Program for support of this work. Additional funding came from the J.W Fulbright Foundation and the Alfred P. Sloan Foundation. Finally, discussions with Paul Mueller at the University of Florida have been very helpful. iv CONTENTS CHAPTER 1: INTRODUCTION…………………………………………………………………1 1.1 The Hadean Eon…………………………………………………………………….....1 1.2 Ancient Zircon Localities………………………………………………………….....2 CHAPTER 2: GEOLOGY AND SAMPLE PREPARATION………………………..……….....7 2.1 Regional geology of the Wyoming Province including the Beartooth Mountains……7 2.2 Methods………………………………………………………………………………11 2.2.1 Field Mapping…………………………………………………...…………12 2.3 Sample preparation…………………………………………………………………..14 2.3.1 Ion microprobe U-Pb geochronology……………………………………...14 2.3.2 Whole rock geochemistry………………………………………………….19 CHAPTER 3: WHOLE ROCK GEOCHEMISTRY…………………………………………….21 3.1 Sample descriptions and results……………………………………………………...21 3.3 Discussion…………………………………………………………………...……….24 CHAPTER 4: ZIRCON GEOCHRONOLOGY……………………………………………..…..30 4.1 Ion microprobe zircon geochronology……………………………………………....30 4.2 Results…………………………………………………………………………….….33 4.2.1 Depth profile of the oldest zircon found at Quad Creek by Trail, 2006…..33 4.3 Discussion…………………………………………………………………………....34 CHAPTER 5: BEARTOOTH ZIRCONS......................................................................................35 5.1 Zircon morphology…………………………………………………………………..35 v 5.2 Zircon chemistry……………………………………………………………………..40 CHAPTER 6: EVIDENCE OF A SUPERCRATON; SCLAVIA?..............................................42 6.1 Detritus from an ancient continent………………………………………………….42 6.1.2 Hadean detrital zircons in other Paleoarchean sedimentary basins in North America…………………………………………………………………………..42 6.2 Craton and supercratons……………………………………………………………..46 6.2.1Slave clan: modern compoents of Sclavia……………………………........47 6.3 Discussion…………………………………………………………………………....48 CHAPTER 7: SUMMARY……………………………………………………………………...50 REFERENCES CITED…………………………………………………………………………..52 vi List of Tables 1. Conventional ion microprobe U-Pb analyses of zircons from Quad Creek, Montana……….15 2. Representative bulk-rock compositions from Quad Creek, southern Montana………………20 vii List of Figures 1. a. Generalized sketch map of Archean and Proterozoic rocks of North America……………..5 1. b. Discovery sample site of Mueller et al. (1992)………………………………………………5 1.c. Location map of the Quad Creek site…………………………………………...…………...5 2. Figure 2. Tera Wasserburg Plot of the Beartooth zircons < 5% disrocdant, showing a large concentration of zircons at the 3300-3200 Ma population……………………………………….9 3. Comparative relative probability plots of Beartooth Mountain zircon 207Pb/206Pb ages from this study, Mueller et al. (1992), and Trail (2006)…………………………………………….…10 4. Geologic map (1:250 scale) of the Quad Creek study area…………………………………..13 5. (a,c) Chondrite-normalized REE plots……………………………………………………….22 5. (b,d) Primitive mantle-normalized multi-element plots of the different lithologies in the Quad Creek study area…………………………………………………...……………...…………..….22 6. Anorthosite-Albite-Orthoclase (An-Al-Or) diagram………………………………………….25 7. Field photographs of described lithologies…………………………………………………...26 8. Alkalis-Fe oxides (AFM) diagram and K2O vs. silicia diagram of BT0602……………...….27 9. Na2O + CaO – Al2O3 – K2O diagram………………………………………………………...29 10a. Tera-Wasserberg plot with a relative probability plot of the Beartooth zircons…………..31 10b. Plot of relative probability and [Th/U]zircon vs. 207Pb/206Pb zircons <30%………………...31 11. Plot of relative probability and [Th/U]zircon vs. 207Pb/206Pb zircons <5%……………….…..32 viii 12. Back scattered electon micrographs of the Beartooth zircons…………………...………….36 13. Diagram of [Th/U]zircon vs. 207Pb/206Pb age from this work, Trail et al. (2007), Mueller et al. (1992), the average oldest, most concordant zircon from the Assean Lake Complex (Böhm et al. 2003), and the oldest most concordant zircon from the Central Slave Cover Group (Sircombe et al. 2001)….....................................................................................................................................45 ix CHAPTER 1 INTRODUCTION 1.1 The Hadean Eon The Hadean (pre-3.85 Ga) geologic record on Earth is limited. Information regarding the formation and evolution of the earliest crust has relied on geodynamical model studies (Reymer and Schubert, 1985), general comparisons with lunar data, and most importantly, pre-3.9 Ga zircons from Western Australia (Froude et al., 1983, Compston and Pidegeon, 1986) and Northern Canada (Bowring and Williams, 1999, Iizuka et al., 2006). Since zircon is fairly resistant to chemical alteration, physical breakdown during weathering, and is somewhat refractory, non-metamict crystals can serve as strong sources of radiogenic Pb* and Th/U ratios. Therefore, the geochemistry of Hadean zircons is an exceptional resource to explore the establishment of crust and the amount of early crustal types. There are two main views on early crustal types; one is that ancient Earth was characterized by appearance of significant volumes of ultramafic/basaltic crust (Galer and Goldstein, 1991, Kamber et al., 2005) and the absence of considerable continental crust before about 4.0 Ga (Taylor and Mclennan, 1995). The other view is that early Earth had considerable continental growth and succeeding recycling (Armstrong, 1981, 1991, Harrison et al., 2005a). The significant role of recycled, granitic crust in the Hadean eon (≥3850 Ma; definition of Bleeker, 2004) has been demonstrated with the discovery and characterization of hundreds of thousands of zircons (Holden et al., 2009) including a population of greater than 4000 Ma grains, found in quartzites from the Narryer Gneiss Complex of the Yilgarn Craton at the Mt. Narryer (Froude et al., 1983) and Jack Hills (Compston and Pidgeon, 1986) localities in Western Australia. The mineral chemistry of the ancient Australian zircons is explicable with granite-granitoid melt compositions (e.g. Maas and 1 McCulloch, 1991; Peck et al., 2001; Blichert-Toft and Albarède, 2008; Harrison, 2009; Hopkins et al., 2008, 2010; Trail et al., 2007). Elsewhere in Western Australia, Wyche et al. (2004) identified detrital zircons from the Southern Cross Granite-Greenstone terrane in the central part of the Yilgarn craton as old as 4350 Ma. Due to the geographic proximity of the Southern Cross with the Narryer Gneisses, it is likely that they shared the same crustal source for old zircons. Because these studies are exclusively of detrital zircon grains orphaned from their parent rocks, only broad inferences may be made about probable source melt compositions that gave rise to them. Detrital zircons of similar >3.9 Ga have also been found in the Beartooth Mountains of southern Montana and in northern Canada with related issues identifying melt source compositions. 1.2 Ancient zircon localities As opposed to detrital zircon studies, direct analyses of Hadean rocks are severely limited to the few locations where actual ~4 billion-year-old crustal components are documented with certainty. These are the Acasta Gneiss Complex (AGC) in the Northwest Territories of Canada (King, 1985; Bowring et al., 1989; Stern and Bleeker, 1998; Bowring and Williams, 1999) and certain components of the Mt. Sones gneisses within the Napier Complex of Antarctica (Harley and Black, 1997). Although both the Acasta and Napier rocks are older than ~3900 Ma (cf. Moorbath, 2005), the geology of their respective outcrops is known only in very broad terms, and further complicated by protracted histories of high grade metamorphism and deformation. Rare >4000 Ma xenocrystic zircons have also been reported occluded in younger gneisses. One such 4183 Ma zircon was found in drill core from near to the Narryer Gneiss Complex (Nelson et al., 2000). A concordant ca. 4200 Ma xenocrystic zircon was discovered by Iizuka et al. (2006, 2 2007) in ~ 3960 Ma tonalitic gneiss from the Acasta area. A 3850 Ma orthogneiss sample from the southern Færinghavn terrane of the Itsaq Gneiss Complex in West Greenland (Nutman et al., 1997), yielded a single concordant 4100 Ma zircon encased in biotite (Mojzsis and Harrison, 2002). It is noteworthy that although the Itsaq Gneiss Complex (IGC) is regarded as the largest, most comprehensively studied Eoarchean-Paleoarchean terrane on Earth (e.g. Nutman et al., 1996), no detrital zircons older than about 3890 Ma have so far been found there (Nutman et al., 2004). Aside from the ancient zircon work, other geochemical evidence has accumulated that previously extensive >3850 Ma Hadean mafic and felsic crust continued to influence the character of supracrustal belts well into the Eoarchean. This viewpoint is based on the enhanced 142 Nd/144Nd values compared to the terrestrial standard of (basaltic) amphibolites – some of which preserve pillow structures – of parts of the ~3770 Ma Isua Supracrustal Belt in West Greenland (Boyet et al., 2003; Caro et al., 2006). Mafic rocks from the IGC tapped re-worked/recycled even older pre-4100 Ma mafic crust when they first formed, which is required to explain the presence of radiogenic 142 Nd from the decay of constraints. Pre-4000 Ma 147 146 Sm (half-life = 103 Myr) in light of U-Pb zircon age Sm/143Nd source model ages have also been reported for basalts of the ca. 3450-3490 Ma Warrawoona Group in Western Australia (Tessalina et al., 2010). Another line of evidence comes from some CaO-poor (cummingtonite-rich) amphibolitic components of the Nuvvuagittuq Supracrustal Belt in northern Québec that record lower 142Nd/144Nd ratios than the terrestrial standard. This result requires that the source melts for the Nuvvuagittuq amphibolites were isolated from 146 Sm before it became extinct, and the data were used by O’Neil et al. (2008) to propose a 4280 Ma age for these rocks. An alternative interpretation, based on U-Pb detrital zircon geochronology (Cates and Mojzsis, 2007, 2009) has argued that the 3 Nuvvuagittuq belt formed at about 3780 Ma. If the latter interpretation is correct, depleted 142Nd values in these amphibolites were inherited from a recycled ancient precursor in ways comparable to the Isua rocks cited above. In sum, any new source of information on the Hadean Earth holds the potential to greatly enhance our understanding of the mechanisms that shaped the young planet. These would include: (i) the role of catastrophic impacts in the destruction of primordial crust; (ii) plausibility of a biosphere at that time; (iii) extent of continental-type crust; and (iv) initiation of plate tectonics. To advance what we know of the geology of those places already identified to contain Hadean detritus, fine-scale mapping to direct sample collection for geochronology and geochemistry of specific outcrops is warranted. With such knowledge, we can begin to build hypothesis tests for the origin of the oldest detrital rocks and minerals. One such example is from a fuchsite- (Cr-muscovite) bearing quartzite which hosts rare (≤2% yield) pre-3900 Ma detrital zircons identified from samples collected at a road cut in the northwestern part of the Wyoming Craton at Quad Creek in southern Montana (Mueller et al., 1992, 1998; Figure 1). The Quad Creek rocks provide an accessible collection of Hadean detrital zircons, but relatively little is known about their specific field relations (e.g. Chamberlain et al., 2003; Chamberlain and Mueller, 2007). Some low-resolution sketch maps of the Quad Creek area have been published (e.g. Mogk and Henry, 1988), but the associations of ancient zirconbearing quartzites with other lithotypes and what those lithologies are, as well as the various modes of zircon occurrence, range of ages, and compositions need definition. To this end, we performed 1:250 scale mapping of the associated supracrustal rocks nearby the ancient zircon discovery site (Mueller et al., 1992, 1996) and used the output to propose a model for the origin 4 5 Figure 1. a. Generalized sketch map of Archean and Proterozoic rocks of North America. b. Discovery sample site of Mueller et al. (1992; sample “HRQ”). FOV = 200 m. c. Location map of the Quad Creek site in the Hellroaring Plateau, southern Montana. of the oldest zircon populations in these and other Paleoarchean metasediments in North America. 6 CHAPTER 2 GEOLOGY AND SAMPLE PREPARATION 2.1 Regional geology of the Wyoming Province including the Beartooth Mountains Well known occurrences of Archean paragneisses in the Wyoming Craton are found as metamorphosed volcano-sedimentary supracrustal enclaves throughout the northern and eastern Beartooth Mountains of southern Montana and northern Wyoming (Chamberlain and Mueller, 2007). At Quad Creek (Figure 1), a diverse suite of tectonically interwoven lithologies comprise mafic granitoid gneisses, quartzites and other paragneisses, iron-formations, amphibolites, and some small outcrops of biotite schists of uncertain affinity. This kind of supracrustal assemblage has been described throughout the northwestern Wyoming Craton, especially for the Montana Metasedimentary province (MMP; Mogk et al., 1992) in the North Snowy block (Mogk et al., 1988), about 75 km northwest of the Quad Creek outcrops. Quartzites from the area have been known for some time to host zircons as old as 3960 Ma (Mueller et al., 1992). In addition, a supracrustal sample from the Tobacco Root Mountains ~175 km northeast of Quad Creek also yielded a discordant 3930 Ma zircon (Mueller et al., 1998), which testifies to the presence of a small background of Hadean detritus in the wider region. Aside from the rare Hadean zircon record documented for various supracrustal enclaves, periods of Eoarchean and Paleoarchean crustal growth have also been inferred based on the compositions of gneisses from throughout the Wyoming Craton by ~4000 Ma Nd model ages and highly radiogenic Pb isotopic compositions (Wooden and Mueller, 1988). This includes rocks of the Sacawee block and MMP (Grace et al., 2006) as well as ~3300-3700 Ma ages for the Granite Mountain region of central Wyoming (Fisher and Stacey, 1986; Langstaff, 1995). 7 The overall dominance of ~3300 Ma ages – which are also the ages of the youngest, most concordant zircons of Th/U composition consonant with equilibrium exchange from magma – in the compiled data set of Quad Creek zircons are also likely representative of the maximum age of deposition of the supracrustals (see below). This is further supported by Figure 2, showing a strong concentration of < 5% discordant zircon ages in the 3300-3200 Ma age population. These rocks later experienced magmatic injections probably associated with the final assembly of the Wyoming Craton (Mueller and Frost 2006). Magmatism occurred in three separate phases in the southern margin of the terrane at 2710-2670, 2650-2620, and 2550-2500 Ma. Proterozoic extension at 2100-2000 Ma and 1500-1400 Ma is also recognized, which saw the emplacement of mafic dike swarms associated with crustal thinning (Chamberlain et al. 2003). The Beartooth rocks were brought to granulite facies metamorphic conditions (T = 750-800°C, P = 5-6 kbar) from 3250-3100 Ma (Henry et al., 1982) based mostly on Rb-Sr and Pb-Pb systematics and other indicators (Wooden et al., 1988a,b; Mogk et al., 1992). Granulite metamorphism was followed by retrograde amphibolite facies and several later episodes of granitoid intrusions (Mogk and Henry, 1988). Archean basement visible throughout the contemporary Wyoming province in the cores of mountain ranges was uplifted during the Cretaceous Laramide Orogeny and sculpted by glaciers in the Pleistocene so that outcrop exposure is good. Various results from prior geochronological work on detrital (igneous) and younger postdepositional (metamorphic) zircons from the northern part of the Wyoming Province have been used to constrain a broader age range of 2700-3300 Ma for the various quartzites (Mueller et al., 1988). In a study of 355 detrital zircons from the Quad Creek (Figure 3) locality by Mueller et al. 8 Figure 2. Tera Wasserburg Plot of the Beartooth zircons < 5% disrocdant, showing a large concentration of zircons at the 3300-3200 population. 9 Figure 3. Comparative relative probability plots of Beartooth Mountain zircon 207Pb/206Pb ages from this study (light grey), Mueller et al. (1992; black), and Trail (2006; dark grey). 10 (1992, 1998), about 20% of the grains were found to be between 3400-4000 Ma and many of those were within 10% of concordia. While the oldest zircons found in the Beartooth quartzites are upward of 3960 Ma, demonstrably detrital zircons of igneous derivation here and from the neighboring Ruby and Tobacco Root uplifts are no younger than about 3250 Ma (Mueller and Frost, 2006). The mostly Paleoarchean igneous (as opposed to younger neoform metamorphic) zircon ages reflect the dominant influence to these sediments from some relatively younger crust shed into the MMP basins. Metamorphic zircons grew either in situ, or as metamorphic overgrowths/rims on older grain cores and have ages between 2700 and 3200 Ma (Mueller et al., 1998); granulite facies metamorphism in the eastern Beartooth Mountains occurred at ca. 3200 Ma (Mogk et al., 1992) which matches well ages for metamorphic zircons and outgrowths in older detrital grains. Samarium-Neodymium model ages and whole-rock and mineral (feldspar) Pb isotope models further indicate that ~3900 Ma crust was present in the northern Wyoming province when the Beartooth quartzites (and probably the whole of the MMT) were deposited (Mueller et al. 2010); Hafnium model ages of ca. 3850 Ma also support this possibility (Stevenson and Patchett, 2000). 2.2. Methods The present study reports on a small (185 210 m) area of outcrop with good exposure (~5075%) at the discovery location of pre-3900 Ma zircons in a road cut described in Mueller et al. (1992). New conventional zircon U-Pb ion microprobe ages (n=118) were combined with a large scale survey (204Pb-corrected) of detrital zircon age determinations (n=1156). Further details of an ion microprobe U-Th-Pb depth-profiled ca. 4000 Ma grain in Trail et al. (2007) are also reported herein. The purpose of the large survey study was to comprehensively constrain the frequency of zircons of specific ages in these Paleoarchean sediments. When available, we 11 compare measured Th/U for individual zircon analyses with U-Pb zircon age against expected values for crystals grown in an igneous environment as a function of (207Pb/235U age vs. 207 Pb/206Pb age) concordance (e.g. Mojzsis and Harrison, 2002). Acquisition of whole-rock major-, minor- and trace element geochemistry is also described below. Collectively these data were used to assess protolith(s) based on mineralogy and composition, and maximum ages of deposition of igneous zircons to suggest geochemical links with other ancient rock localities mentioned above. 2.2.1 Field Mapping The Quad Creek exposure was mapped with a 25 25 m E-W oriented grid (Figure 4) annotated to show sample locations with photo-documentation of specific outcrops discussed in the text. Initial mapping was done by Stephen Mojzsis and Dustin Trail in 2006 and that draft map was refined and corrected in 2010 with more samples collected in the same locations for geochemistry. The focus was on quartzites which contain Hadean detrital zircons. Overall, the exposure is dominated by strongly deformed isoclinally folded supracrustal rocks comprising massive quartzites (Aqm) and banded paragneisses (Aqb) with locally highly-variable fuchsite contents (labeled f on the map where locally enriched in Cr-mica). Paragneisses in the field range from massive to banded and patchily garnetiferous (gt), and are cut by at least one generation of mafic dikes (Md). Subordinate units include a finely banded but strongly deformed and sheared quartz magnetite rock (banded iron-formation s.l.) restricted to the easternmost mapped area that is in contact with a garnet-bearing amphibolite (Amg). No carbonates were noted at this particular site (cf. Mogk et al., 1992). The sheared east-west dipping isoclinal folding within the supracrustal sequence does not appear to be a structure shared by the 12 Figure 4. Geologic map (1:250 scale) of the Quad Creek study area is within grey-green quartzites along a road cut of Montana Rte. 212 at the southern boundary of the mapped area. 13 underlying massive intrusive granitoid (trondhjemitic) gneisses (Gb). To the best of our knowledge, the original sampling locality of old detrital zircons reported by Mueller et al. (1992) 2.3 Sample Preparation 2.3.1 Ion Microprobe U-Pb Zircon Geochronology Ion microprobe U-Pb zircon geochronological analyses of zircons from five supracrustal lithologies (three quartzites Aqm, and three paragneisses Agb) are reported in Table 1. Rock samples were prepared for geochemistry and zircon separations using standard techniques (e.g. Cates and Mojzsis, 2006) and a brief summary is provided here: Approximately 5 kg of Aqm samples BT0606, -08 and -11 and Agb samples BT0603, -04 and -10 from the mapped area were chosen for zircon extractions. These large samples were reduced by crushing and sieving to >350 µm. Geochemistry for sample BT1 is not reported here but mineralogically it is identical to Aqm sample BT0606. Powders for zircon extractions went through two stages of clean heavy liquid separations, rinsing in reagent-grade acetone and ultrapure water, followed by hand-magnet and Frantz magnetic separations of the densest mineral residues. The least magnetic grain fractions were handpicked under a binocular microscope for individual zircons with a wide spectrum of morphologies and aspect ratios, placed on double-sided adhesive tape and cast in a 2.52 cm diameter mould with Buehler© epoxide resin alongside standard zircon AS3 (Paces and Miller, 1993; Black et al. 2003). The sample disk was then beveled and polished to brilliance in stages to 0.25 µm alumina until grain centers were exposed. Transmitted and reflected optical micrograph sample maps were created followed by imaging with back-scattered electrons of polished centers on a JEOL JXA-8600 electron microprobe at the University of Colorado under standard operating conditions. To reduce common Pb contamination prior to ion microprobe analysis, all . 14 Table 1. Geochronology 15 Table 1. Geochronology 16 Table 1. Geochronology 17 TABLE 1 GEOCHRONOLOGY 18 grain mounts were pre-cleaned in 1N HCL solution to remove common-Pb contamination, washed in ultrapure water, air dried in a 50 ºC oven, and coated with ~100 Å of Au to facilitate conductivity. Conventional zircon U-Pb ion microprobe geochronology on the sample zircons was performed using the UCLA Cameca ims1270 high-resolution SIMS (e.g. Cates and Mojzsis, 2006), a brief synopsis is provided here: a ~6-nA O2-primary beam was focused to a 25 -µm spot and operated at a mass resolving power of ΔM ~6000 to exclude molecular interferences. To increase the Pb+ ion yields, oxygen flooding to a pressure of 2.7x10-5 torr was used (Schumacher et al., 1994). Ages for unknown zircons were determined by comparison with a working curve defined by multiple measurements of standard AS3 that yields concordant 207 206 Pb/238U and Pb/235U ages of 1099.1+ 0.5 Ma. Data were reduced and the output used to construct Tera- Wasserburg diagrams (207Pb/206Pb vs. 238 U/206Pb) with the Isoplot software package (Ludwig, 2001). All conventional ion microprobe ages are reported at the 1σ level based on asymmetric weighted regressions except when noted. 2.3.2 Whole-rock geochemistry Duplicate whole-rock sample splits were prepared by subdividing ~2 kg unweathered rock pieces and powdering in pre-cleaned ceramic mills. Analysis of major-, minor- and trace elements (Table 2) was performed by XRF and ICP-MS at Activation Laboratories (ACTLABS; Ontario, Canada). Comparison of recommended and analyzed standards from ACTLABS show small deviations (<1.5%) for oxides, except MnO (6.3%), Na2O (6.5%) and P2O5 (23.1%) and for trace elements <20% relative, except Ni (21.4%), Pr had a 47.6% relative error. Granitoid rock assignments are from Barker (1979) based on normative modes of albite (Ab), orthoclase (Or) and anorthite (An); (trond = trondhjemite). 19 Table 2. Representative bulk-rock compositions from Quad Creek, southern Montana 20 CHAPTER 3 WHOLE ROCK GEOEMISTRY 3.1. Geochemical results and sample descriptions Quartzites (Aqm) BT0606, BT0608, BT0611 The dominant rock type exposed in the mapped area is a quartzite with local dark green banding. Bulk compositions of the Aqm rocks are strongly enriched in SiO2 (93.2-95 wt. %) befitting their protolith (probably as “dirty” quartz sands) so that the narrow variability in SiO2 content is due to different relative abundances of heavy minerals. Most are massive, grey to greenish in color. However certain sections locally preserve distinctive “sedimentary” bands rich in chromite, sulfide, Cr-mica, zircon and other minerals. The quartzites have Al2O3 contents that vary by a factor of ~2 (2.86-4.57 wt. %). Chromium abundances for different samples show an order-ofmagnitude range (40-360 ppm) but overall lower Zr concentrations (37-81 ppm) compared to granitoids. Primitive mantle normalized multi-element plots (Figure 5) show negative Nb, Sr, and Ti anomalies consistent with prevailing felsic sources to the sediment with a small mixture of mafic component. Chondrite normalized REE patterns also display enriched LREE and depleted HREE, with a negative Eu anomaly observed only in sample BT0608. Paragneiss (Agb) BT0610 A suite of banded quartz±garnet paragneisses appear in the field as white to grey rocks with bands of darker more mafic minerals rich in garnet. In outcrop, they appear to show evidence of partial melting and recrystallization, and tend to weather to a beige-orange color. Sample BT0610 has a similar Cr-enrichment (230) ppm to some quartzites, but has lower SiO2 contents 21 22 Figure 5. (a,c) Chondrite-normalized REE plots, and (b,d) Primitive mantle-normalized multi-element plots of the different lithologies in the Quad Creek study area. (75.09%) and is relatively enriched in alumina (13.66%). Zirconium content is greatly elevated in the paragneiss with respect to the typical Aqm lithologies (348 ppm) which may denote its origin from weathered granite. In a multi-element plot, (Figure 5) the Agb rocks show negative Sr and Ti anomalies along with a concave-upward trend from Dy-to Yb. Overall REE contents show enriched LREE compared to HREE with a positive Eu anomaly. Quartz-magnetite rock (BIF) BT1006 A small and narrow outcrop of silicate-facies BIF is sandwiched between sections of an amphibolite which defines a tight fold axis pinching out northwards. On the map, the amphibolite limbs are bounded by quartzites on one side and trondhjemite on the other (Figure 3). The amphibolite was not sampled for geochemistry. The BIF unit is very iron rich (51 wt. % Fe2O3) with 44% SiO2 and minor Al2O3 (~3%). It also contains 50 ppm Cr and 26 ppm Zr consistent with a small detrital component. Trends in primitive mantle-normalized multi-element REE plots reflect this in negative Sr and Ti anomalies and show that the BIF has some similarities to the quarztites and paragneiss for which it is presumably co-genetic in the supracrustal sequence. In a chondrite normalized REE plot, (Figure 5) LREE are enriched over HREE and show a small negative Eu anomaly. With respect to Y/Ho ratio, the Quad Creek quartz-magnetite rock is low (28.7) in comparison to most Archean BIFs (Bolhar et al., 2004), as well as relatively low in molar Ni/Fe (Konhauser et al., 2009) and Cr contents (Konhauser et al., 2011) compared to averages for other BIFs of this age. Trondhjemitic Granitoid Body (Gb) BT1004 and quartz-biotite schist (Aqbc) BT1002 In several places within the supracrustal succession, pockets of a granitoid penetrate through the outcrop, and the whole locality is underlain by a large trondhjemitic body (BT1004) as defined 23 by normative An-Al-Or compositions (Barker, 1979; Figure 6). We also identified a quartzbiotite schist (conglomerate? BT1002) near the base of the Rte. 212 road cut west of sample site HRQ (Figure 7d). As this rock was not found to crop out in the mapped area it is not marked on the map. Both the quartz-biotite schist and the trondhjemite show similar SiO2 and Al2O3 values to the paragneiss with 77.3% SiO2, and 14.3% Al2O3 for the conglomerate and 72.6% SiO2 and 15.4% Al2O3 for the trondhjemite, respectively. The quartz-biotite schist contains 70 ppm Cr and 49 ppm Zr, whereas the trondhjemite is Cr-poor (<D.L.), and relatively low in Zr (167 ppm) compared to granites but is entirely consistent with trondhjemite. Mafic Dike (Md) BT0602 Sample BT0602 is a black to dark grey unit bordered by paragneisses. It contains 47.58% SiO 2, 14.46% Al2O3, 12.54% Fe2O3, and 10.93% CaO with 310 ppm Cr and 45 ppm Zr. To determine if the mafic dike is part of the calc-alkaline or tholeiite series, we plotted its composition on a AFM diagram but this was inconclusive as lies between the determining lines of Kuno (1968) and Irvine and Baragar (1971) (Figure 8). However, on a K2O vs. silica diagram, using the subdivisions of Rickwood (1989) the mafic dike appears more akin to calc-alkaline with medium to low K (Figure 8). 3.3 Discussion Geochemistry and protolith assignment Sample BT0608 was mapped as a paragneiss, however, the geochemistry shows its protolith is quartzitic; it should be noted then that the quarztite may extend down further in this location or the paragneiss and quarztite are intermixed here. In a multi-element diagram (Figure 5), felsic 24 Figure 6. Granitic assignment using the Anorthorsite-Albite-Orthoclase (An-Al-Or) composition. The black division lines from Barker (1979). 25 26 Figure 7. Field photographs of the key lithologies described in the text. a. Fuchsitic quartzite (Aqm). b. Banded paragneisses (Agb). c. Mafic dike (Md). d. possible conglomerate. Note scales. Figure 8 a. Alkalis-Fe oxides-Mg oxides (AFM) diagram showing the boundary between the calc-alkaline field and the tholeiitic field from Kuno (1968; solid black line) and Irvine and Baragar (1971; dashed black line). Mafic dike BT0602 (black square) plots ambiguously in between the boundaries. b. Black lines showing the subdivision of subalkalic rocks from Rickwood (1989) using a K2O vs. silica diagram. BT0602 (black square) plots low in the medium K, calc-alkaline series region. 27 lithologies (detrital and igneous) show enriched large ion lithophiles, and negative Nb anomalies with depletions in Sr and Ti for all lithologies except the mafic dike. The quartzites follow expected weathering trends (Nesbitt and Young, 1989) for a mixed granitoid + mafic source (Figure 9). The quartzites show general variability in Cr and low have Zr values compared to the single paragneiss sample, which is elevated in both Cr and Zr (Table 2). The quartzites are shown in Figure 9 to be more weathered than the paragneiss as it is depleted in K2O and enriched Al2O3 compared to the paragneiss. This could provide an explanation for why the quartzite is depleted in Zr compared to the paragneiss. Sediments also have a natural variability in elements and it could be that this particular area of quartzites have stochastically depleted abundances of Cr and Zr. Alternatively, the paragneiss could be a product of a mix of rock types. To explore this further, more analysis of other paragneiss samples would be required. 28 Figure 9. Na2O + CaO – Al2O3 – K2O diagram showing weathering trends for the samples in this work. Quartzites (BT0606, -08, 11) are shown as gray asterisks, the paragneiss (BT0610) is shown as an open triangle, the conglomerate an open circle, the BIF an open square, the trondhjemite (BT1004) a filled gray square, and the mafic dike (BT0602) a filled black diamond. The arrow shows the average weathering trend predicted for upper continental granite from Nesbitt and Young (1984). 29 CHAPTER 4 ZIRCON GEOCHRONOLOGY 4.1 Conventional U-Pb zircon ion microprobe geochronology This chapter reports data for zircons from both the quartzites and paragneisses of the Quad Creek outcrops. Zircons were extracted from three fuchsitic quartzites (BT0606, -08 and -11) and three (banded) paragneisses (BT0603, BT0604, and BT0610) from the mapped area. Values for (Th/U)zircon vs. age (Figure 10) for data < 30% discordant appear similarly scattered for different age bins and vary by several orders of magnitude in Th/U with distinct age groupings at (i) ~2800 Ma with a Th/U composition of .011 (ii) ~3300-3100 Ma (Th/U=0.14-1.5); (iii) ~34003500 Ma (Th/U=0.32-.57) and another older grouping at ≥3700 Ma with Th/U ratios of 0.350.45. Filtering data to the least disturbed ages (<5% discordant; Figure 11) yields distinct age groupings at ~3300-3100 Ma (Th/U=0.14-1.5), ~3400-3500 Ma (Th/U=0.32-.57) and another older grouping at ≥3700 Ma with Th/U ratios of 0.35-0.45. The younger ages are not <5% discordant attributed to significant periods of metamorphism and granitod intrusions after 3200 Ma. Eoarchean to Hadean zircons were found in several of the banded paragneiss samples (3650-3902 Ma), but the majority of ages reside in the ca. 3300 Ma population. Younger zircons show a large scatter in Th/U. Figure 11 shows that there is a pronounced age grouping at ca. 3250 Ma and ca. 3450 Ma with Th/U ratios consonant with igneous derivation (0.4-0.5) for the peak around 3450 Ma, and highly variable ratios (0.4-1.2). A small subset of 3200 Ma and younger zircons is also evident with highly variable Th/U as well as internal textures (Corfu et al. 2003; Hoskin and Schaltegger, 2003) attributable to in situ growth during granulite facies metamorphism at 3200 Ma (Figure 11). The igneous-derived Th/U ratios reported here place a 30 Figure 10 a. Tera Wasserburg Plot of the Beartooth zircons. b. Relative probability and Th/U vs. age plot of zircons that are <30% discordant. The Th/U values of the quartzites are marked as hollow squares and the paragneisses as filled squares. The different shades of gray for a correspond to different age populations as indicated in the legend. 31 Figure 11. Plot of relative probability and [Th/U]zircon vs. 207Pb/206Pb zircon age filtered for analyses that are <5% discordant. 32 putative maximum age for the quartzites and paragneisses at 3300 Ma, in agreement with previous work (Chamberlain and Mueller, 2007). Out of the 118 zircons analyzed in this work by conventional U-Pb zircon geochronology, 70 were less than 30% discordant. One of the four oldest zircons in our analysis pool (3866 Ma) is from the paragneiss (Agb sample BT0610), and the three others (3902, 3756, and 3690 Ma) are from two fuchsitic quartzites (Aqm sample, BT0608 and BT0611). Collectively our ages from conventional ion microprobe geochronology are somewhat younger than the oldest zircon ages reported by Mueller et al. (1992) and in subsequent publications (Figure 3). 4.2 Results 4.2.1 U-Th-Pb depth profile of the oldest zircon at Quad Creek Approximately 10 kg of sample BT-1 (HRQ of Mueller et al., 1992) yielded thousands of zircon grains of diverse morphologies (Trail, 2006). Large scale geochronological surveys of two polished zircon grain mounts prepared from this sample yielded two zircons with ages ≥3960 Ma later verified by depth profile analysis (Trail, 2006) on one grain (BEAR-2_5-6) to be 3997±5 Ma (2; mswd = 1.9) with a weighted mean (Th/U)Zirc = 0.503±0.002 Zircon sample BEAR-2_56 is the oldest zircon thus far documented from Quad Creek. In the data set of Mueller et al. (1992), two pre-3900 Ma HRQ zircons were within 1% of concordia: their “grain 6” was 3964 Ma (Th/U=0.403) and “grain 45” had an average age of 3936 Ma and Th/U = 0.513. That these oldest grains are large (~120-160 µm), concordant zircons with a limited range of igneous (Th/U)zircon can potentially provide a clue as to their source rocks. Such (Th/U) values for zircon are typical of, but not limited to, those derived from granitoid melts (e.g. Mojzsis and Harrison, 2002). Based on partitioning rules (Blundy and 33 Wood, 2003) these values may be used to approximate the whole rock Th/U that would have been sourced these zircons to be in the range 1.3-2.7, assuming a partition of ~2 for Th/U in zircon vs. melt (Mahood and Hildreth, 1983). Unfortunately, the value is not particularly diagnostic of any single rock type. 4.3 Discussion Zircon geochronology for the Quad Creek rocks shows evidence of three different events, the precise geological order and context of which are problematic due to the deformational overprint and the limited information from beyond the mapped area. Based on what is known so far, the first event in its history was the establishment in the Paleoarchean of a sedimentary depocenter typified by psammitic quartzites rich in detrital placer minerals (chromite, minor sulfides), emplacement of the protolith to the amphibolite (as extrusive basalt?) and deposition of the banded iron-formation sometime around 3300 Ma. The stratigraphic context and order of these events remains unclear. Next appears to have been a period of deformation coinciding with metamorphism at the granulite facies at ~3200 Ma. Afterwards, crustal thinning saw the intrusion of mafic dikes. Subsequently, an intrusive trondjhemite/granodiorite was responsible for local partial melting of the sediments. Finally, a quartz and biotite leucogranitoid was emplaced in and around the supracrustal rocks. Pervasive hydrothermal alteration affected much of the area. Uplift was followed by denudation and erosion of younger rocks to expose the oldest core of the Beartooth Mountains. 34 CHAPTER 5 BEARTOOTH ZIRCONS 5.1 Zircon morphology Uncertainties between zircons of (primary) igneous or (secondary) metamorphic and hydrothermal origin befuddle age analysis, especially in ancient complexes. However, different petrogenetic origins of different zircon populations in the same rock can sometimes be reconciled on crystal chemical grounds (Th/U, REE partitioning, Ti-thermometry; e.g. Cates and Mojzsis, 2009) coupled with morphological analysis of grain habit and internal texture (Corfu et al., 2003). Igneous zircons tend to have relatively high aspect ratios and are euhedral; detrital zircons of whatever petrogenetic origin tend to be rounded from sedimentary transport, and metamorphic zircons can have a host of textures depending to a large degree on the host rock composition and degree of metamorphism. Zircons in this study which were collected from sedimentary protoliths tend to have subhedral habit with variable aspect ratios; some grew in situ during metamorphism, others have overgrowths on rounded cores, and still others show complex internal morphologies indicative of resorption/recrystallization (Figure 12). Back-scattered electron images of the oldest and most concordant zircons in this study (>3.6 Ga) show that they are subhedral and display clear overgrowths consistent with neoform zircon growth, remelting/resorption and/or recrystallization (Hay et al., 2010). For example, sample grain #1 of BT0611 (a.k.a. BT0611_1), as well as BT0608_6, and BT0610_5, are cracked with rounded tips, while one of the oldest zircons in our sample suite: BT0608_4 (3902 Ma) shows little to no fracturing with quite angular (sub-rounded) tips. One unusual grain (BT0608_4) is a penetration twin. 35 Figure 12. Back scattered electron micrographs of the Beartooth zircons showing <30% discordant (not labeled) and >30% discordant (labeled) zircons morphologies 36 Figure 12. Backscattered electron micrographs of the Beartooth zircons. 37 Figure 12. Back scatter electron micrographs of the Beartooth zircons. 38 Figure 12. Back scatter electron micrographs of the Beartooth Mountain zircons. 39 Gross morphological analysis in BSE and optical microscopy of some of the youngest demonstrably igneous age populations (from 3400-3500 Ma) shows that they are sub-rounded inclusion-rich grains with obvious rim overgrowths; zircons of this age population are generally within a few percent of concordia. The youngest igneous zircons from within the ~3300 Ma population tend to be euhedral with metamorphic rims, and are generally concordant. All zircons equal to or younger than ca. 3200 Ma are stubby crystals with some fracturing, and are rich in inclusions. A significant proportion of grains from this grouping are >30% discordant and show evidence for radiation damage in BSE images. The youngest metamorphic populations (27002900 Ma) show a wide variety of habit from highly rounded to broken with fracturing and overgrowths with highly variable concordance. Many of the zircons appear igneous, however, while images are informative, interpretations can be subjective and the suites of textures inferred as magmatic or igneous can be ambiguous (Corfu et al., 2003) Many of the younger grains appear to contain igneous morphology including oscillatory zoning. However, many studies have shown that zircon formed during high-temperature metamorphism can exhibit a wide range of morphologies and internal zoning features. Pre-existing zircon can be partially or completely transformed or changed in situ in response to interactions with fluids and melts. Harley et al., 2007 detail several metamorphic zircons displaying planar and sector zoning common in high temperature rocks and similar zoning patters found in zircon crystallized from partial melt. The new growth can be extensive forming on the older zircon including the core (Harley et al., 2007). For this reason, the zircons in this study have been largely classified as igneous or metamorphic based on their concordance and/or known metamorphic events or widespread intrusions. 5.2 Zircon chemistry 40 Chemically distinguishing the magmatic zircons from metamorphic zircons from sediments is problematic. Detrital zircon REEs show an extremely limited abundance range with no systematic difference in pattern shape and are not generally useful as indicators of provenance (Hoskin and Ireland, 2000). However, there have been some improvements through the use partition coefficients vs. ionic radius using a lattice-strain model, the results of which are said to be readily interpretable with magmatic exchange (Trail, 2007). This technique should be considered for further study on these samples. The Th/U values of the oldest zircons from the paragneiss and quartzites are comparable (Figure 11) and suggestive, though not conclusive of the same Hadean sediment source. Known metamorphic events occurred from 3200 Ma and younger, which may provide a reason for the greater discordance and therefore absence on the younger zircon populations in Figure 11. Likewise, the overall highly variable < 30% discordant zircons with inverse Th/U ratios (Figure 10) remains problematic unless separated out in terms of U-Pb age populations. The scatter in Th/U of the 3200 Ma age grouping is interpreted as representative of the first widespread metamorphic event that triggered zircon growth at the granulite facies (Mueller et al., 1998), a feature that has been widely documented for other ancient supracrustal rocks which have experienced similar protracted and high grade metamorphic histories (e.g. Manning et al., 2006). More conclusive provenance information may be obtained by study of the occurrence and chemistry of inclusions hosted by the detrital zircons and is a priority for future studies. 41 CHAPTER 6 EVIDENCE OF A SUPERCRATON; SCLAVIA? 6.1 Detritus from an ancient Continent Soon after their discovery, it was realized that the oldest ages for the Beartooth zircons were akin to those of the oldest tonalitic and granodioritic gneisses in the ca. 3960 Ma Acasta Gneiss Complex (AGC) ~2250 km to the north at the westernmost margin of the Slave craton, Northwest Territories, Canada (Bowring and Williams, 1999). The AGC comprises an exposure of strongly foliated and transposed amphibolitic to granitic gneisses that may be larger than 180 km2 or the known extent of ancient basement rock in the area (Iizuka et al., 2007 and references therein). The AGC probably represents the last unequilibrated remnant of a much larger block (Bleeker, 2003) that in principal could have been the source of widepread contamination by ancient zircons. Presently, the main exposure of the AGC is dominated by layered mafic gneisses and felsic gneisses with blocks of mafic- intermediate gneisses (Iizuka et al., 2007). The oldest components of the AGC were emplaced 4030 – 3940 Ma, but record evidence for possibly older rocks from a 4200 Ma inherited zircon (Iizuka et al., 2006). The initial AGC magmatic emplacement was followed by two Eoarchean igneous intrusions at 3740-3720 Ma and 36603590 Ma. The last of these events was accompanied by crustal anatexis and recrystallization of older units (Bowring and Housh, 1995; Iizuka et al., 2007). Subsequent intrusions and modifications of the AGC continued into the Proterozoic, culminating with regional metamorphism at 1900-1800 Ma related to the Wopmay Orogen (King, 1985; Sano et al., 1999). Many of these ages find complements in the zircon age spectra for Quad Creek. 6.1.2 Hadean detrital zircons in other Paleoarchean sedimentary basins in North America? 42 Approximately 1500 km to the northeast of the Beartooth Mountain locality other ~3900 Ma ages for detrital zircons have also been reported in the Assean Lake Complex (ALC). The ALC is a ~120 km2 west-northwest trending slice of largely Neo- to Mesoarchean rocks bounded to the north by the Paleoproterozoic Trans-Hudson Orogen and the Neoarchean Northwest Superior Province to the south (Böhm et al., 2000, 2003, 2007). Within the ALC there are at least two ca. 3200 Ma supracrustal packages of quartz-biotite schists (“meta-greywacke” s.l.) that yield Eoarchean and Hadean detrital zircons. These supracrustals were intruded by ~3100 Ma tonalites, which in turn were modified by later granitoids. Neodymium model ages of the oldest tonalites and various schists are all >3500 Ma, and some are as old as 4200 Ma, strongly indicative of reworked Hadean/Eoarchean crust. The Assean Lake rocks underwent multiple metamorphic episodes and migmatization. A solitary detrital 3940 Ma zircon was reported from a drill core in the Thelon Basin in Nunavut (Palmer et al., 2004), however, it is important to note the Thelon Basin is not a Paleoarchean sedimentary basin but younger (~1.7 Ga). Also, a fuchsitic quartzite of the Rae Province in northern Canada to the east of the AGC likewise contains old detrital zircons with age distrubution peaks at 3860, 3760, and 3720 Ma (Hartlaub et al., 2006). The fact that the dominant population of pre-3800 Ma ages for detrital zircons from ca. 3300 Ma Narryer Gneisses as tabulated in Holden et al. (2009) resembles the published ages of the oldest (igneous) components of the Acasta Gneiss Complex (ca. 4020-4050 Ma; e.g. Stern and Bleeker, 1998) is interesting, but lacks compelling linkage with any other rock outside of the terranes captured within the Yilgarn Craton. The long tail in the distribution of older ages reported in Holden et al. (2009) from the Jack Hills to 4380 Ma is so far unmatched by any other detrital zircon locality yet discovered (Nutman et al., 2001). Such a substantial difference in the (albeit 43 small number) age distributions between the Australian and North American Paleoarchean basin detrital zircon record maybe used to build the case that another different and very old continental core with diverse primordial crustal components (accreted terrane?) existed in the vicinity of what is now the Yilgarn block in Western Australia about 3300 million years ago. The oldest components of this particular source seem not to have affected Laurentian Paleoarchean sedimentary basins. Other clues about the ultimate origin(s) of the Quad Creek zircons may be found in the composition and population statistics of zircons from the ~2800 Ma Central Slave Cover Group (CSCG) in Canada (Sircombe et al., 2001). Quartzites of CSCG uncomformably overlay 30004000 Ma tonalitic to dioritic gneisses and foliated granites of the Central Slave Basement Complex (Bleeker et al., 1999; Pietranik et al., 2008). The oldest zircon noted from the CSCG comes from fuchsitic quartzite at Dwyer Lake and was dated at 3918±5 Ma (spot 65.1, 101% concordant, Th/U = 0.6 reported in Sircombe et al., 2001) (Figure 13). The remainder of the Dwyer Lake zircons range from highly discordant (66%) minimum ages of 2439 Ma to within 6% of concordia at 3885 Ma (Th/U=0.560) and 3901 Ma (Th/U=0.486). Like the Quad Creek zircons in this study, the oldest CSCG ages from Dwyer Lake are evocative of an ancient “Slave Continent” source (Bleeker 2003; Reddy and Evans, 2009). Comparisons with these studies and those of the Beartooth zircons including this, Trail, 2006 and Mueller et al., 1992, 1998 can be summed up in Figure 13. 44 Figure 13. Diagram of [Th/U]zircon vs. 207Pb/206Pb age from this work, Trail et al. (2007), Mueller et al. (1992), the average oldest, most concordant zircon from the Assean Lake Complex (Böhm et al. 2003), and the oldest most concordant zircon from the Central Slave Cover Group (Sircombe et al. 2001). Similar ages and Th/U ratios are evident for the oldest Quad Creek zircons in this study to those reported from other ancient detrital zircon localities. 45 6.2 Cratons and Supercratons Archean cratons are complex fragments of pre-2500 Ma crust with commonly genetically linked mantle. Supercratons, as defined by Bleeker (2003), are large ancestral landmasses of Archean age with a stabilized core that upon break up spawned several independently drifting cratons. Bleeker (2003) found a potential 35 cratons giving three possibilities of how these cratons may have originated and if they can be matched to one another. The first mechanism to explain their origin is that all 35 cratons came from a single supercontinent called Kenorland (Williams et al., 1991, Hoffman, 1992, 1997). The evidence for this comes from the fact that most cratons have rifted and margins and must have come from a greater older landmass. The second explanation of origin is that a limited set of somewhat independent late Archean supercratons formed that were similar in nature but with substantial differences between most of the preserved cratons. Finally, the last mechanism is that there were many more cratons than the 35 preserved today, but they were destroyed by erosion or crustal recycling. Bleeker (2003) considered that the most likely scenario is the second, where there are a few cratons, some of which share certain likenesses and can be related. The greatest evidence for a limited number of cratons lies in the knowledge that Archean Earth would have had twice enhanced heat production with a greater fraction of primordial heat budget being retained, and was most likely characterized by more dynamic mantle convection and smaller, faster, plates (Pollack, 1997). This would favor several independent supercratons, and smaller continental accumulations (Bleeker, 2003). Using this evidence, it can be supposed that several of the 35 known cratons formed from the breakup of the same supercraton. Of these cratons, numerous can be matched to three distinct ‘clans’ of cratons, a Slave-like clan, a Superior-like clan, and a Kaapvaal clan, each of these likely traces its ancestry to a different supercraton (Bleeker 2003). The main difference between these craton 46 groups are their relative cratonization ages. The Kaapvaal craton was cratonized around 3000 Ma (Moser et al., 2001), the majority of the Superior by 2680 and 2640 Ma (Card, 1990), and the Slave craton ca. 2600 to 2580 Ma (Davis et al., 2003). Other differences between these craton clans are the chronology and stratigraphy of Paleoproterozoic cover sequences and the emplacement of mafic dike swarms at different periods. The Superior craton is intruded by the Matachewan dike swarm at 2473 and 2446 Ma (Heaman, 1997) and draped with glaciogenic deposits ca. 2200 and 2400 Ma in the Huronian Supergroup (Young et al., 2001). The Slave on the other hand, does not contain mafic dikes around 2470-2440 Ma or 2200-2400 Ma glaciation events. The Kaapvaal craton is overlain by extensive Paleoproterozic cover sequences of basins (Transvaal and Hamersley) along with banded iron formation deposits and carbonate deposits (Cheney, 1996). These differences between the craton clans advocate an origin from different supercratons. Bleeker (2003) names three main supercratons that would have resulted in the Kaapvaal, Superior and Slave clans after their breakup, the Vaalbara (Kaapvaal), the Sclavia (Slave), and the Superia (Superior). For the purposes of this thesis the focus will be on the Slave clan and Sclavia supercraton 6.2.1 The Slave clan: Modern component of a Sclavia supercraton The Slave craton is characterized by an extensive Hadean to Mesoarchean basement complex including the oldest known whole rocks in the Acasta Gneiss Complex (Bowring and Williams 1999) discussed in section 6.1. Apart from minor regional differences, the overall stratigraphy and granitoid geochronology of the Slave is reasonably similar across the craton and can be characterized by a pre-2860 Ma basement covered by a thin sequence of banded iron formation and fuchsitic quartzites (Bleeker et al., 1999a,b), largely similar to those observed in the eastern 47 Beartooth Mountains. There is also pervasive 2730-2700 Ma basaltic volcanism across the basement complex with minor komatiitic rocks and some intercalations of rhyolite tuff (dated 2722-2700 Ma), and volcaniclastic rocks that have been reworked (Isachsen and Bowring, 1997). Furthermore, the Slave contains widespread tonalite-trondhjemite-granodiorite-type subvolcanic plumes and turbidite sedimentation (2670-2650 Ma) across the craton (Bleeker and Villeneuve, 1995; Isachsen and Bowring, 1994). There are abundant tonalite-granodiorite plutons that are 2630-2610 Ma (Davis and Bleeker, 1999), and 2605-2580 Ma evolved granites (two-mica granites). The Beartooth Mountains and overall Wyoming Province show a similar plume history with late Archean magmatism composed of high-Na tonalities and trondjemites to high-K granites from 2950-2550 Ma (Chamberlain et al., 2003). Similar to the Wyoming Province, there is little evidence of dike swarms in the Slave at ca. 2470-2410 Ma. The oldest known Paleoproterozoic mafic dikes in the craton (Malley swarm) have been dated at 2230 Ma (LeCheminant and van Breemen, 1994), with several other swarms dated between 2200-2000 Ma, suggesting that the breakup of the Sclavia supercraton was occurring during this time. The broad similarities between the Slave and Wyoming necessitate an explanation, being the modern components of an ancient supercraton provides a fitting rationalization if it is assumed that the modern Slave and Wyoming craton were once part of the same Slave clan. 6.3 Discussion There exists that possibility that the Wyoming craton contains sediments from the Slave craton and that the Slave and Wyoming cratons are the modern components of what was once the Slave clan and supercraton Sclavia. They both contain ancient detrital zircons of similar >3.9 Ga age and rock types including fuchsitic quartzites, thin banded iron formation and trondhjemtic 48 bodies. Other locations like the Assean Lake and Thelon Basin also contain ancient detritus and may be linked to the Slave, Wyoming, and therefore Sclavia as well. High quality paleomagnetic data sets on similar lithologic units across several cratons are needed to test for further potential correlation. 49 CHAPTER 7 SUMMARY Fine-scale geology and geochemistry of the eastern Beartooth Mountains provide a useful snapshot in miniature of the long and complex history of the Wyoming Craton. The Quad Creek rocks show evidence of three different events with the establishment of a sedimentary depo center and BIF deposit in the Paleoarchean, then a period of deformation coinciding with metamorphism at granulite facies at 3200 Ma, and later hydrothermal alteration, uplift, and subsequent denudation and erosion of the younger sediments allowing the oldest core of the Beartooth Mountains to be exposed. Detrital/xenocrystic zircons with Eoarchean to Hadean ages confirm that older crustal components of the Wyoming Craton existed in the vicinity of these quartzites and are comparable to the oldest components of the North American Craton. The geochemistry of the quartzites and paragneiss shows a general variability in Cr and Zr in the quartzites and elevated Cr and Zr in the paragneiss suggesting a mixed mafic and granitic source. This work bolsters the case for a genetic link between the Beartooth quartzites and the geology of the Western Slave Province as previously suggested by Mueller et al. (1992). The Thelon Basin in Nunavut (Palmer et al., 2004), Assean Lake Complex (Bohm et al., 2000) and Central Slave Cover Group (Sircombe et al., 2001) may have also been linked. Comparison of the most concordant grains (<5% discordant) of this study’s Th/U ratios to others from the Beartooth Mountains, Assean Lake Complex and Central Slave Cover Group and shows highly variable Th/U ratios around 3200 Ma, but similar ages and Th/U ratios for the oldest and most concordant grains from Quad Creek with the other localities mentioned in section 6.1.1 (Figure 13). 50 The new data from this study further lead to a better relation of the ages with the regional geology of this part of the Wyoming Craton and provide additional information towards correlating ancient terranes (Reddy and Evans, 2009). Paleoarchean sedimentary basins occupy much of the supracrustal inventory of southwestern Montana (e.g. Tobacco Root Mountains). The oldest sediment source(s) of the Beartooth quartzites may share a common origin with the supracrustals of the Assean Lake Complex (Manitoba), the Thelon Basin (Nunavut) and other quartzites on the Slave Craton also known to contain pre-3900 Ma zircons. If this hypothesis is correct, it would mean that some Paleoarchean basins of northern North America, like those captured as supracrustal enclaves in the Wyoming craton, were at one time in the vicinity of a larger “Slave continent,” Sclavia that shed Hadean and Eoarchean zircons into them. If the ca. 3960 Ma Acasta Gneisses are the core remnants of Sclavia ( Bleeker, 2003), it would be the logical source of this detritus. Future investigation should include the study of mineral inclusions in the zircons, high quality paleomagnetic data of the mafic dikes in the Wyoming craton, and the search for other Paleoarchean sedimentary basins in North America. 51 REFERENCES CITED Armstrong, R.L., 1981. 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