Bucknell University Bucknell Digital Commons Honors Theses Student Theses 2016 Do Proterozoic Metamorphic Rocks in Northern New Mexico Record One Metamorphic Event At 1.4 Ga, or Two Overprinting Events At 1.65 GA and 1.4 GA Sara Vivienne Stotter Bucknell University, [email protected] Follow this and additional works at: https://digitalcommons.bucknell.edu/honors_theses Recommended Citation Stotter, Sara Vivienne, "Do Proterozoic Metamorphic Rocks in Northern New Mexico Record One Metamorphic Event At 1.4 Ga, or Two Overprinting Events At 1.65 GA and 1.4 GA" (2016). Honors Theses. 355. https://digitalcommons.bucknell.edu/honors_theses/355 This Honors Thesis is brought to you for free and open access by the Student Theses at Bucknell Digital Commons. It has been accepted for inclusion in Honors Theses by an authorized administrator of Bucknell Digital Commons. For more information, please contact [email protected]. iv ACKNOWLEDGEMENTS I would first like to thank my thesis advisor, Dr. Chris Daniel, for his unwavering guidance, encouragement, patience, and support throughout the course of my research project. Sophomore year, when I first started doing work with Dr. Daniel, I never imagined that I would accomplish and learn so much, and I am truly grateful for his insight and mentoring. He always pushed me to do my best, and continually challenged me to think critically which allowed me to reach my goals and excel beyond them. I would also like to thank Dr. Mary Beth Gray for her kind words and endless motivation that kept me going even in the most stressful of times. Similarly, I want to extend thanks to all of the geology faculty, as they helped further cultivate my passion for the geosciences, and helped me grow both as a student and a person over the course of my undergraduate career. I would like to give a special thanks to Brad Jordan and Carliee Dill for organizing trips as well as assisting me with gear and the equipment at Bucknell University. I would also like to thank Chris Macfarlane and his staff at the LA ICP-MS lab at the University of New Brunswick for helping me in conducting geochronological analyses as well as assisting me in data processing. I am furthermore especially grateful for my peers and fellow seniors, Alex Mackay, Izzy Bristol, and Rae Donofrio for spending long nights in the O’Leary computer lab with me and for their relentless and unwavering support. I would also like to give special thanks to Kim Nagotko for mentoring me and guiding me throughout the entire honors thesis process. Also to my fellow “Geopals”, I can’t imagine where I would be today without your constant support v and encouragement. As stressful as it may have been at times, I will miss our late night second home in the O’Leary computer lab, and I will always remember the countless laughs and memories that we shared there. Finally, I would like to thank my family for supporting me through all of these years, especially during my time at Bucknell. To my mom, you are one of the strongest people I know, and you kept me sane and kept me going even through the most difficult times. To my brother Alex, thanks for being my source of comic relief, as well as for the late night phone calls when I needed a break after working consecutive hours on my research. Lastly, I would like to thank my Dad for cultivating a love for the natural world in me at a young age that eventually led me to pursue geology. I know that he would be extremely proud of me and my work. Support for this project is funded by the National Science Foundation grant EAR – 1250220. vi TABLE OF CONTENTS ACKNOWLEDGEMENTS …………………………………………………………... iv TABLE OF CONTENTS ……………………………………………………………... vi LIST OF TABLES ………………………………………………………………...…. viii LIST OF FIGURES …………………………………………………………………... ix ABSTRACT ……………………………………………………………………………. 1 INTRODUCTION ……………………………………………………………………... 3 GEOLOGIC BACKGROUND ……………………………………………………….. 6 Proterozoic geology of the southwestern United States ……………………….... 6 Regional geologic setting of the Taos Range and Picuris Mountains …………. 10 METHODS ……………………………………………………………………………. 15 Sample collection ………………………………………………………………. 15 Monazite U-Pb geochronology methods ……………………............................. 15 RESULTS …………………………………………………………………………....... 21 Picuris Mountains in-situ U-Pb monazite geochronology and textural analysis...22 Marqueñas Formation ............................................................................. 22 Piedra Lumbre Formation ....................................................................... 22 Pilar Formation ....................................................................................... 26 Vadito Formation ..................................................................................... 26 Taos Range in-situ U-Pb monazite geochronology and textural analysis ........... 27 SS15-01 Aluminous quartzite ................................................................... 27 vii SS15-03 Sillimanite k-spar bearing gneiss .............................................. 27 SS15-08 Sillimanite k-spar bearing gneiss .............................................. 30 DISCUSSION ................................................................................................................. 30 Monazite geochronology …………………………............................................. 31 Comparison of monazite ages ………………..................................................... 34 Implications ………………………………………............................................. 42 SUMMARY AND CONCLUSION .............................................................................. 45 REFERENCES .............................................................................................................. 47 viii LIST OF TABLES Table 1. Picuris Mountains sample locations ......................................................... 16 Table 2. Taos Range sample locations ................................................................... 17 Table 3. Picuris U-Pb age summaries ………………………………...…………. 23 Table 4. Taos U-Pb age summaries ……………………………………………... 28 ix LIST OF FIGURES Figure 1. Proposed metamorphic models for the region ........................................... 5 Figure 2. Simplified geologic map of the southwestern U.S. ................................... 7 Figure 3. Simplified regional provinces map............................................................. 8 Figure 4a. Simplified geologic map of the Taos Range ............................................ 11 Figure 4b. Simplified geologic map of the Taos Range inset ................................... 12 Figure 5. Simplified geologic map of the Picuris Mountains ................................. 14 Figure 6. Outcrop photographs from the Taos Range and Picuris Mountains …... 18 Figure 7. Back scatter electron photomicrographs from the Picuris Mountains .... 19 Figure 8. Back scatter electron photomicrographs from the Taos Range ………...20 Figure 9. Concordia diagram and weighted average plot Picuris Mountains …… 24 Figure 9 cont. Concordia diagram and weighted average plot Picuris Mountains …... 25 Figure 10. Concordia diagram and weighted average plot Taos Range .................. 29 Figure 11. Daniel and Pyle (2006) concordia diagram ……………….................... 32 Figure 12: Compiled data concordia diagram this study …………………………. 35 Figure 13: Proposed P-T path for the Picuris Mountains and Taos Range ……….. 36 Figure 14: Compiled data location map …………………………………………... 37 Figure 15: Weighted average comparison diagram ……………………………….. 43 Figure 16: Probability distribution diagram ……………………………………..... 44 1 ABSTRACT In-situ LA-ICPMS U-Pb monazite geochronology was used to determine the timing of regional metamorphism in the Picuris Mountains and northern Taos range, New Mexico. Monazite ages from aluminous quartzite and sillimanite-kspar bearing gneisses in Cedro Canyon, northern Taos Range, yield intercept ages of ca. 1380 Ma. Subhedral to anhedral monazite range in size between 10 µm and 50 µm, are typically aligned parallel/sub-parallel to the foliation, and generally occur along mineral grain boundaries; a few grains occur as inclusions within Fe-Ti oxides. Backscatter electron imaging revealed zoning in several monazite grains, but little to no compositional zoning is exhibited in the analyzed grains. Monazite do not appear to preserve any record of ca. 1650 Ma near-granulite facies metamorphism as previously proposed for this region. Monazite in the Picuris Mountains, northern New Mexico, yield LA-ICPMS, U- Pb intercept growth ages from a variety of bulk compositions: a metarhyolite clast from the Marqueñas Fm. (1386 ± 11 Ma), a micaceous quartzite from the underlying Vadito Fm. (1362 ± 3 Ma), a garnet-biotite-staurolite schist from the Piedra Lumbre Fm. (1357 ± 6 Ma), and a metatuff layer from the Pilar Fm. (1359 ± 19 Ma). The majority of monazite grains are euhedral though some irregular boundaries are apparent due to the partial inclusion of matrix grains. Backscatter electron imaging showed little to no compositional zoning within the grains. Monazite in all samples are generally aligned parallel to the dominant regional foliation, but some clearly overgrow the foliation at a high angle and are interpreted as post-tectonic. 2 Monazite record no evidence of an older, 1.65 Ga metamorphic and deformational event as proposed by previous studies. Younger, ca. 1430-1360 Ma monazite across northern New Mexico are interpreted to reflect a single regional high temperature metamorphic event and/or pervasive