TIMING CONSTRAINTS AND SIGNIFICANCE OF PALEOPROTEROZOIC METAMORPHISM WITHIN THE PENOKEAN OROGEN, NORTHERN WISCONSIN AND MICHIGAN (USA) A thesis presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Master of Science Shellie R. Rose June 2004 This thesis entitled TIMING CONSTRAINTS AND SIGNIFICANCE OF PALEOPROTEROZOIC METAMORPHISM WITHIN THE PENOKEAN OROGEN, NORTHERN WISCONSIN AND MICHIGAN (USA) BY SHELLIE R. ROSE Has been approved for the Department of Geological Sciences and the College of Arts and Sciences by David Schneider Assistant Professor of Geological Sciences Leslie A. Flemming Dean, College of Arts and Sciences Rose, Shellie R. M.S. June 2004. Geological Sciences Timing constraints and significance of Paleoproterozoic metamorphism within the Penokean orogen, northern Wisconsin and Michigan (USA). (84 p.) Director of Thesis: David Schneider The Paleoproterozoic Penokean orogen of the Lake Superior region, records a dynamic and protracted tectonic history of terrane accretion and orogenic collapse. Metamorphic features, located within a narrow corridor of deformed supracrustal rocks, include gneiss domes, fault-bounded structural panels, and concentric metamorphic isograds (nodes). This investigation has led to constraining the timing of metamorphism by utilizing U-Th-Pb monazite geochronometric and Ar-Ar muscovite thermochronometric techniques from samples collected across the orogen. Metamorphic and cooling ages reveal peak Penokean (M1) metamorphism at ~1830 Ma, a weaker 1800 Ma (M2) thermal pulse, tectonic collapse, M3 metamorphism, and rapid unroofing at 1760 Ma related to Yavapai convergence. Tectonic extrusion of a mid-crustal block, and basement diapirism via a density inversion are suggested as the mechanism. South of the deformation corridor, Penokean and Yavapai events are overprinted by 1630 Ma Mazatzal and 1470 Ma Wolf River Batholith deformational and thermal events. Approved: David Schneider Assistant Professor of Geological Sciences ACKNOWLEDGMENTS I would like to express my sincerest thanks to my advisor, David Schneider for all of his academic, financial, and personal support. Although he may not realize it, Dave has taught me many lessons, both scholastically and personally. His perseverance to constantly challenge my abilities has created a more observant, open-minded, tougher individual than the meek spirit I was two years ago. I will carry the newfound knowledge he has given me for the rest of my life; as a result I have become a better person and scientist. A special thanks goes to Daniel Holm for being such a supportive and caring committee member. His astounding knowledge has provided countless insight into this project and sparked a whole new interest in the fascinating dynamics of Precambrian geology. Daniel’s encouragement for me to partake in this project, invaluable input, in addition to Dave’s guidance, have successfully brought me to the end of this journey. I am forever grateful. Last but not least, I would like to send my regards to Damian Nance who also contributed his time and efforts by serving on my committee, in addition to the National Science Foundation for financially supporting this project. Thanks to Mom, Dad, Nen, Rob, Ethan, and Joshua for their much needed support and humor at all times. Mare, thanks for your help in the field and labs, and for your friendship. Let’s party! 5 TABLE OF CONTENTS Page Abstract…………………………………………………………………………………3 Acknowledgments………………………………………………………………………4 List of Figures…………………………………………………………………………..6 List of Tables..…………………………………………………………………………. 7 Introduction……………………………………………………………………………..8 Geologic Setting………………………………………………………………………...11 Penokean Orogeny……………………………………………………………...11 Post Penokean Orogenic Events……………………………………………….. 16 Methodology…………………………………………………………………………....20 Monazite Geochronometry…………………………………………………….. 20 Muscovite Thermochronometry………………………………………………...27 Sampling Strategy………………………………………………………………30 Results…………………………………………………………………………………..32 Park Falls Panel…………………………………………………………………32 Republic Node…………………………………………………………………. 33 Peavy Node…………………………………………………………………….. 45 Niagara Fault Zone…………………………………………………………….. 49 Discussion………………………………………………………………………………52 Geochronometry and Thermochronometry……………………………………..52 Implications for the Development of the Gneiss Dome Corridor………………58 Conclusions……………………………………………………………………………..64 References Cited……………………………………………………………………….. 66 Appendix A: Electron Microprobe Chemical Compositions of Monazite…………….. 78 Appendix B: Sample Descriptions and Locations……………………………………... 83 6 LIST OF FIGURES Page 1. Geologic Map of Paleoproterozoic Laurentia……………………………………….9 2. Reconstructed Tectonic Map of the Lake Superior Region…………………………12 3. Modified geologic and thermochronologic map of Wisconsin and Michigan……... 14 4. Metamorphic map pattern of the Penokean orogen………………………………… 16 5. SEM/BSE images of monazite in thin section………………………………………23 6. X-ray elemental maps of sample MIST……………………………………………..25 7. Ar-Ar laser probe cooling ages of muscovite grains……………………………….. 29 8. Ion microprobe age results of sample 96-17, Park Falls terrane………………….…34 9. Ion microprobe age results of sample MIST, Republic node………………………. 39 10. EMPA age results of sample MIST, Republic node……………………………….41 11. Ion microprobe age results of sample CREP, Republic node……………………...43 12. Monazite elemental maps of sample CREP, Republic node…………………….…44 13. EMPA age results of sample CREP, Republic node…………………………….…46 14. Ion microprobe age results of sample PVD, Peavy node…………………………..47 15. EMPA age results of sample HRR, Peavy node…………………………………...48 16. X-ray elemental monazite maps of sample PG-03, Niagara fault zone……………50 17. EMPA age results of sample PG-03, Niagara fault zone…………………………..51 18. Temperature vs. Time graph of Penokean orogen…………………………………53 19. Suggested model for gneiss dome corridor formation……………………………..62 7 LIST OF TABLES Table Page 1. Ion microprobe Pb-Pb monazite isotopic data, metamorphic ages of Penokean orogen………………………………………………………………………….. 35 2. Ar-Ar laser probe muscovite data, cooling ages of Penokean orogen………………37 3. Summary of EMPA Th-U-total Pb monazite ages…………………………………. 42 4. Summary of 1900-1400 Ma geologic events in the southern Lake Superior Region, U.S.A………………………………………………………………………………. 64 8 INTRODUCTION Metamorphism is a complex process of pressure and temperature dynamics that can be heterogeneous in any given time and place during orogenesis (Zeitler, 1989). Presently, the determination of peak metamorphic conditions of a rock suite is difficult, especially when multiple thermal episodes are involved. For this reason, it is important to understand how particular metamorphic events are related to one another in space and time, and how the final product is represented in the observed metamorphic map pattern. Many Precambrian orogenic belts notably lack intact preservation of primary tectonic events due to subsequent thermal and deformational episodes that have overprinted the original signatures. By dating metamorphic and associated accessory minerals, it is possible to identify metamorphic and cooling episodes that occur across an ancient orogen with respect to crustal thickening and unroofing cycles. Moreover, well- constrained timing of metamorphism in fossil orogenic belts is important in understanding the fundamentals of tectonics, as this information can provide time and depth dimensions unobtainable in contemporary orogens (Murphy and Keppie, 2003). There are a variety of reasons as to why temperature and pressure change within the crust, but for the Lake Superior region of northern Wisconsin and Michigan, I propose as the cause a long-lived convergent margin and related multiple metamorphic events. The conventional explanation for the observed metamorphism in the Lake Superior region attributes it to the result of a single accretionary event that occurred as the Archean Superior craton collided with an island arc (Sims et al., 1989). This region, however, lies within a belt of successive Proterozoic accretionary zones that extend from 9 erior Sup L. THO Superior GFtz GLtz Wyoming P ? nY ATION RM FO DE Ga 5 ST 1.6 U sY CR Mv a M 0 NORTH 0 6 1 - E R P Mz F T O EN 300 km XT N E ER ST EA ? Figure 1. Simplified geologic map of crustal provinces which constitute Laurentia throughout the Paleoproterozoic. Five major tectonic events are exhibited here, including (shaded) the Trans-Hudson, Penokean, and Mojave orogenies, (2.0-1.8 Ga), Yavapai orogeny (1760-1700 Ma), and the Mazatzal orogeny (1660-1600 Ma). Note the parallelism of the accreted terranes, suggesting long-lived convergence along the southern margin (after Karlstrom et al., 2001). Black box = study area. Provinces: P = Penokean, THO = Trans-Hudson Orogeny, GFtz = Great Falls tectonic zone GLtz = Great Lakes tectonic zone, Mv = Mojave, nY = northern Yavapai, sY = southern Yavapai, Mz = Mazatzal. Wyoming and Superior terranes are Archean provinces. Mexico to Scandinavia (Karlstrom et al., 2001; Figure 1). The three major Paleoproterozoic accretionary episodes preserved in the North American mid-continent include the Penokean-Mojavian (1875-1835 Ma), Yavapai (~1760-1700 Ma), and Mazatzal (~1660-1600 Ma) orogenies, and have been suggested to be part of a series of accretionary events during the growth of Laurentia (Karlstrom et al., 2001). The purpose of this investigation is to document timing constraints on the complex metamorphic map pattern