Louisiana State University LSU Digital Commons LSU Master's Theses Graduate School 2016 Metamorphic Conditions of Aluminous Gneisses in the Sawtooth Metamorphic Complex, Idaho, USA: Implications for the Middle-Lower Crust Eleanor Wesley-Anne Smith Louisiana State University and Agricultural and Mechanical College, [email protected] Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_theses Part of the Earth Sciences Commons Recommended Citation Smith, Eleanor Wesley-Anne, "Metamorphic Conditions of Aluminous Gneisses in the Sawtooth Metamorphic Complex, Idaho, USA: Implications for the Middle-Lower Crust" (2016). LSU Master's Theses. 3964. https://digitalcommons.lsu.edu/gradschool_theses/3964 This Thesis is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Master's Theses by an authorized graduate school editor of LSU Digital Commons. For more information, please contact [email protected]. METAMORPHIC CONDITIONS OF ALUMINOUS GNEISSES IN THE SAWTOOTH METAMORPHIC COMPLEX, IDAHO, USA: IMPLICATIONS FOR THE MIDDLE- LOWER CRUST A Thesis Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the of the requirements for the degree of Master of Science in The Department of Geology and Geophysics by Eleanor Smith B.S., Louisiana State University, 2015 May 2016 ACKNOWLEDGMENTS I would like to thank my committee members Dr. Barbara Dutrow, Dr. Darrell Henry, and Dr. Karen Luttrell for sharing their invaluable expertise with me during my graduate career and for their time, patience, and encouragement as I completed this project. I would especially like to thank Dr. Dutrow providing me challenging research opportunities early in my career, for her constant motivation, and for teaching me the skills necessary to succeed in my future endeavors. I owe my successes during my undergraduate and graduate careers to her dedicated guidance. I would also like to thank Mr. Rick Young in the LSU rock lab for creating my thin sections, Dr. Nele Muttik for assistance with the electron microprobe, and Ashley Thrower for help collecting samples in the field. Sample collection was made possible through the National Forest Service by David Fluetsch and Lieze Dean of the Sawtooth National Recreation Area. Funding was provided by NSF-Tectonics #1145073 awarded to Drs. Barbara, Dutrow, Paul Mueller, and David Foster and a Rocky Mountain Section Geological Society of America Undergraduate Research Grant awarded to me. I would also like to thank the LSU Department of Geology and Geophysics for awarding me several undergraduate scholarships and a graduate teaching assistantship. ii TABLE OF CONTENTS ACKNOWLEDGMENTS………………..……………………………………………….ii ABSTRACT………………………………………………………………………...……iv INTRODUCTION………………………………………………..……………………….1 Regional geologic Setting…………….………………………………..……………….…4 Regional basement exposures…..……….........………………...…………...……….…..12 METHODS………………………….……………………………………….……..……17 Petrographic Analysis…………………………………….....………………..……...…..17 Electron microprobe analysis…………………….……..…………………………...…...18 Geothermobarometry…………………….………………………………………......…..19 Whole rock geochemical analysis…………………………………………….…….....…21 Mineral assemblage diagrams…………………………..………….…………...………..21 RESULTS…………………………………………………………………………….….24 Petrographic analysis…………….…………………….…….……………...…….....…..29 Whole rock geochemical analysis…………..…...……………………….…......……..…35 Mineral Chemistry……………….………………………………….…………...……....41 P-T determination…..…………………………………….………………………...…....51 Mineral assemblage diagrams…………………………………………….……...………57 DISCUSSION………………………………………………………………….…….......66 Metamorphic pathway of SMC aluminous gneisses……...……..………...….………….69 Protolith of SMC aluminous gneisses……………………….….…..…………...…...…..84 Tectonic setting of SMC aluminous gneisses……………………………………………88 CONCLUSIONS…………………………………………………………….….….……90 REFERENCES…………………………………………………………………….....….92 APPENDICES……………………………………………………………………….....100 APPENDIX A: EMPA ANALYSES-GARNET……………..………………………...100 APPENDIX B: EMPA ANALYSES-PLAGIOCLASE……………..……………..…..111 APPENDIX C: EMPA ANALYSES-BIOTITE……………..………………………....124 APPENDIX D: ELECTRON BACKSCATTERED IMAGES WITH EMPA ANALYSIS POINTS………………………………………………………………………...144 VITA…………………………………………………………………………….……...158 iii ABSTRACT The Sawtooth Complex (SMC) of central Idaho contains metasedimentary units that elucidate the pressure-temperature conditions and potentially, the evolution of Precambrian crust in the northwestern United States. Petrographic analysis, whole rock geochemistry, and geothermobarometry combined with thermodynamic phase equilibrium modeling record a regional metamorphic pathway for SMC aluminous gneisses characterized by burial to middle-lower crustal levels with at least two deformational events, followed by a retrograde overprint. P-T conditions are generally consistent with the peak assemblage of SMC aluminous gneisses of bt + grt + sil + ilm + zrn + mnz ± pl ± qtz ± kfs ± ap ± xn ± py ± po ± ccp ± gr. This assemblage constrains P-T conditions to be below the biotite dehydration-melting reaction, within sillimanite zone, and above the thermal stability of muscovite. Abundant leucosomes suggest melt. Peak conditions are associated with two deformational events evidenced by crenulation cleavage and rotated inclusions in garnet. Classic geothermobarometry combined with thermodynamic phase equilibrium modeling constrain peak pressure conditions. These pressures decrease from ~11 kbars in the south to ~7.5 kbars in the north at similar temperatures of ~800 °C. Mineral assemblage diagrams are used to identify likely post-peak reequilibration features and used to determine retrograde conditions. SMC samples share a similar retrograde path to conditions near ~4-6 kbars and ~600-650 °C into the stability field of muscovite. iv Peak pressures from south the north suggest burial depths between 33 and 23 km (3 km /kbar). This range in burial depth indicates that the SMC records different crustal levels that may have been juxtaposed by faulting. Whole rock geochemical data is consistent with a mature shale protolith having undergone significant weathering of feldspar to clay minerals with varying amounts of potassium metasomatism and sedimentary sorting. These shales were likely deposited in a passive margin environment and may represent a deep-water continuation of calc- silicates found adjacent to aluminous gneisses. P-T conditions of SMC aluminous gneisses are consistent with burial to middle crust with subsequent uplift. P-T conditions of SMC aluminous gneisses may elucidate metamorphic conditions along the southwestern margin of the Lauretian craton during the development of Rodinia. v INTRODUCTION The North American continent offers one of the most complete geologic records of continental formation. It resided at the center of both the Nuna and Rodinia supercontinents, offering invaluable insight into the supercontinent cycle (e.g. Whitmeyer and Karlstrom, 2007). The geologic core of North America, the Laurentian craton, formed through a series of plate collisions between Archean microcontinents, 2.0-1.8 Ga, known as the Trans-Hudson Orogeny (e.g. Hoffman, 1988; Whitmeyer and Karlstrom, 2007). These microcontinents include mostly reworked Archean crust but also entrap slivers of juvenile volcanic arcs at suture zones (e.g. Mogk et al., 1992; Foster et al., 2006). Following the formation of the Archean-Paleoproterozoic core of North America, accretion of juvenile volcanic arcs and oceanic terranes built the growing continent from its southern margins during the Yavapai (1.71-1.68 Ga), Mazatzal (1.70-1.65 Ga), and Grenville orogenies (1.30-0.95 Ga; e.g. Whitmeyer and Karlstrom, 2007). Middle-lower crustal rocks are key to understanding the accretion of North American Archean-Proterozoic terranes and the associated orogenic events that created the Laurentian Craton. Lack of outcrops of Precambrian middle-to-lower crustal rocks renders the tectonic reconstructions of Laurentia largely difficult to constrain. In the Idaho area, the Idaho Batholith and volcanic rocks from the Yellowstone hotspot produced during Cretaceous-Tertiary magmatic activity cover large expanses of Precambrian basement rock (Mueller et al., 2005; Gaschnig et al., 2011). Also masking Precambrian rocks in this region are thick sedimentary units such the Windermere Supergroup (> 780 Ma) and the 1.47-1.44 Ga Belt-Purcell Supergroup (Fig. 1; e.g. Lund et al., 2003; Mueller et al., 2005; Foster et al., 2006). 1 Within this enigmatic region, in the Sawtooth Range in central Idaho, lies the Sawtooth Metamorphic Complex (SMC). The SMC contains high-grade metamorphic rocks suspended as a roof pendant within the Idaho and Sawtooth batholiths (Anderson, 1995; Dutrow at al., 1995; Metz, 2010; Bergeron, 2012; Fukai, 2013). The SMC is located at the intersection of Archean-Proterozoic terranes i.e. the >2.5-1.5 Ga Priest River block, the 2.6-3.3 Ga Medicine Hat block, the >2.5 Ga Wyoming craton, and the >2.5 Ga Grouse Creek block (Fig. 1; e.g., Hoffman, 1988; Mogk et al., 1992; Foster et al., 2006) and occur within the proposed 2.4-1.6 Ga Selway terrane (Foster et al., 2006). Characterization of SMC lithologies and metamorphic conditions may elucidate a relationship between the SMC and the surrounding terranes. Limited studies on various SMC lithologies have yielded upper greenschist to lower-granulite facies conditions but the relationship