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Re-Thinking the Laramide: Investigating the Role of Fluids In

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Re-Thinking the Laramide: Investigating the Role of Fluids In RE-THINKING THE LARAMIDE: INVESTIGATING THE ROLE OF FLUIDS IN PRODUCING SURFACE UPLIFT USING XENOLITH MINERALOGY AND GEOCHRONOLOGY By Lesley Ann Butcher B.A., Brown University, 2010 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Master of Science Department of Geological Sciences 2013 This thesis entitled: Re-thinking the Laramide: Investigating the role of fluids in producing surface uplift using xenolith mineralogy and geochronology written by Lesley Ann Butcher has been approved for the Department of Geological Sciences Dr. Kevin H. Mahan Dr. Craig H. Jones 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. iii Butcher, Lesley Ann (M.S., Geological Sciences) Re-thinking the Laramide: Investigating the role of fluids in producing surface uplift using xenolith mineralogy and geochronology Thesis directed by Dr. Kevin H. Mahan ABSTRACT High-temperature, high-pressure mineral assemblages preserved in much of the North American lithosphere owe their origins to Archean and Proterozoic tectonic processes. Whether subsequent mechanical, thermal, or chemical modification of ancient lithosphere affects overlying crust and the extent to which such processes contribute to anomalous deformation and topography in the interior of continents is poorly understood. This study addresses the occurrence and effects of hydration on continental crust in producing regionally elevated topography in the Colorado Plateau since the Late Cretaceous. Mineralogical characteristics of two deep crustal xenoliths (GR-11 and RM-21) from the Four Corners Volcanic field record varying degrees of hydrous alteration including extensive replacement of garnet by hornblende, secondary albite and phengite growth at the expense of primary plagioclase, and secondary monazite growth in association with fluid-related allanite and plagioclase breakdown. Results from forward petrological modeling for both deep crustal xenoliths are consistent with hydration at 20 km depth prior to exhumation in the ~20 Ma volcanic host. In situ Th/Pb dating provides evidence for a finite period of fluid-related monazite crystallization in xenolith RM-21 from 91 ± 2.8 Ma to 58 ± 4 Ma, concurrent with timing estimates of low-angle subduction of the Farallon slab. Hydration-related reactions at depth lead to a net density decrease as low-density hydrous phases (hbl±ab±phg) grow at the expense of high-density, anhydrous minerals (gt±pl) abundant iv in unaltered Proterozoic crust. If these reactions are sufficiently pervasive and widespread, reductions in lower crustal density would provide a significant and quantifiable source of lithospheric buoyancy. Calculations for density decreases associated with extensive hydration recorded in xenolith GR-11 for an ~25 km thick crustal layer yield uplift estimates on the order of hundreds of meters associated with phase changes at depth. The results of this study substantiate the hypothesis that chemical alteration of lower continental crust by slab-derived fluids played a role in producing Laramide-related surface uplift of the Colorado Plateau and establishes chemical modification of continental lithosphere as a credible possibility for producing elevated regional topography in continental interiors. v ACKNOWLEDGEMENTS I would first of all like to thank Kevin Mahan for giving me the opportunity and encouragement to complete this work, for his help in keeping me focused on the task at hand, for his constructive criticism along the way, and for his enthusiasm towards his students and the scientific process. Thanks also to my other committee members: Lang Farmer for his keen interest in science and Craig Jones for his ability to keep me an honest and always critical researcher. I would like to thank Julien Allaz for his ongoing patience and assistance in negotiating the electron microprobe and for his role as an enthusiastic and encouraging independent study advisor and thanks to Paul Boni for his help and expertise in the rock lab. I’d also like to acknowledge Joanne Brunetti, Barbara Easter, Marcia Kelly and Susan Pryor for their enduring efforts to keep me on track, organized, and sane. Thank you! Additionally, I would like to extend gratitude to Rita Economos and Axel K. Schmitt at the W.M. Keck Center for Isotope Geochemistry at the University of California, Los Angeles for their expertise and tireless patience in helping me navigate my way as a geochronologist on the ion microprobe. I am deeply grateful for the my hugely intelligent and witty office mates Justin Ball, Melissa Bernardino, Danny Feucht, Will Levandowski, Colin O’Rourke, Will Yeck, and Dan Zietlow. Their presence, support, and constant banter made me smile and carry on even in the most frustrating of times. Lastly, thank you to my family, Molly, Brian, Della and Eugene Butcher, and to Ellen Wilcox and David Thuline for your tireless help, support, and encouragement. vi CONTENTS ABSTRACT ....................................................................................................................... iii ACKNOWLEDGEMENTS .................................................................................................v INTRODUCTION ...............................................................................................................1 GEOLOGIC HISTORY AND XENOLITH BACKGROUND ...........................................4 Geologic history of western North America ............................................................4 Geologic history of the Navajo Volcanic Field, Colorado Plateau ..........................7 Results of prior xenolith studies ..............................................................................8 ANALYTICAL METHODS .............................................................................................15 QEMSCAN mineralogical mapping ....................................................................15 Electron microprobe ..............................................................................................16 Petrological modeling ............................................................................................16 Monazite geochronology .......................................................................................17 SEM analysis .........................................................................................................18 RESULTS ..........................................................................................................................20 Sample descriptions ...............................................................................................20 Pressure-temperature histories...............................................................................26 Monazite geochronology .......................................................................................31 Elevation calculations ............................................................................................36 DISCUSSION ....................................................................................................................40 Mineralogy ............................................................................................................40 Pressure-temperature histories...............................................................................42 Monazite geochronology .......................................................................................43 The case for the Farallon slab ................................................................................47 Elevation and density changes ..............................................................................52 Timing of uplift .....................................................................................................53 CONCLUSIONS................................................................................................................59 REFERENCES ..................................................................................................................60 vii TABLES Table 1. Petrologic models used in pseudosection analysis ..................................17 Table 2. Mineral compositions ..............................................................................21 Table 3. Bulk compositions ...................................................................................22 Table 4. Monazite age data ....................................................................................33 viii FIGURES Figure 1. Map of hydrated xenolith localities .........................................................7 Figure 2. Simplified modal mineralogy maps of RM-21 and GR-11 ...................15 Figure 3. X-ray maps of Garnet 1 (RM-21) ..........................................................23 Figure 4. X-ray maps of Garnet 2 (RM-21) ..........................................................24 Figure 5. Graphs of elemental zoning patterns in garnet (RM-21) .......................25 Figure 6. Pseudosection for RM-21 ......................................................................27 Figure 7. Peak assemblage pseudosection for RM-21 ..........................................29 Figure 8. Secondary assemblage pseudosection for RM-21 .................................30 Figure 9. BSE images of analyzed monazites .......................................................32
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