AGE AND TECTONIC EVOLUTION OF THE AMDO BASEMENT: IMPLICATIONS FOR DEVELOPMENT OF THE TIBETAN PLATEAU AND GONDWANA PALEOGEOGRAPHY

Transcription

1 AGE AND TECTONIC EVOLUTION OF THE AMDO BASEMENT: IMPLICATIONS FOR DEVELOPMENT OF THE TIBETAN PLATEAU AND GONDWANA PALEOGEOGRAPHY by Jerome Hamilton Guynn A Dissertation Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY In the Graduate College THE UNIVERSITY OF ARIZONA 2006

2 2 THE UNIVERSITY OF ARIZONA GRADUATE COLLEGE As members of the Dissertation Committee, we certify that we have read the dissertation prepared by Jerome Guynn entitled Age and Tectonic Evolution of the Amdo Basement: Implications for Development of the Tibetan Plateau and Gondwana Paleogeography and recommend that it be accepted as fulfilling the dissertation requirement for the Degree of Doctor of Philosophy Date: 11/20/06 Dr. Paul Kapp Date: 11/20/06 Dr. Pete DeCelles Date: 11/20/06 Dr. George Zandt Date: 11/20/06 Dr. Mihai Ducea Date: 11/20/06 Dr. Clem Chase Final approval and acceptance of this dissertation is contingent upon the candidate s submission of the final copies of the dissertation to the Graduate College. I hereby certify that I have read this dissertation prepared under my direction and recommend that it be accepted as fulfilling the dissertation requirement. Date: 11/20/06 Dissertation Director: Dr. Paul Kapp

3 3 STATEMENT BY AUTHOR This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be grated by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. Signed: Jerome H. Guynn

4 4 ACKNOWLEDGMENTS Many, many people have supported me throughout the last seven years as I switched careers from engineering to geology. First and foremost is my mom, Trish Casper, who wholly supported my decision to go back to school and has always been there to talk to during bumps along the way. My sister, Sierra, and I have been in graduate school about the same time as me and so she has been a great resource and a wonderful support during this time. She also was the first to suggest I go back to school in geology after the great trip we had to New Zealand. My friend, Mimi Ashcraft, has always wanted me to pursue geology, ever since I earned the geology merit badge with her in high school. She is always interested in hearing about my research and has been highly supportive. I want to give a big thank you to my advisor, Paul Kapp, who has provided me with opportunities, advice, constructive criticism and encouragement throughout the time I have earned my Ph.D. He has always been available for discussing my research and helping me through problems and I have learned a great deal about doing science from him. I have also learned a lot from Pete DeCelles, in classes, in the field and in discussions about Himalayan and Tibetan geology. George Gehrels has been a great help and I am indebted to him for his time and assistance with U-Pb geochronology, which has formed a large part of my research. I would also like to thank my other committee members: George Zandt, Mihai Ducea and Clem Chase. I also want to thank Matt Heizler of New Mexico Tech for his 40 Ar/ 39 Ar analyses. Finally, I thank Ding Lin of the Chinese Academy of Sciences in Beijing for his support with our field work, including permits, colleagues and travel arrangements. A special thanks goes to Alex Pullen and Ross Waldrip who assisted me in the field in Tibet. John Volkmer, Shundong He and Ross Waldrip have been terrific graduate student colleagues and friends. Other fellow graduate students and undergraduates who have provided assistance and discussion include Joel Saylor, Dave Pearson, Jen Fox, Jen McGraw, Kelley Stair and Jen Pullen. Ken Dominik of the Lunar and Planetary Laboratory provided invaluable assistance with the electron microprobe. My roommates for the past year and a half, Kevin Anchukaitis and Tm Shanahan, have been a great support as we have all struggled through finishing our Ph.D. I will miss the commiserating and comradery that we have shared over many beers and burgers at Gentle Ben s the past year. Thanks to Katie Davis for our many conversations and the good advice she always has. I also have a long list of Arizona friends who have been very supportive and a great social network for escaping the craziness of grad school. Many thanks to Aly, Andy, Anna, Britt, Dave, Jessica R., Jessica C., Kevin J., Lynette, Scott, Toby, Tom, Megan, Lara, Tashana, Jana and Morgen.

5 5 DEDICATION This dissertation is dedicated to my mom, Trish Hamilton Casper; my sister, Dr. Sierra Guynn; and my friend, Mimi Ashcraft.

6 6 TABLE OF CONTENTS ABSTRACT...12 INTRODUCTION...13 PRESENT STUDY...17 Timing of Bangong suture tectonics as revealed by the Amdo basement...17 Metamorphism of the Amdo basement...18 Jurassic and Cretaceous evolution of the Bangong suture...19 Geochronology of the Amdo gneisses and paleogeography of the Lhasa and Qiangtang terranes...20 REFERENCES CITED...21 APPENDIX A: Permission for Reproduction from the Geological Society of America...25 APPENDIX A: Tibetan basement rocks near Amdo reveal missing Mesozoic tectonism along the Bangong suture, central Tibet...27 ABSTRACT...28 INTRODUCTION...29 GEOLOGY...31 GEOCHRONOLOGY...32 HISTORY OF METAMORPHISM AND COOLING...33 DISCUSSION AND CONCLUSIONS...35 ACKNOWLEDGEMENTS...37 REFERENCES CITED...37

7 7 TABLE OF CONTENTS - Continued FIGURE CAPTIONS...42 FIGURES...45 Data Repository Item DR Supplementary Geochronologic and Thermochronologic Data...49 Summary of analytical methods for zircon and titanite U-Pb analysis (University of Arizona)...49 Summary of analytical methods for mica 40 Ar/ 39 Ar analyses (University of California, Los Angeles) Summary of analytical methods for hornblende and feldspar 40 Ar/ 39 Ar analysis (New Mexico Geochronological Research Laboratory)...50 K-Feldspar multi-domain diffusion modeling methods...51 References Cited...52 Figures and Tables...54 APPENDIX B: Metamorphism and exhumation of the Amdo basement, Tibet: Implications for the Jurassic tectonics of the Bangong Suture zone...74 ABSTRACT...75 INTRODUCTION...76 REGIONAL GEOLOGY AND PREVIOUS WORK...80 Basement Rocks...82 THERMOBAROMETRY...84 Garnet-kyanite schist thermobarometry...86

8 8 TABLE OF CONTENTS - Continued Mafic amphibolite thermobarometry...88 Thermometry of non-garnet bearing amphibolites...94 Samples without thermobarometry...95 U-Pb METAMORPHIC GEOCHRONOLOGY...97 DISCUSSION Thermobarometric results Tectonic implications ACKNOWLEDGEMENTS REFERENCES FIGURE CAPTIONS FIGURES TABLES APPENDIX C: Jurassic and Cretaceous Tectonic Evolution of the Bangong Suture Zone near Amdo, central Tibet ABSTRACT INTRODUCTION REGIONAL GEOLOGY GEOCHRONOLOGY Igneous Zircon Analysis Detrital Zircon Analysis Jurassic flysch...158

9 9 TABLE OF CONTENTS - Continued Red arenites and shale GEOLOGY OF THE AMDO AREA Rock Types and Ages Structure of the Amdo Basement Region DISCUSSION Jurassic Tectonic Evolution of the Bangong Suture at Amdo Cretaceous Tectonic Evolution of the Bangong Suture at Amdo CONCLUSIONS ACKNOWLEDGMENTS REFERENCES CITED FIGURE CAPTIONS FIGURES TABLES APPENDIX D: U-Pb geochronology of basement rocks in central Tibetan and paleogeographic implications Abstract Introduction Regional Geology and Paleozoic Paleogeography Amdo Basement Geology U-Pb Geochronology Methods...235

10 10 TABLE OF CONTENTS - Continued 4.2. Igneous Zircon Analysis JG JG PK A JG JG PK B JG PK A JG Detrital Zircon Analysis JG JG AP A Discussion Tibetan Paleozoic Detrital Zircon Signature Regional Detrital Zircon Record Regional Basement Ages Implications for Paleogeography of the Lhasa-Qiangtang terrane Conclusions Acknowledgements...267

11 11 TABLE OF CONTENTS - Continued Appendix A.1 Description of Rock Samples A.1.1. JG orthogneiss A.1.2. JG orthogneiss A.1.3. PK A orthogneiss A.1.4. JG orthogneiss A.1.5. JG orthogneiss A.1.6. PK B orthogneiss A.1.7. JG orthogneiss A.1.8. PK A orthogneiss A.1.9. JG orthogneiss A JG paragneiss A JG quartzite A AP A quartzite References Figure Captions Figures Tables...309

12 12 ABSTRACT The elucidation of the geologic processes that led to the creation of the Tibetan Plateau, a large area of thick crust and high elevation, is a fundamental question in geology. This study provides new data and insight on the geologic history of central Tibet in the Jurassic and Cretaceous, prior to the Indo-Asian collision, as well as the Gondwanan history of the Lhasa and Qiangtang terranes of the plateau. This investigation is centered on the Bangong suture zone near the town of Amdo and I present new geochronology, thermochronology, thermobarometry and structural data of the Amdo basement, an exposure of high-grade gneisses and intrusive granitoids. Using a range of thermochronometers, I show there were two periods of cooling, one in the Middle-Late Jurassic after high-grade metamorphism and a second in the Early Cretaceous. I attribute Middle-Late Jurassic metamorphism, magmatism, and initial cooling of the Amdo basement to arc related tectonism that resulted in tectonic or sedimentary burial of the magmatic arc. I propose that a second period of cooling, nonmarine, clastic sediment deposition and thrust faulting in the Early Cretaceous is related to the Lhasa-Qiangtang collision. The thermochronology reveals limited denudation between the Cretaceous and the present, indicating the existence of thickened crust when India collided with Asia in the early Tertiary. U-Pb geochronology of the orthogneisses and detrital zircon geochronology of metasedimentary rocks suggests that the Lhasa and Qiangtang terrane were located farther west along Gondwanan s northern margin than most reconstructions depict.

13 13 INTRODUCTION The Tibetan Plateau is the largest and, on average, highest orogenic feature on the earth, with an area of approximately 5,000,000 km 2 and an average elevation of 5 km (Fielding et al., 1994). The events and processes that led to its creation are an area of ongoing research. A variety of end-member models have been proposed for the formation of the plateau due to India s collision with Asia, including distributed shortening (Dewey and Burke, 1973), underthrusting of Indian lithosphere (Argand, 1924; Powell and Conaghan, 1973), continental injection (Zhao and Morgan, 1985), crustal flow (Royden et al., 1997), and oblique continental subduction and sedimentary basin infilling (Meyer et al., 1998; Tapponnier et al., 2001). However, all these models assume that Tibet was near sea-level at the time of the Indo-Asian collision in the early Tertiary and that the plateau is solely due to Tertiary tectonics. In addition, many of these models are based on processes that may be currently active on Tibet s margins but may not have contributed to it s earlier growth (Kapp et al., 2003a). While much of the research concerning the geology of the plateau has focused on the time period since the Indo-Asian collision in the Early Tertiary, much less work has been done on the prior geologic history of Tibet and the influence of that previous tectonism on its development. Some recent geologic investigations of Tibet s Mesozoic history have suggested that prior deformation played an important part in forming the plateau. These studies have focused on the Lhasa and Qiangtang terranes which make up the central and southern region and collided along the Bangong suture in the Early Cretaceous (Guynn et al., 2006; Kapp et al., 2003a; Yin and Harrison, 2000). Murphy et al. (1997) documented a

14 14 Cretaceous thrust belt in the central Lhasa terrane near the town of Coqen and suggested a km thick crust in the region during the early Tertiary. Further Cretaceous and early Tertiary shortening was documented just north of Coqen and showed that shortening was decoupled between the upper (Cretaceous) and lower (late Paleozoic) sedimentary rocks (Volkmer et al., accepted). Several studies have revealed thrust belts in the northern Lhasa terrane along the Bangong suture that were active in the Cretaceous and Early Tertiary (Kapp et al., 2003a; Kapp et al., 2005). These results imply a large part of the Lhasa terrane has been thrust underneath the Qiangtang terrane and may be the cause of a regional anticline in the central Qiangtang terrane (Kapp et al., 2003b; Yin and Harrison, 2000). Finally, field studies have documented the presence of an Andean-style retro-arc thrust belt and associated foreland basin in the southern Lhasa terrane in the Late Cretaceous and early Tertiary (Kapp et al., submitted; Leier et al., in press). Collectively these studies indicate significant shortening and resultant crustal thickening throughout the southern Qiangtang and Lhasa terranes during the Cretaceous and early Tertiary, right up until the Indo-Asian collision. The southward propagating thrust belts of the northern and southern Lhasa terrane are thought to be a result of the Lhasa-Qiangtang collision. At the western end of the suture, this collision has been documented to have occurred during the Early Cretaceous, just prior to thrust belt development (Kapp et al., 2003a; Matte et al., 1996). In the east around the town of Amdo, however, the collision is thought to have occurred in the Late Jurassic based on ophiolite obduction and some limited thermochronology (Dewey et al., 1988; Girardeau et al., 1984; Xu et al., 1985). Resolving the time of collision is

15 15 important for understanding thrust belt development and for relating deformation due to the Lhasa-Qiangtang collision to tectonics associated with the Indo-Asian collision. The Lhasa-Qiangtang collision is a poorly understood event and the Bangong suture zone has several enigmatic aspects, including a lack of arc-related igneous rocks despite subduction of the Meso-Tethys Ocean (Allégre et al., 1984; Dewey et al., 1988) and the occurrence of ophiolite fragments up to 200 km across parts of the suture zone (Girardeau et al., 1984; Matte et al., 1996). Better defining the tectonic history of the suture zone could help our knowledge of accretionary processes. In order to understand the evolution of the suture zone and the Cretaceous tectonics of the Tibetan Plateau, I studied an area along the suture zone referred to as the Amdo basement or Amdo gneiss. The Amdo basement is the only exposure of crystalline basement along the suture zone, which makes it an ideal location for several geologic techniques, including geochronology, thermochronology and thermobarometry. The gneisses within the exposure have experience high-grade metamorphism that indicates a major tectonic event (Harris et al., 1988), but that metamorphism has been attributed to Cambrian tectonics and not the Jurassic-Cretaceous Lhasa-Qiangtang collision (Coward et al., 1988; Dewey et al., 1988; Xu et al., 1985). The Amdo basement is also unique because it is the only exposure of Precambrian, crystalline basement within the central part of the Tibetan Plateau (Dewey et al., 1988; Yin and Harrison, 2000). Thus it provides an opportunity to study the deeper crust of the plateau and to constrain the older geologic history of the Lhasa and Qiangtang terranes during and prior to formation of Gondwana in the late Neoproterozoic. Despite its importance, the Amdo basement has

16 16 received only minor attention in past studies of the Tibetan Plateau (Coward et al., 1988; Harris et al., 1988; Xu et al., 1985).

17 17 PRESENT STUDY The research presented in this dissertation is an addition to our understanding of the geologic history of the Tibetan Plateau prior to the Indo-Asian collision. Specifically, I address several issues related to the Lhasa and Qiangtang terranes of Tibet, including the tectonic evolution of the Bangong suture zone, timing and extent of shortening and denudation related to the Lhasa-Qiangtang collision, geochronology of the gneisses that comprise the bulk of the Amdo basement exposure and the composition and location of the Lhasa and Qiangtang terranes prior to their rifting from Gondwana. The methods, results and conclusions of this study are presented in the papers appended to this dissertation and the following is a summary of the methods used and the most important results. Timing of Bangong suture tectonics as revealed by the Amdo basement I applied a wide variety of thermochronometers to elucidate the cooling history of the Amdo basement: U-Pb for zircon and titanite and 40 Ar/ 39 Ar for hornblende, mica and K- Feldspar. In addition, I performed extensive dating of the granitoids that intrude the Amdo gneisses using U-Pb analysis on zircon. These results, together with my initial mapping, are presented in Appendix A. This initial work revealed two distinct periods of cooling for the Amdo basement; one during the Middle-Early Jurassic ( Ma) and another during the Early Cretaceous ( Ma). Furthermore, titanite U-Pb ages and new zircon growth revealed by U-Pb analyses show that the high-grade metamorphism occurred during the Middle-Jurassic and not the Cambrian. In addition, the ages of the

18 18 intruding granitoids are Middle-Early Jurassic, coeval with the metamorphism and initial cooling, and the only documented igneous rocks along the suture that could be related to subduction of the Meso-Tethys Ocean along the Qiangtang margin. Together, these results demonstrate a major period of tectonism during the Middle-Early Jurassic which I suggest is related to the development of a continental arc along the southern margin of the Qiangtang terrane. I further propose that this tectonism resulted in burial or underthrusting of the arc as an explanation for the lack of subduction related igneous rocks along the suture zone. Finally, I propose that the second period of cooling represents the time of Lhasa-Qiangtang collision, similar to the timing of collision to the west, and this resulted in thrusting of the basement over unmetamorphosed sedimentary rocks and the deformation of Cretaceous (?) nonmarine sedimentary rocks. The lowtemperature thermochronology of the K-Feldspar 40Ar/39Ar analysis implies that there was limited denudation (< 5 km) since the mid-cretaceous. Metamorphism of the Amdo basement In Appendix B, I present a detailed investigation of the timing and degree of metamorphism of the Amdo basement. Thermobarometry and mineral assemblages reveal that the basement experienced peak temperatures of C and pressures of kbar. These conditions were experienced by the entire ~50 km x 50 km exposure of gneisses that I mapped, which places an important constraint on the exhumation of the basement rocks. U-Pb ages of metamorphic zircon growth show that peak conditions occurred Ma. Metasedimentary rocks along the southern edge of the basement

19 19 record slightly lower conditions, around 600 C and 8 kbar, indicating that the highergrade gneisses were thrust over them. The lithologies and the metamorphic conditions are similar to those seen in the Coast Plutonic Complexes of the northwestern U.S. and western Canadian Cordillera, which provides additional support for an arc-related tectonic environment. Jurassic and Cretaceous evolution of the Bangong suture I spent two summers mapping and taking structural measurements of the Amdo basement and deformed sedimentary rocks just south of the basement. I also collected and analyzed several sedimentary rocks for detrital zircon analysis to determine provenance and to provide constraints on depositional ages. The results of this work and the development of a tectonic model for the Bangong suture zone in the late Mesozoic are the subject of Appendix C. The detrital zircon record reveals marine sedimentation on the southern margin of the Qiangtang terrane through the Jurassic, followed by nonmarine deposition of coarse, clastic sediments in the mid-cretaceous. Structural measurements demonstrate southward directed thrusting within the Amdo basement, including the Jurassic granitoids, as well as thrusts placing basement over lower-grade metasedimentary rocks and metamorphic rocks over sedimentary rocks. I dated a granite pluton (~117 Ma) that is cut by a thrust fault and another intrusion (~106 Ma) that cuts a thrust fault. These demonstrate shortening in the Early Cretaceous, coeval with or just following the period of cooling in the Amdo basement revealed by K-Feldspar 40 Ar/ 39 Ar analysis ( Ma). A simplified cross-section of the region suggests ~100 km of

20 20 shortening at a rate typical of many fold and thrust belts. The data suggest that the Amdo basement region was the hinterland to the Cretaceous, southward propagating thrust belts of the northern Lhasa terrane and lend more support to the deposition of Aptian-Albian limestones and clastic sediments in a foreland basin rather than in a rift environment. Geochronology of the Amdo gneisses and paleogeography of the Lhasa and Qiangtang terranes I performed extensive U-Pb geochronology of Amdo orthogneisses and detrital zircon geochronology of several metasedimentary rocks associated with the orthogneisses. This work and its implications are presented in Appendix D. Geochronology reveals two periods of granitoid emplacement for the orthogneiss protoliths, one in the Cambrian- Ordovician (~ Ma) and one in the early Neoproterozoic (~ Ma). Detrital zircon (DZ) geochronology of two quartzites indicates that they were deposited in the Paleozoic, possibly the Carboniferous-Permian based on the similarity of their DZ spectra to DZ spectra from Carboniferous-Permian Tibetan sandstones. A paragneiss was probably deposited in the Neoproterozoic based on its DZ spectrum. Comparison of the DZ signature of Tibetan Paleozoic rocks, which were deposited prior to rifting of the Lhasa and Qiangtang terranes from Gondwana, to those of Paleozoic sedimentary rocks from the Nepalese Himalaya reveal some distinct differences. Comparing these signatures to others from Gondwana, I suggest that the Lhasa and Qiangtang terranes were not directly north of India as depicted in most reconstructions, but were located farther to the west, closer to northwest India and eastern Africa.

21 21 REFERENCES CITED Allégre, C.J., Courtillot, V., Tapponnier, P., Hirn, A., Mattauer, M., Coulon, C., Jaeger, J.J., Achache, J., Scharer, U., Marcoux, J., Burg, J.P., Girardeau, J., Armijo, R., Gariepy, C., Gopel, C., Li, T.D., Xiao, X.C., Chang, C.F., Li, G.Q., Lin, B.Y., Teng, J.W., Wang, N.W., Chen, G.M., Han, T.L., Wang, X.B., Den, W.M., Sheng, H.B., Cao, Y.G., Zhou, J., Qiu, H.R., Bao, P.S., Wang, S.C., Wang, B.X., Zhou, Y.X., and Ronghua, X., 1984, Structure and Evolution of the Himalaya- Tibet Orogenic Belt: Nature, v. 307, p Argand, E., 1924, La tectonique de L'Asie: Proceedings of the 13th International Geologic Conference, p Coward, M.P., Kidd, W.S.F., Yun, P., Shackleton, R.M., and Hu, Z., 1988, The Structure of the 1985 Tibet Geotraverse, Lhasa to Golmud: Philosophical Transactions of the Royal Society of London Series A-Mathematical Physical and Engineering Sciences, v. 327, p Dewey, J.F., and Burke, K.C.A., 1973, Tibetan, Variscan, and Precambrian Basement Reactivation - Products of Continental Collision: Journal of Geology, v. 81, p Dewey, J.F., Shackleton, R.M., Chang, C.F., and Sun, Y.Y., 1988, The Tectonic Evolution of the Tibetan Plateau: Philosophical Transactions of the Royal Society of London Series A-Mathematical Physical and Engineering Sciences, v. 327, p

22 22 Fielding, E., Isacks, B., Barazangi, M., and Duncan, C., 1994, How flat is Tibet?: Geology, v. 22, p Girardeau, J., Marcoux, J., Allégre, C.J., Bassoullet, J.P., Tang, Y.K., Xiao, X.C., Zao, Y.G., and Wang, X.B., 1984, Tectonic environment and geodynamic significance of the Neo-Cimmerian Donqiao ophiolite, Bangong-Nujiang suture zone, Tibet: Nature, v. 307, p Guynn, J.H., Kapp, P., Pullen, A., Heizler, M., Gehrels, G., and Ding, L., 2006, Tibetan basement rocks near Amdo reveal "missing" Mesozoic tectonism along the Bangong suture, central Tibet: Geology, v. 34, p Harris, N.B.W., Holland, T.J.B., and Tindle, A.G., 1988, Metamorphic Rocks of the 1985 Tibet Geotraverse, Lhasa to Golmud: Philosophical Transactions of the Royal Society of London Series A-Mathematical Physical and Engineering Sciences, v. 327, p Kapp, P., DeCelles, P.G., Leier, A., Fabijanic, J.M., He, S., Pullen, A., and Gehrels, G., submitted, The Gangdese Retroarc Thrust Belt Revealed: Geological Society of America Bulletin. Kapp, P., Murphy, M.A., Yin, A., Harrison, T.M., Ding, L., and Guo, J.H., 2003a, Mesozoic and Cenozoic tectonic evolution of the Shiquanhe area of western Tibet: Tectonics, v. 22, p. doi: /2002tc Kapp, P., Yin, A., Harrison, T.M., and Ding, L., 2005, Cretaceous-Tertiary shortening, basin development, and volcanism in central Tibet: Geological Society of America Bulletin, v. 117, p

23 23 Kapp, P., Yin, A., Manning, C.E., Harrison, T.M., Taylor, M.H., and Ding, L., 2003b, Tectonic evolution of the early Mesozoic blueschist-bearing Qiangtang metamorphic belt, central Tibet: Tectonics, v. 22(4), p. doi: /2002tc Leier, A., DeCelles, P.G., Kapp, P., and Ding, L., in press, The Takena Formation of the Lhasa terrane, southern Tibet: The record of a Late Cretaceous retroarc foreland basin: Geological Society of America Bulletin. Matte, P., Tapponnier, P., Arnaud, N., Bourjot, L., Avouac, J.P., Vidal, P., Liu, Q., Pan, Y., and Wang, Y., 1996, Tectonics of Western Tibet, between the Tarim and the Indus: Earth and Planetary Science Letters, v. 142, p Meyer, B., Tapponnier, P., Bourjot, L., Metivier, F., Gaudemer, Y., Peltzer, G., Shunmin, G., and Zhitai, C., 1998, Crustal thickening in Gansu-Qinghai, lithospheric mantle subduction, and oblique, strike-slip controlled growth of the Tibet plateau: Geophysical Journal International, v. 135, p Murphy, M.A., Yin, A., Harrison, T.M., Durr, S.B., Chen, Z., Ryerson, F.J., Kidd, W.S.F., Wang, X., and Zhou, X., 1997, Did the Indo-Asian collision alone create the Tibetan plateau?: Geology, v. 25, p Powell, C.M.A., and Conaghan, P.J., 1973, Plate Tectonics and Himalayas: Earth and Planetary Science Letters, v. 20, p Royden, L.H., Burchfiel, B.C., King, R.W., Wang, E., Chen, Z., Shen, F., and Liu, Y., 1997, Surface Deformation and Lower Crustal Flow in Eastern Tibet: Science, v. 276, p

24 24 Tapponnier, P., Zhiqin, X., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G., and Jingsui, Y., 2001, Oblique Stepwise Rise and Growth of the Tibet Plateau: Science, v. 294, p Volkmer, J., Kapp, P., Guynn, J., and Lai, Q., accepted, Cretaceous-Tertiary structural evolution of the north-central Lhasa terrane, Tibet: Tectonics. Xu, R.H., Schärer, U., and Allégre, C.J., 1985, Magmatism and metamorphism in the Lhasa block (Tibet): A geochronological study: Journal of Geology, v. 93, p Yin, A., and Harrison, T.M., 2000, Geologic evolution of the Himalayan-Tibetan orogen: Annual Review of Earth and Planetary Science, v. 28, p Zhao, W.L., and Morgan, W.J., 1985, Uplift of Tibetan Plateau: Tectonics, v. 4, p

25 25 APPENDIX A: Permission for Reproduction from Geological Society of America The following discourse gives permission for the reproduction of the Geology article Tibetan basement rocks near Amdo reveal missing Mesozoic tectonism along the Bangong suture, central Tibet by J.H. Guynn, P. Kapp, A. Pullen, M. Heizler, G. Gehrels and L. Ding published in June, 2006 by the Geological Society of America (GSA), as part of this dissertation. discourse is an acceptable form of permission for UMI s parent company Proquest. The original request for permission is included in the response and permission letter. Subject: RE: Geology article permissions From: "Jeanette Hammann" <jhammann@geosociety.org> Date: Wed, 4 Oct :10: To: <jhguynn@ .arizona.edu> Dear Mr. Guynn, Thank you for your message. Permission is granted for your use of the article in your dissertation as you describe below. Best regards, Jeanette Jeanette Hammann GSA Editorial Manager P.O. Box Penrose Place Boulder, CO (303) fax jhammann@geosociety.org -----Original Message----- From: Jerome Guynn [mailto:jhguynn@ .arizona.edu] Sent: Tuesday, October 03, :38 PM To: Editorial - Internet Mailbox Subject: Geology article permissions To whom it may concern: I am a PhD student who will be graduating this semester and I would like to include a Geology article that was published in June, I am the first author of the article, "Tibetan basement rocks near Amdo reveal "missing" Mesozoic tectonism along the Bangong suture, central Tibet", and the work presented in this paper is a significant portion of my PhD. The University of Arizona publishes its theses through University

26 26 Microfilms Incorporated (UMI) and they will accept published papers as part of the appendix of my dissertation, but I must include a permission form from GSA to include the material. UMI needs to be able to sell, on demand, single copies of the dissertation, including the published paper, for scholarly purposes. Is it possible to get such permission from GSA? Sincerely, Jerome Guynn -- PhD Candidate Department of Geosciences Gould-Simpson Bldg. #77 University of Arizona jguynn@geo.arizona.edu Approximate truth is the only truth attainable, but at least one must strive for that, and not wade off into arbitrary falsehood. George Eliot

27 27 APPENDIX A: Tibetan basement rocks near Amdo reveal missing Mesozoic tectonism along the Bangong suture, central Tibet Reprint from Geology

28 28 Tibetan basement rocks near Amdo reveal missing Mesozoic tectonism along the Bangong suture, central Tibet Jerome H. Guynn Paul Kapp, Alex Pullen, Department of Geosciences, University of Arizona, Tucson, Arizona, 85721, USA Matthew Heizler, New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico, 87801, USA George Gehrels, Department of Geosciences, University of Arizona, Tucson, Arizona, 85721, USA Lin Ding Institute of Tibetan Plateau Research and Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing China ABSTRACT U-Pb and 40 Ar/ 39 Ar studies of a unique exposure of crystalline basement along the Jurassic - Early Cretaceous Bangong suture of central Tibet reveal previously unrecognized records of Mesozoic metamorphism, magmatism, and exhumation. The basement includes Cambrian and older orthogneisses that underwent amphibolite facies

29 29 metamorphism coeval with extensive granitoid emplacement at Ma. The basement cooled to ~300 C by 165 Ma and was exhumed to upper crustal levels in the hanging wall of a south-directed thrust system during Early Cretaceous time. We attribute Jurassic metamorphism and magmatism to the development of a continental arc during Bangong Ocean subduction and Early Cretaceous exhumation to northward continental underthrusting of the Lhasa terrane beneath the Qiangtang terrane. We speculate that a Jurassic arc extended regionally along the length of the Bangong suture but in all other places in Tibet has been buried, either depositionally or structurally, beneath supracrustal assemblages. Keywords: Tibet, Bangong suture, terrane accretion, continental arcs, continental collision INTRODUCTION While it is widely assumed that the high elevation and thick crust of Tibet are largely a consequence of the Cenozoic Indo-Asian collision, the importance of older tectonism in building the Tibetan plateau and influencing its subsequent development must be considered (e.g., Yin and Harrison, 2000). Of particular relevance is the Jurassic - Cretaceous collision between the Lhasa and Qiangtang terranes along the Bangong suture in central Tibet (Fig. 1). Southward obduction of ophiolitic fragments onto the northern margin of the Lhasa terrane during Middle to Late Jurassic time is generally taken to mark the cessation of north-dipping oceanic subduction beneath the southern Qiangtang terrane and the onset of Lhasa-Qiangtang collision (Girardeau et al., 1984;

30 30 Smith and Xu, 1988; Leeder et al., 1988; Zhou et al., 1997). The apparent absence of a Jurassic arc and major mid-mesozoic tectonism along the Bangong suture has contributed to the notion that Bangong Ocean closure and subsequent Lhasa-Qiangtang collision were relatively insignificant events in the development of central Tibet (e.g., Coward et al., 1988; Dewey et al., 1988; Schneider et al., 2003). In contrast, thick accumulations of northerly-derived Lower Cretaceous clastic strata in the northern Lhasa terrane (Leeder et al., 1988; Leier et al., 2004; Zhang, 2004) together with Early Cretaceous growth of an enormous, east-west trending structural culmination in the central Qiangtang terrane (Fig. 1) (Kapp et al., 2003, 2005) have been attributed to large-magnitude northward underthrusting of the Lhasa terrane during Lhasa-Qiangtang collision. Furthermore, extensive mid-mesozoic magmatism and exhumation have been documented along a possible extension of the Bangong suture zone in the Pamirs (Rushan-Pshart zone; Schwab et al., 2004). In an attempt to better constrain the tectonic evolution of central Tibet, we conducted geologic mapping and U-Pb and 40 Ar/ 39 Ar thermochronologic studies on unique exposures of orthogneisses and cross-cutting granitoids located along the Bangong suture near Amdo (Fig. 1). The Amdo basement is the only established exposure of pre-mesozoic crystalline basement rock within the interior of Tibet. Earlier studies showed that the ~100 km long by ~50 km wide basement exposure consists of amphibolite-facies orthogneisses and subordinate metasedimentary rocks intruded by undeformed granitoids (Xu et al., 1985; Harris et al., 1988b; Kidd et al., 1988; Coward et al., 1988). Conventional ID-TIMS U-Pb dating of zircon and titanite fractions from one orthogneiss sample yielded discordant

31 31 ages for both minerals which were combined into a single discordia (Xu et al., 1985). The upper intercept age (531 ± 14 Ma) was taken to represent the crystallization age of the granitoid protolith, while the lower intercept age (171 ± 6 Ma) was interpreted to mark the timing of low-grade metamorphism due to Lhasa-Qiangtang collision; high-grade metamorphism was assumed to be related to the Cambrian magmatic emplacement (Xu et al., 1985; Coward et al., 1988). Younger, intrusive granitoids (Harris et al., 1988a) were interpreted to be Early Cretaceous based on discordant U-Pb zircon ages ( Ma) from one sample (Xu et al., 1985). GEOLOGY The Amdo basement is predominately composed of strongly foliated orthogneisses which contain small but abundant mafic amphibolite gneiss pods with the assemblage: amphibole + plagioclase + quartz + ilmenite ± biotite ± garnet. There are also sporadic outcrops of sillimanite ± garnet ± K-feldspar paragneisses and migmatites. The presence of sillimanite and K-feldspar, as well as the migmatites, are suggestive of upper amphibolite facies metamorphic conditions. Metasedimentary rocks are exposed to the south of the orthogneiss (Fig. 2) and include marble, quartz-mica schist, phyllite, quartzite, and garnet-kyanite schist. The basement was intruded by widespread, generally undeformed granitoids (Fig. 2) of variable composition (granite, monzonite, granodiorite, quartz-syenite and diorite). The western boundary of the Amdo basement is a west-dipping normal fault and the northwestern boundary is a left-lateral strike-slip fault (Fig. 2), both of which cut

32 32 Quaternary deposits (Coward et al., 1988; Kidd et al., 1988). The Mesozoic tectonic contacts of the northern and western exposures of the Amdo basement are buried beneath Neogene-Quaternary basin fill. Some of the granitoids, particularly in the south, have meters to tens of meters wide north-dipping shear zones that display mylonitic fabrics and bookshelf microfaulting of feldspar-phenocrysts showing a top-to-the-south sense-ofshear. The gneisses and metasedimentary rocks along the southern margin are in the hanging walls of north dipping thrust fault zones with Jurassic marine shales and turbiditic sandstones and Cretaceous (?) red beds and conglomerates in the footwall. GEOCHRONOLOGY We dated eight granitoid samples and one orthogneiss sample using U-Pb laserablation multicollector inductively coupled plasma-mass spectrometry (LA-MC-ICPMS) analyses on zircon (see footnote 1). The crystallization ages reported in this study are based on weighted averages of concordant, clustered 206 Pb*/ 238 U ages of individual zircons because low 207 Pb concentrations in young (< 1000 Ma) granitoids result in large uncertainties in the 207 Pb*/ 235 U and 207 Pb*/ 206 Pb* ages. The assigned uncertainties (2σ) on the ages include all known random and systematic errors. The eight granitoid samples define a relatively narrow range of mid-jurassic ages from ~185 Ma to ~170 Ma (Table 1; Table DR1; Fig. DR1a-h) 1. The zircons from the orthogneiss sample (PK A) show signs of young zircon growth due to metamorphic processes. Zircon resorption can be seen in cathodeluminescence images as bright, irregular zones only a few microns in thickness on the

33 33 edges of the crystals (Fig. 3a). The wide range of discordant zircon ages (Fig. 3b; Table DR1) 1 is interpreted to be due to the laser beam ablating a mix of Precambrian cores and the younger rims. The younger ages correlate well with lower Th/U ratios (Fig. 3c), a typical indication of zircon (re)crystallization in equilibrium with a metamorphic fluid (Mojzsis and Harrison, 2002). The lower intercept of a discordia through the points indicates a Mesozoic age for zircon resorption. The interpreted crystallization age of 852 ± 18 Ma (Fig. 3b) provides the first direct documentation of Precambrian basement in central Tibet. HISTORY OF METAMORPHISM AND COOLING We interpret the discordant analyses and metamorphic zircon rims in sample PK A to indicate that the amphibolite facies metamorphism of the Amdo basement is Mesozoic. This interpretation is consistent with results of 40 Ar/ 39 Ar analysis of hornblende and U-Pb analysis of titanite from orthogneiss samples. Hornblende sample PK A provides generally monotonically increasing apparent ages from ~160 Ma to ~187 Ma with an integrated age of ~180 Ma, while hornblende sample PK A yields an integrated age of ~175 Ma (Fig. DR2; Table DR2). 1 We interpret these results to indicate that the Amdo gneisses cooled to below the bulk closure temperature for hornblende (500 ± 50 C; McDougall and Harrison, 1999) at 180 ± 5 Ma. U-Pb analysis on titanite from one of these samples (PK A) provides a mean 206 Pb*/ 238 U age of ± 4.2 Ma (Fig. DR1j; Table DR1) 1, which is statistically indistinguishable from the integrated 40 Ar/ 39 Ar hornblende age from the same sample.

34 34 Given the higher closure temperature for titanite (~ C; see Frost et al., 2000 and references therein), this may indicate rapid cooling of Amdo basement at ~180 Ma. Alternatively, the titanite may have crystallized at conditions below the closure temperature, possibly as low as 500 C (Frost et al., 2000). The lower temperature cooling history of Amdo basement rocks is inferred from 40 Ar/ 39 Ar thermochronologic studies on mica and K-feldspar from the orthogneiss samples (Fig. 2 for location) and the combined results are shown in Figure 4. Two biotite samples (PK A and PK A) and one muscovite sample (PK ) yield complexly varying apparent ages over the last ~80% cumulative 39 Ar released, but all fall within the 165 ± 5 Ma age range (Fig. DR3; Table DR3) 1, indicating regional cooling to below ~300 C at this time. The K-feldspar samples have complex spectra with age gradients that appear to be due largely to excess argon contamination (Fig. DR4) 1. Isochron analysis was used to estimate initial 40 Ar/ 36 Ar trapped compositions (Fig. DR5) 1 which were subsequently used to calculate what we refer to as isochron corrected age spectra. These isochron corrected age spectra were used to extract thermal histories using the multi-diffusion domain model (Fig. DR6-8). 1 Samples PK A and PK A show cooling from ~300 C to ~100 C between 135 and 115 Ma, while PK shows an older and slower episode of cooling from 155 to 125 Ma (Fig. 4). The difference in the timing of cooling initiation could be due to internal disruption of the gneiss by Early Cretaceous faults or shear zones that were not recognized in the field. Alternatively, PK displays an intermediate hump in the age spectra (both corrected and uncorrected) that

35 35 could be indicative of low-temperature recrystallization, which would negate the significance of the calculated thermal history (Lovera et al., 2002). It is important to note that the absolute temperatures of the thermal histories are strongly dependent on the choice of the activation energy used, which is estimated from the step heating experiments, and may be systematically shifted to lower or higher values. This is the most probable explanation for the difference between timing of the 300 C isotherm as determined by K-feldspar and biotite for sample PK A. However, the form of the cooling histories is not affected by the choice of activation energy. DISCUSSION AND CONCLUSIONS Our results show that upper amphibolite-facies metamorphism of the Amdo basement was the result of a tectonothermal event during the Early-Middle Jurassic and not the Cambrian as previously inferred. Metamorphism and subsequent cooling of the Amdo basement to ~500 C were coeval with emplacement of extensive granitoids between 185 and 170 Ma. These granitoids are the only known igneous rocks along the Bangong suture in Tibet with ages that overlap with the timing of Bangong Ocean subduction. Jurassic magmatism and exhumation have also been documented in the Pamirs along and to the north of the Rushan-Pshart suture zone, a likely westward extension of the Bangong suture zone that has been offset by the right-lateral Karakoram fault (Schwab et al., 2004). As has been suggested for the Pamir (Schwab et al., 2004), we attribute Early - Middle Jurassic metamorphism and magmatism to the development of a continental arc along the southern Qiangtang terrane due to north-dipping subduction

36 36 of oceanic lithosphere. The opening and subsequent closing of a back-arc oceanic basin during arc development (Fig. 5a-b) could explain the presence of ophiolitic fragments north of the Amdo gneiss (Fig. 1) as well as extensive Early - Middle Jurassic marine sedimentation in the southern Qiangtang terrane and Bangong suture zone, which is apparently lacking in the Lhasa terrane (Leeder et al., 1988; Yin et al., 1988; Schneider et al., 2003). The Amdo basement and superimposed arc remained in the mid-crust until relatively rapid exhumation to upper crustal levels during the Early Cretaceous, most probably due to a south-directed thrust system (Fig. 5c-d). This exhumation was coeval with growth of the large antiformal structural culmination in the central Qiangtang terrane (Kapp et al., 2005) and the accumulation of thick, northerly-derived, clastic deposits in the Lhasa terrane (Leeder et al., 1988; Leier et al., 2004; Zhang, 2004) and is therefore attributed to Lhasa-Qiangtang continental collision. Continued northward underthrusting of the Lhasa terrane led to a southward propagation of upper crustal deformation into the northern Lhasa terrane during the Late Cretaceous, with the magnitude of regional shortening exceeding 40% (Murphy et al., 1997; Kapp et al., 2003) and leading to significant crustal thickening in central Tibet (Fig. 5d). As our K-feldspar thermochronologic results indicate minimal denudation along the Bangong suture since the Early Cretaceous, this thick crust would have persisted until the onset of India s Cenozoic collision with Asia. While the Jurassic tectonic evolution of the Bangong suture zone remains speculative, a robust conclusion is that the Amdo region provides unambiguous evidence for Jurassic high grade metamorphism and extensive magmatism. We suggest that the

37 37 Jurassic granitoids represent exhumed portions of a continental arc that paralleled the length of the entire Bangong suture from the Amdo area to the Pamir. This arc is "missing" in central Tibet because it was either buried depositionally beneath Upper Jurassic and younger supracrustal assemblages or underthrust northward beneath the Qiangtang terrane along with Lhasa terrane basement. This interpretation begs the more general question to what extent are entire metamorphic/magmatic belts and other important records of tectonism missing beneath supracrustal assemblages in Tibet and in other contractional orogens worldwide? ACKNOWLEDGMENTS Preliminary sampling of the Amdo region was undertaken in 1997 with the assistance of A.Yin and M. Murphy. We thank R. Waldrip for assistance in the field and J. Fox and F. Guerrero for sample preparation. The manuscript benefited from discussions with M. Ducea and P.G. DeCelles and reviews by C. Burchfiel, L. Ratschbacher and A.Yin. This research was supported by NSF grant EAR , University of Arizona start-up funds, and student research grants from ChevronTexaco and the Geological Society of America. REFERENCES CITED Coward, M.P., Kidd, W.S.F., Yun, P., Shackleton, R.M., and Hu, Z., 1988, The structure of the 1985 Tibet Geotraverse, Lhasa to Golmud: Philosophical Transactions of the Royal Society of London: Ser. A, v. 327, p

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