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
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
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
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 40Ar/39Ar 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
DEDICATION
This dissertation is dedicated to my mom, Trish Hamilton Casper; my sister, Dr. Sierra Guynn; and my friend, Mimi Ashcraft.
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
TABLE OF CONTENTS - Continued
FIGURE CAPTIONS ...... 42
FIGURES...... 45
Data Repository Item DR2006094 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 40Ar/39Ar analyses (University of
California, Los Angeles)...... 50
Summary of analytical methods for hornblende and feldspar 40Ar/39Ar
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
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 ...... 100
Thermobarometric results ...... 100
Tectonic implications ...... 104
ACKNOWLEDGEMENTS ...... 108
REFERENCES ...... 108
FIGURE CAPTIONS ...... 116
FIGURES ...... 122
TABLES ...... 137
APPENDIX C: Jurassic and Cretaceous Tectonic Evolution of the Bangong Suture
Zone near Amdo, central Tibet ...... 145
ABSTRACT ...... 147
INTRODUCTION ...... 149
REGIONAL GEOLOGY ...... 151
GEOCHRONOLOGY ...... 155
Igneous Zircon Analysis ...... 156
Detrital Zircon Analysis ...... 157
Jurassic flysch ...... 158 9
TABLE OF CONTENTS - Continued
Red arenites and shale ...... 158
GEOLOGY OF THE AMDO AREA ...... 160
Rock Types and Ages ...... 160
Structure of the Amdo Basement Region ...... 168
DISCUSSION ...... 172
Jurassic Tectonic Evolution of the Bangong Suture at Amdo ...... 173
Cretaceous Tectonic Evolution of the Bangong Suture at Amdo ...... 175
CONCLUSIONS ...... 179
ACKNOWLEDGMENTS ...... 180
REFERENCES CITED ...... 181
FIGURE CAPTIONS ...... 188
FIGURES ...... 195
TABLES ...... 209
APPENDIX D: U-Pb geochronology of basement rocks in central Tibetan and paleogeographic implications ...... 225
Abstract ...... 227
1. Introduction ...... 228
2. Regional Geology and Paleozoic Paleogeography ...... 230
3. Amdo Basement Geology ...... 233
4. U-Pb Geochronology ...... 234
4.1. Methods ...... 235 10
TABLE OF CONTENTS - Continued
4.2. Igneous Zircon Analysis ...... 239
4.2.1. JG053104-1 ...... 239
4.2.2. JG061504-2 ...... 240
4.2.3. PK970604-3A ...... 240
4.2.4. JG060504-2 ...... 240
4.2.5. JG061504-1 ...... 241
4.2.6. PK970604-1B ...... 241
4.2.7. JG061604-1 ...... 241
4.2.8. PK970604-1A ...... 242
4.2.9. JG053104-2 ...... 242
4.3. Detrital Zircon Analysis ...... 243
4.3.1. JG061504-4 ...... 243
4.3.2. JG062504-3 ...... 244
4.3.3. AP061304-A ...... 244
5. Discussion ...... 244
5.1. Tibetan Paleozoic Detrital Zircon Signature ...... 245
5.2. Regional Detrital Zircon Record ...... 247
5.3. Regional Basement Ages ...... 251
5.4. Implications for Paleogeography of the Lhasa-Qiangtang terrane ...... 261
6. Conclusions ...... 265
Acknowledgements ...... 267 11
TABLE OF CONTENTS - Continued
Appendix A.1 Description of Rock Samples...... 267
A.1.1. JG053104-1 orthogneiss ...... 267
A.1.2. JG061504-2 orthogneiss ...... 267
A.1.3. PK970604-3A orthogneiss ...... 268
A.1.4. JG060504-2 orthogneiss ...... 268
A.1.5. JG061504-1 orthogneiss ...... 268
A.1.6. PK970604-1B orthogneiss ...... 269
A.1.7. JG061604-1 orthogneiss ...... 269
A.1.8. PK970604-1A orthogneiss ...... 269
A.1.9. JG053104-2 orthogneiss ...... 269
A.1.10. JG061504-4 paragneiss ...... 269
A.1.11. JG062504-3 quartzite ...... 270
A.1.12. AP061304-A quartzite ...... 270
References ...... 270
Figure Captions ...... 290
Figures ...... 297
Tables ...... 309 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
INTRODUCTION
The Tibetan Plateau is the largest and, on average, highest orogenic feature on the
earth, with an area of approximately 5,000,000 km2 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
Cretaceous thrust belt in the central Lhasa terrane near the town of Coqen and suggested
a 60-65 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
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 received only minor attention in past studies of the Tibetan Plateau (Coward et al., 1988;
Harris et al., 1988; Xu et al., 1985). 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 40Ar/39Ar 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 (180-165 Ma) and another during the Early Cretaceous (130-115 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
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 low-
temperature 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 700-750°C and pressures of
10-12 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 185-175 Ma. Metasedimentary rocks along the southern edge of the basement 19
record slightly lower conditions, around 600°C and 8 kbar, indicating that the higher-
grade 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 40Ar/39Ar analysis (130-115 Ma). A simplified cross-section of the region suggests ~100 km of 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 (~530-480 Ma) and one in the early Neoproterozoic (~910-850 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
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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. 41-
57.
Yin, A., and Harrison, T.M., 2000, Geologic evolution of the Himalayan-Tibetan orogen:
Annual Review of Earth and Planetary Science, v. 28, p. 211-280.
Zhao, W.L., and Morgan, W.J., 1985, Uplift of Tibetan Plateau: Tectonics, v. 4, p. 359-
369.
25
APPENDIX A: Permission for Reproduction from Geological Society of America
The following email 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. Email 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"
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 9140 3300 Penrose Place Boulder, CO 80301-9140 (303) 357-1048 fax 303-357-1073 [email protected]
-----Original Message----- From: Jerome Guynn [mailto:[email protected]] Sent: Tuesday, October 03, 2006 8:38 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, 2006. 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
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.
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-- PhD Candidate Department of Geosciences Gould-Simpson Bldg. #77 University of Arizona [email protected]
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27
APPENDIX A: Tibetan basement rocks near Amdo reveal “missing” Mesozoic tectonism along the Bangong suture, central Tibet
Reprint from Geology
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 100029 China
ABSTRACT
U-Pb and 40Ar/39Ar 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 metamorphism coeval with extensive granitoid emplacement at 185-170 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
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 40Ar/39Ar 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
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 (140-120 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
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-of-
shear. 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 laser-
ablation 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 206Pb*/238U ages of individual zircons because low 207Pb concentrations in young (< 1000 Ma) granitoids result in large uncertainties in the 207Pb*/235U and 207Pb*/206Pb* 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 (PK97-6-4-3A) show signs of young zircon growth due to metamorphic processes. Zircon resorption can be seen in cathode- luminescence images as bright, irregular zones only a few microns in thickness on the 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
PK97-6-4-3A to indicate that the amphibolite facies metamorphism of the Amdo
basement is Mesozoic. This interpretation is consistent with results of 40Ar/39Ar analysis of hornblende and U-Pb analysis of titanite from orthogneiss samples. Hornblende sample PK97-6-4-1A provides generally monotonically increasing apparent ages from
~160 Ma to ~187 Ma with an integrated age of ~180 Ma, while hornblende sample
PK97-6-4-3A 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 (PK97-6-4-1A) provides a mean
206Pb*/238U age of 179.0 ± 4.2 Ma (Fig. DR1j; Table DR1)1, which is statistically indistinguishable from the integrated 40Ar/39Ar hornblende age from the same sample. 34
Given the higher closure temperature for titanite (~600-700 °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
40Ar/39Ar 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 (PK97-6-4-1A and PK97-6-4-3A) and one muscovite sample (PK97-6-4-2) yield complexly varying apparent ages over the last ~80% cumulative 39Ar 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 40Ar/36Ar 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 PK97-6-4-1A and PK97-6-4-3A show cooling from ~300 °C to ~100 °C between 135 and 115 Ma, while PK97-6-4-2 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, PK97-6-4-2 displays an intermediate hump in the age spectra (both corrected and uncorrected) that 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 PK97-6-4-3A. 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
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
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-0309844,
University of Arizona start-up funds, and student research grants from
ChevronTexaco and the Geological Society of America.
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42
FIGURE CAPTIONS
Figure 1. Map of Tibetan terranes and suture zones. Ophiolite exposures related to the
Bangong suture zone are shown in dark green. Note that ophiolites occur both north and south of the Amdo basement. Traditionally, the Bangong suture is placed at the northernmost ophiolite outcrops, but our model suggests it occurs to the south.
Abbreviations: SGFC—Songpan-Ganzi flysch complex; MBT—Main Boundary Thrust;
STD—South Tibetan Detachment.
Figure 2. Simplified geologic map of the western Amdo basement region. Compiled from our mapping at 1:100,000 scale and the more regional geologic maps of Kidd et al.
(1988) and Pan et al. (2004).
Figure 3. U-Pb analysis of orthogneiss PK97-6-4-3A. (a) Cathode-luminescence images of two zircons with thin, bright resorption rims. (b) U-Pb concordia diagram for orthogneiss sample PK97-6-4-3A; error ellipses are at the 95% confidence level. Plotting and discordia regression are from Isoplot 3.00 (Ludwig, 2003). Analyses include laser ablation spots in the cores and at the tips of the individual crystals. The crystallization age is based on a weighted mean of 206Pb*/238U ages for 13 concordant analyses clustered near the upper intercept of the discordia (> 830 Ma; Fig. DR1i);1 these ellipses are shown in black. (c) Variation in Th/U with 206Pb*/238U ratio (i.e., age), showing a systematic decrease in Th/U values with decreasing age.
43
Figure 4. Cooling history of the Amdo basement based on thermochronologic data from three basement samples. Ttn - titanite; Hbl - hornblende; Ms - muscovite; Bi - biotite;
Kspar - K-feldspar. The lower limit of the temperature range for titanite includes the possibility of crystallization below the closure temperature. Total gas ages of hornblende
and mica are shown with assigned uncertainties of 2%. The thermal histories calculated
from multi-domain diffusion (MDD) modeling of K-feldspar 40Ar/39Ar age spectra represent a 90% confidence window about the mean of at least 20 solutions.
Figure 5. Tectonic model for the evolution of the Amdo basement and Bangong suture zone. (a) The Amdo basement rifted from the Qiangtang terrane by Early Jurassic time, possibly due to slab rollback, creating an oceanic back-arc basin. (b) Middle Jurassic closure of the back-arc basin resulted in high-grade metamorphism of the Amdo basement and ophiolite obduction along the northern edge of the Bangong suture zone.
Arc magmatism is coeval with metamorphism at this time. (c) During Early Cretaceous time, the Bangong Ocean closes and the Lhasa terrane collides with the Qiangtang terrane. A normal fault is inferred on the north side of the Amdo gneiss to place unmetamorphosed sedimentary rocks against the high grade Amdo basement. A foreland basin develops south of the Amdo basement. (d) Continued convergence between the
Lhasa and Qiangtang terranes in the Cretaceous results in underthrusting of the Lhasa terrane, exhumation of the Amdo basement to upper-crustal levels, and a fold-thrust belt along the convergence zone.
44
1GSA Data Repository item 2006094, Supplementary Geochronologic and
Thermochronologic Data, is available online at www.geosociety.org/pubs/ft2006.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140,
Boulder, CO 80301, USA. 45
FIGURES
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Data Repository Item DR2006094 Supplementary Geochronologic and Thermochronologic Data
Summary of analytical methods for U-Pb analysis (University of Arizona)
Zircons and titanite were separated from 1-2 kg samples of fresh rock using standard crushing and separation techniques, including a water table, magnetic separator and heavy liquids. Individual zircon crystals and titanite fragments were than hand-picked under a binocular microscope to select large, inclusion free grains. Approximately 50 grains were than mounted in epoxy and polished to approximately 2/3 of the crystal thickness. The grains were ablated with a New Wave DUV193 Excimer laser, operating at a wavelength of 193 nm and using a spot diameter of 25, 35, or 50 microns, depending on the size of the crystal. The ablated material is then carried in argon gas into the plasma source of a Micromass Isoprobe multicollector ICP-MS and ionized. The Micromass Isoprobe is equipped with a flight tube of sufficient width that U, Th, and Pb isotopes are measured simultaneously using nine Faraday collectors, an axial Daly detector and four ion-counting channels. All measurements were made in static mode, using Faraday detectors for 238U, 232Th, and 208-206Pb and an ion-counting channel for 204Pb. 235U is determined from the 238U measurement assuming 238U/235U = 137.88. Ion yields are ~1 mV per ppm. Each analysis consists of one background run with 20-second integration on peaks and the laser off, 20 1-second integrations with the laser firing, and a 30-second delay to purge for the next analysis. The laser ablates at ~1 micron/second, resulting in an ablation pit ~20 microns in depth. A common lead correction was performed by using the measured 204Pb and assuming an initial Pb composition from Stacey and Kramers (1975) (with uncertainties of 1.0, 0.3, and 2.0 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, respectively). Measurement of 204Pb is unaffected by the presence of 204Hg because backgrounds are measured on peaks (thereby subtracting any background 204Hg and 204Pb) and because very little Hg is present in the argon gas. The errors in determining 206Pb/238U and 206Pb/204Pb result in a measurement uncertainty of several percent (at 2-σ level) in the 206Pb*/238U age. The low concentrations of 207Pb in younger samples (approximately < 1000 Ma), due to the low concentration of 235U relative to 238U, result in a substantially larger measurement uncertainty for 206Pb/207Pb. The 207Pb*/235U and 206Pb*/207Pb* ages for younger grains accordingly have larger uncertainties. Inter-element fractionation of Pb/U is generally <20%, whereas isotopic fractionation of Pb is generally <5%. These fractionations were corrected by analysis of a standard, with a known, concordant ID-TIMS age, between every three unknown analyses. The zircon standards are fragments of a large zircon crystal from a Sri Lankan pegmatite (e.g. Dickinson and Gehrels, 2003) with an age of 564 ± 4 Ma (2σ), while the titanite standards are fragments of a large titanite crystal from the Bear Lake region, Canada, with an age of 1050 ± 2 Ma (2σ) (Mark Schmitz and John Aleinikoff, 50 written communication, 2003). The uncertainty resulting from the calibration correction is generally ~3% (2σ) for both 207Pb/206Pb and 206Pb/238U ages. U and Th concentrations were determined by analyzing a piece of NIST 610 glass with ~500 ppm U and Th and using the resulting measured intensities to calibrate the sample measured U and Th intensities. The crystallization ages reported in this paper (Table 1; Fig. 3a; Fig. DR1) are weighted averages of individual, concordant spot analyses using the 206Pb*/238U ages, since the 207Pb*/235U and 207Pb/206Pb* ages are less precise for these younger granitoids. The stated uncertainties (2σ) on the assigned crystallization ages are absolute values and include contributions from all known random and systematic errors. Random errors are included in the data tables. Systematic errors (2σ) are as follows: PK97-6-4-3A, 2.06%; JG062004-4, 1.00%; JG062204-1, 1.00%; JG061504-7, 0.81%; AP061504-B, 1.02%; AP061604-B, 1.00%; AP060604-A, 1.06%; AP052904-A, 1.06%; AP062104-A, 0.96%; PK97-6-4-1A Titanite, 2.16%. All U-Pb plots and weighted average calculations were made using Isoplot 3.00 (Ludwig, 2003).
Summary of analytical methods for mica 40Ar/39Ar analyses (University of California, Los Angeles).
High-purity mica mineral separates for 40Ar/39Ar analysis were obtained using standard mineral separation techniques. They were irradiated for 45 hours at the Ford Reactor, University of Michigan, along with Fish Canyon sanidine (27.8 ± 0.3 Ma; Renne et al., 1994) or Taylor Creek sanidine (28.1 ± 0.3
Ma) flux monitors to calculate J-factors and K2SO4 and CaF2 salts to calculate correction factors for interfering neutron reactions. Samples were step-heated in a Ta crucible within a double-vacuum furnace, and 40Ar/39Ar isotopic measurements were conducted on a VG 1200S or VG 3600 mass spectrometer at UCLA. Apparent ages were calculated using conventional decay constants and isotopic abundances. The uncertainties for apparent ages listed in Table A1 are at the one sigma level and do not include uncertainties in the J factor or decay constants.
Summary of analytical methods for hornblende and feldspar 40Ar/39Ar analyses (New Mexico Geochronological Research Laboratory).
Feldspar and hornblende separates were obtained by standard heavy liquid and magnetic separation methods. The separates were loaded into machined Al discs and irradiated for 75 mW hours in 5-c position at the McMaster reactor, Hamilton, Ontario along with the Fish Canyon Tuff sanidine (FC-2) as a neutron flux monitor. The FC-2 standard has an assigned age = 27.84 Ma (Deino and Potts, 1990) relative to Mmhb-1 at 520.4 Ma (Samson and Alexander, 1987). 51
The hornblendes and K-feldspars were step heated in a Mo resistance furnace, the former for 10 minutes while the latter ranged from 15 to 180 minutes. Isotopes were measured on a Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system. Reactive gases were removed during heating with an SAES GP-50 getter operated at ~450°C. Additional cleanup (5 minutes hornblendes; 2 minutes K-feldspar) following the step heating with two SAES GP-50 getters, one operated at ~450°C and one at 20°C. The gas was also exposed to a W filament operated at ~2000°C. The electron multiplier sensitivity averaged 2.87x10-16 moles/pA. The total system blank and background for hornblendes was 70, 1.0, 0.4, 0.4, 0.5 x 10-17 moles for masses 40, 39, 38, 37, 36, respectively. The total system blank and background for K-feldspars was 45, 1.2, 0.3, 0.4, 0.3 x 10-17 moles for masses 40, 39, 38, 37, 36, respectively. The J-factors were determined to a precision of ± 0.1%
by CO2 laser-fusion of 6 single crystals from each of 4 radial positions around the irradiation tray. The
Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as 40 39 36 37 39 37 follows: NM-173: ( Ar/ Ar)K = 0.02895±0.00059; ( Ar/ Ar)Ca = 0.000284±0.000006; ( Ar/ Ar)Ca = 38 39 0.00074±0.00002; ( Ar/ Ar)K = 0.0129.
K-Feldspar multi-domain diffusion modeling methods
Thermal histories are calculated from the feldspar data using the multi-domain diffusion (MDD) model (Figs. DR6d, DR7d, DR8d). The diffusion domain distribution for each feldspar is obtained by modeling the release of 39Ar relative to the laboratory heating schedule. In all cases, the activation energy (E) is assumed to be constant for all diffusion domains of a given sample and seven diffusion domains were imposed on each model (Table DR4). The model Arrhenius and log(r/ro) plots show that the measured data can be well approximated by seven diffusion domains of varying proportions (Figs DR6b,c; DR7b,c; DR8b,c). For each sample, two thermal histories are shown: first the measured age data that have only been corrected for an assumed atmospheric 40Ar/36Ar initial value are modeled and secondly, apparent ages corrected for implied trapped components from isochron analysis are modeled (Figs. DR6a,d; DR7a,d; DR8a,d). Because the feldspar age spectra climb above the hornblende apparent ages we can be reasonably certain that the final heating steps for each feldspar are contaminated with excess 40Ar. However, it is not obvious that the initial age gradients, which appear to climb in a steady fashion and are younger than coexisting hornblendes, are contaminated by excess 40Ar. These age gradients are well fit by model age spectra that are created by cooling the samples from about 300°C at 150 Ma to about 100°C by 120 Ma (Figs. DR6a,d; DR7a,d; DR8a,d). The accuracy of these models depends critically upon the assumption that the apparent ages are not influenced by excess 40Ar, however the isochron data appear to challenge this assumption (Fig. DR5). Admittedly the isochron data define poor linear trends as revealed by high MSWD values; however we still 52 assign overall meaning to their indication of excess argon (Fig. DR5). The probable cause for the scatter on the isochron arrays is incomplete separation of multiple trapped components (i.e. Heizler and Harrison, 1988) and potential true 40Ar* gradients related to complex thermal histories. We recognize problems with rigorous quantitative use of the isochron data, but feel that it is more appropriate to model age spectra that have been corrected with corresponding isochron trapped components rather then choose models that only correct of an atmospheric trapped initial 40Ar/36Ar value. Therefore, each apparent age is calculated using the inferred isochron trapped values (Fig. DR5) and recast as “isochron corrected” age spectra in Figures DR6a, DR7a, and DR8a. These spectra are much flatter than the atmosphere corrected spectra and thus produce model thermal histories that require cooling at a younger time and faster rate (Figs. DR6d, DR7d, DR8d). The “flattening” of the age spectra is most pronounced in samples PK-97-6-4-1A and PK-97-6-4- 3A, whereas sample PK-97-6-4-2 is less sensitive to choice of the initial trapped 40Ar/36Ar component. This is mainly because the apparent radiogenic yields for samples PK-97-6-4-1A and 3A are significantly lower than that for PK-97-6-4-2 (Table DR2). As expected from the high MSWD values calculated from the isochron arrays, the corrected age spectra show significant scatter, but are overall much flatter than the original spectra and we assert that these isochron corrected spectra yield more accurate thermal histories. The thermal histories determined from the isochron corrected age spectra indicate two periods of rapid cooling. Samples PK-97-6-4-1A and 3A suggest cooling from about 300°C to 100°C occurred between 125 and 120 Ma; whereas PK-97-6-4-2 suggests this same cooling range took place at about 140 Ma (Figs. DR6d, DR7d, DR8d). We have the least amount of confidence in sample PK-97-6-4-2 feldspar as both versions of the age spectra reveal a significant intermediate age hump that is suggested to be caused by post-argon closure modification of the diffusion domains (Lovera et al., 2002). For instance, Lovera et al. (2002) showed that complex age spectra with intermediate age humps could result from relatively low- temperature recrystallization and cautioned against assigning significance to model thermal histories in some instances.
References Cited Deino, A., and Potts, R., 1990, Single-Crystal 40Ar/39Ar dating of the Olorgesailie Formation, Southern Kenya Rift: Journal of Geophysical Research, v. 95, p. 8,453-8,470. Dickinson, W.R., and Gehrels, G.E., 2003, U-Pb ages of detrital zircons from Permian and Jurassic eolian sandstones of the Colorado Plateau, USA: paleogeographic implications: Sedimentary Geology, v. 163, p. 29-66. Heizler, M.T., and Harrison, T.M., 1988, Multiple trapped argon isotope components revealed by 40Ar/39Ar analysis. Geochimica Cosmochima Acta, v. 52, p. 1,295-1,303. Lovera, O. M., Grove, M., Harrison, T. M., 2002, Systematic analysis of K-feldspar 40Ar/39Ar step heating results; II, Relevance of laboratory argon diffusion properties to nature: Geochimica Cosmochimica 53
Acta, v. 66, p. 1,237-1,255. Ludgwig, K.R., 2003, Isoplot 3.00: Berkeley Geochronology Center Special Publication No. 4, 70 p. Renne, P.R., Deino, A.L., Walter, R.C., Turrin, B.D., Swisher, C.C., Becker, T.A., Curtis, G.H., Sharp, W.D., and Jaouni, A.R., 1994, Intercalibration of astronomical and radioisotopic time: Geology, v. 22, p. 783-786. Samson, S.D., and, Alexander, E.C., Jr., 1987, Calibration of the interlaboratory 40Ar/39Ar dating standard, Mmhb-1: Chemical Geology, v. 66, p. 27-34. Stacey, J.S., and Kramers, J.D., 1975, Approximation of Terrestrial Lead Isotope Evolution by a 2-Stage Model: Earth and Planetary Science Letters, v. 26, p. 207-221. 54
Figures and Tables
0.035 a) AP061504-B 210 190 0.033 Zircon
0.031 190 180 U 0.029 238
0.027 Pb*/ 170 170 U Age (Ma) 206 238 0.025
150 160 Pb*/ 0.023 Age = 175.0 ± 3.4 Ma 206 n = 17 MSWD = 5.5 0.021 n = 11 0.0 0.1 0.2 0.3 0.4 150
0.032 200 b) JG062004-4 200 Zircon 0.030 190 190
U 180 0.028 180 238
170 U Age (Ma) Pb*/ 0.026 238 170 206 160 Pb*/ 0.024
150 206 160 n = 19 Age = 170.7 ± 2.7 Ma 0.022 MSWD = 2.5 n = 19 0.0 0.1 0.2 0.3 0.4 150
200 0.035 220 c) JG062204-1 Zircon 210 0.033 190
200 0.031 U 190 180 238 0.029
180 U Age (Ma) Pb*/
238 170
206 0.027 170 Pb*/ 160
0.025 206 160 Age = 177.7 ± 1.8 Ma 150 n = 17 MSWD = 1.4 0.023 n = 16 0.05 0.15 0.25 0.35 150
0.036 220 d) AP052904-A 220 0.034 Zircon 210
0.032 200 200 U 0.030 190 238 U Age (Ma)
180 238
Pb*/ 0.028 180 206 Pb*/
0.026 206 170 160
0.024 160 Age = 177.8 ± 4.1 Ma n = 20 MSWD = 6.8 n = 20 0.022 150 0.0 0.1 0.2 0.3 0.4 0.5 207Pb*/235U Figure DR1. U-Pb concordia plots and weighted averages for Jurassic granitoids a) AP061504-B, b) JG062004-4, c) JG062204-1, and d) AP052904-A. Error ellipses and error bars in this and all subsequent plots are at the 2-σ level. Note that mean age only includes random errors; see Table 1 for random & systematic errors. 55
0.032 e) AP062104-A 200 195 Zircon
0.030 185
180 0.028
U 175 238 0.026
U Age (Ma) 165
Pb*/ 160 238 206 0.024
Pb*/ 155 206 0.022 140 145 n = 15 Age = 171.8 ± 3.8 Ma 0.020 MSWD = 4.9 n = 15 0.0 0.1 0.2 0.3 0.4 135 207Pb*/235U
0.034 f) AP060604-A 210 205
0.032 Zircon 200 195
0.030 190
U 185
238 180 0.028
U Age (Ma) 175 Pb*/ 170 238 206 0.026 165 160 Pb*/ Age = 179.4 ± 2.8 Ma
206 MSWD = 2.3 0.024 150 155
n = 18 n = 18 0.022 145 0.0 0.1 0.2 0.3 0.4
207Pb*/235U
205 g) AP061604-B 0.032 Zircon 200 195
0.030 190 185
U 180 0.028 238
U Age (Ma) 175 170 Pb*/ 238 0.026 206
160 Pb*/ 165 206 0.024 150 155 n = 17 Age = 174.0 ± 3.3 Ma 0.022 MSWD = 4.1 n = 17 0.0 0.1 0.2 0.3 0.4 0.5 145 207Pb*/235U
0.0325 202 h) JG061504-7 206 202 Zircon 198 0.0315 198 194 0.0305 194 U 190 190 238 0.0295
186 U Age (Ma) 186 Pb*/ 238
206 182 0.0285 182 Pb*/ 178 206 178 0.0275 174
174 170 n = 16 Age = 182.9 ± 2.2 Ma 0.0265 MSWD = 4.8 n = 16 0.15 0.17 0.19 0.21 0.23 0.25 170
207Pb*/235U Figure DR1 continued. U-Pb concordia plots and weighted averages for Jurassic granitoids e) AP062104-A, f) AP060604-A, g) AP061604-B, and h) JG061504-7 56
j) PK97-6-4-3A 890 Zircon
870
U Age (Ma) 850 238 Pb*/ 830 206 Age = 852.0 ± 3.5 Ma MSWD = 1.16 n = 13 810
220 i) PK97-6-4-1A 0.032 Titanite 210 200
200
0.030 190 190 U 238 180 180 0.028 U Age (Ma) Pb*/ 238 170
206 170
Pb*/ 160 0.026 Age = 178.8 ± 1.6 Ma 206 160 MSWD = 0.69 150 n = 27 0.024 n = 27 140 0.0 0.1 0.2 0.3 0.4 0.5
207Pb*/235U
Figure DR1 continued. i) Weighted average of selected, concordant zircons from orthogneiss PK97-6-4-3A to define a crystallization age and j) U-Pb concordia plot and weighted average for Titanite from orthogneiss PK97-6-4-1A. 57
Table DR1. U-Pb data
Sample spot numbers in bold were used to calculate the weighted averages
Isotopic ratios Apparent ages (Ma)
Sample U 206Pb U/Th 207Pb* ± (%) 206Pb* ± (%) error 206Pb* ± (Ma) 207Pb* ± (Ma) 206Pb* ± (Ma) spot (ppm) 204Pb 235U 238U corr 238U 235U 207Pb*
PK97-6-4-3A Zircon 1 74 23236 3.8 1.04585 2.28 0.11275 2.01 0.88 689 14.6 727 23.9 846 11 2 49 82186 3.9 1.35389 1.50 0.14203 0.75 0.50 856 6.9 869 20.3 903 13 3 42 64844 3.8 1.31590 2.37 0.14004 0.83 0.35 845 7.5 853 31.2 873 23 6 26 28827 4.1 1.36611 3.82 0.14298 0.75 0.20 861 6.9 874 51.6 908 39 9 29 31145 32.5 0.48372 9.15 0.05947 5.00 0.55 372 19.1 401 44.0 567 83 10t 66 197058 10.9 0.83463 3.53 0.09433 3.09 0.88 581 18.8 616 29.4 747 18 11 34 119041 3.8 1.33345 2.38 0.14349 1.15 0.48 864 10.6 860 31.7 850 22 12 67 58200 31.5 0.24671 6.32 0.03185 1.21 0.19 202 2.5 224 15.7 459 69 16 103 63471 4.2 0.85294 1.53 0.08796 1.16 0.76 543 6.6 626 13.2 938 10 17 23 133570 5.1 1.31926 2.51 0.14272 0.95 0.38 860 8.8 854 33.1 839 24 17t 52 49148 31.9 0.28783 8.27 0.03364 1.35 0.16 213 2.9 257 23.9 676 87 18 24 134179 4.2 1.24968 3.61 0.13489 1.09 0.30 816 9.5 823 44.9 844 36 19 20 81693 3.8 1.37145 3.22 0.14348 1.45 0.45 864 13.4 877 43.9 908 30 20 35 180701 3.1 1.22229 2.19 0.12916 0.80 0.36 783 6.6 811 26.8 888 21 22t 50 68318 6.6 0.71205 3.77 0.07248 1.99 0.53 451 9.3 546 26.9 965 33 25t 65 629740 5.5 0.99926 2.79 0.10830 2.16 0.77 663 15.1 703 27.9 835 18 27t 41 95493 12.0 0.37717 6.80 0.04607 2.14 0.32 290 6.4 325 25.7 581 70 29 26 381676 4.3 1.23819 2.40 0.13362 0.86 0.36 808 7.4 818 29.7 844 23 31 18 64114 12.0 0.82039 7.25 0.09058 3.88 0.54 559 22.6 608 58.7 797 64 32 76 852773 6.4 1.02338 1.45 0.11115 0.53 0.37 679 3.8 716 15.0 831 14 35 24 169750 4.0 1.24817 3.68 0.13182 0.82 0.22 798 6.9 823 45.6 889 37 2-1tr 24 57085 4.8 1.09325 4.86 0.11497 1.44 0.30 702 10.7 750 52.5 898 48 2-2c 14 8430 3.4 1.30092 3.46 0.14046 0.93 0.27 847 8.4 846 44.7 843 35 2-2tl 26 17008 3.7 0.92843 2.70 0.10475 1.44 0.53 642 9.7 667 25.2 751 24 2-2tr 19 6676 3.2 0.84426 5.01 0.09903 1.11 0.22 609 7.1 621 42.1 668 52 2-3c 14 33488 4.9 1.39342 4.73 0.14128 1.34 0.28 852 12.2 886 64.8 973 46 2-4c 35 20342 2.3 1.28120 2.89 0.14070 0.67 0.23 849 6.0 837 36.9 808 29 2-4tl 52 5798 10.2 0.50719 3.30 0.06164 2.15 0.65 386 8.5 417 16.9 592 27 2-5c 27 15189 2.4 1.29755 4.10 0.14248 0.90 0.22 859 8.3 845 52.6 808 42 2-5cr 25 68770 4.0 1.03891 3.37 0.10743 1.20 0.36 658 8.3 723 34.9 932 32 2-5tr 39 2117 10.9 0.50971 6.73 0.06138 2.51 0.37 384 9.9 418 34.2 612 67 2-6c 35 16064 2.5 1.34398 1.86 0.14398 0.97 0.52 867 9.0 865 25.0 859 16 2-7c 24 42070 4.3 1.04769 4.48 0.11452 3.71 0.83 699 27.4 728 46.6 818 26 2-10c 13 27809 2.6 1.37472 4.98 0.13872 1.06 0.21 837 9.5 878 67.2 982 50 2-10tr 19 6864 4.3 0.64831 6.30 0.07839 2.11 0.33 486 10.6 507 40.7 603 64 2-11c 24 29165 1.8 1.29767 4.60 0.13613 1.43 0.31 823 12.5 845 58.8 903 45 2-11tr2 25 8302 6.4 0.76238 6.10 0.08854 5.03 0.82 547 28.6 575 46.2 689 37 2-13c 26 3544 3.3 1.36904 2.60 0.14074 0.30 0.12 849 2.8 876 35.5 944 26 2-13tl 43 4932 7.5 0.71289 5.21 0.07458 4.37 0.84 464 21.0 546 37.0 909 29 2-14c 41 166873 3.4 1.24897 4.40 0.13447 2.08 0.47 813 18.0 823 54.3 849 40 2-17tl 17 6702 5.3 0.78529 6.08 0.09252 1.54 0.25 570 9.2 588 47.4 659 63 2-17tr 25 3192 11.9 0.50105 5.95 0.05328 2.71 0.46 335 9.3 412 29.8 875 55 58
Table DR1. U-Pb data
Isotopic ratios Apparent ages (Ma)
Sample U 206Pb U/Th 207Pb* ± (%) 206Pb* ± (%) error 206Pb* ± (Ma) 207Pb* ± (Ma) 206Pb* ± (Ma) spot (ppm) 204Pb 235U 238U corr 238U 235U 207Pb*
AP061504-B Zircon 1R 554 6834 1.4 0.21990 11.49 0.03228 0.74 0.06 205 1.5 202 21.0 167 269 1C 718 4756 0.7 0.17190 12.09 0.02589 2.37 0.20 165 3.9 161 18.0 107 281 2T 998 5831 1.8 0.17999 10.35 0.02764 0.70 0.07 176 1.2 168 16.0 61 247 2C 340 2575 1.7 0.18240 15.62 0.02995 4.60 0.29 190 8.6 170 24.5 -101 368 3R 1168 2968 2.2 0.18909 8.06 0.02645 1.63 0.20 168 2.7 176 13.0 279 181 3C 835 352 1.0 0.22565 17.56 0.02525 3.11 0.18 161 4.9 207 32.8 768 367 4C 465 1621 1.2 0.22924 24.92 0.02653 4.45 0.18 169 7.4 210 47.2 697 530 5 304 3547 1.2 0.20124 27.17 0.02656 3.38 0.12 169 5.6 186 46.3 410 613 6 1219 6951 2.0 0.19277 4.72 0.02652 1.10 0.23 169 1.8 179 7.7 317 104 7 301 2102 1.1 0.18288 11.89 0.02760 1.99 0.17 176 3.4 171 18.7 102 278 8 1641 1138 2.1 0.18608 5.88 0.02671 1.90 0.32 170 3.2 173 9.4 220 129 9 1082 3471 2.2 0.20390 5.05 0.02848 0.74 0.15 181 1.3 188 8.7 282 114 10 294 1879 1.1 0.15854 13.06 0.02801 3.02 0.23 178 5.3 149 18.1 -284 325 12 363 2641 1.3 0.16708 10.25 0.02852 2.49 0.24 181 4.5 157 14.9 -197 250 13 1368 1446 2.5 0.18713 7.34 0.02737 1.88 0.26 174 3.2 174 11.7 176 166 16 587 4443 1.9 0.18269 8.52 0.02655 1.88 0.22 169 3.1 170 13.4 190 194 18 1031 6906 2.3 0.18375 7.34 0.02770 1.38 0.19 176 2.4 171 11.6 105 170
JG062004-4 Zircon 1R 390 2558 1.4 0.18947 9.49 0.02673 2.80 0.29 170 4.7 176 15.3 259 209 1C 340 2854 2.7 0.21016 26.14 0.02676 3.35 0.13 170 5.6 194 46.1 490 581 2R 241 2699 1.2 0.18777 10.21 0.02617 2.49 0.24 167 4.1 175 16.4 287 227 2C 208 1458 1.7 0.16727 25.77 0.02674 2.08 0.08 170 3.5 157 37.5 -36 632 3T 107 872 0.6 0.41713 12.09 0.02939 4.01 0.33 187 7.4 354 36.2 1677 211 3C 226 2345 0.9 0.20325 17.08 0.02679 2.20 0.13 170 3.7 188 29.3 413 381 4 147 937 0.5 0.20923 36.50 0.02669 3.33 0.09 170 5.6 193 64.2 486 828 5 886 711 1.1 0.16684 9.13 0.02567 1.19 0.13 163 1.9 157 13.3 57 216 6 661 2961 1.2 0.20293 11.19 0.02784 5.07 0.45 177 8.9 188 19.2 323 227 7 1514 1884 1.0 0.16104 4.41 0.02522 2.01 0.45 161 3.2 152 6.2 14 95 8 320 932 2.0 0.20361 19.60 0.02709 2.13 0.11 172 3.6 188 33.7 392 441 10 361 2375 1.3 0.17926 13.29 0.02738 2.06 0.16 174 3.5 167 20.5 74 313 11 258 2021 0.3 0.14503 26.19 0.02673 2.23 0.09 170 3.7 138 33.7 -393 689 12 262 1711 1.7 0.17450 21.34 0.02751 2.47 0.12 175 4.3 163 32.2 -2 516 13 693 506 0.3 0.18709 19.00 0.02714 3.50 0.18 173 6.0 174 30.4 194 437 14 788 8207 1.5 0.19645 8.87 0.02772 1.61 0.18 176 2.8 182 14.8 259 201 15 788 8207 1.5 0.19645 8.87 0.02772 1.61 0.18 176 2.8 182 14.8 259 201 18 1107 1243 1.2 0.20630 15.23 0.02746 1.59 0.10 175 2.7 190 26.5 391 342 19 206 2702 0.9 0.19609 18.35 0.02780 2.52 0.14 177 4.4 182 30.6 248 421 20 690 365 0.8 0.22546 6.16 0.02594 1.37 0.22 165 2.2 206 11.5 709 128 21 745 7149 1.2 0.19716 8.70 0.02858 2.72 0.31 182 4.9 183 14.5 196 192 59
Table DR1. U-Pb data
Isotopic ratios Apparent ages (Ma)
Sample U 206Pb U/Th 207Pb* ± (%) 206Pb* ± (%) error 206Pb* ± (Ma) 207Pb* ± (Ma) 206Pb* ± (Ma) spot (ppm) 204Pb 235U 238U corr 238U 235U 207Pb*
JG062204-1 Zircon 1R 493 7111 1.2 0.19904 14.43 0.02729 1.28 0.09 174 2.2 184 24.3 324 328 1C 303 403 0.7 0.19484 19.18 0.02731 2.96 0.15 174 5.1 181 31.8 274 438 2R 458 2980 1.5 0.19928 11.21 0.02794 0.76 0.07 178 1.3 185 18.9 274 257 2C 607 2249 1.0 0.19292 13.48 0.02829 1.56 0.12 180 2.8 179 22.1 169 314 3 600 3654 1.0 0.20976 11.06 0.02895 1.47 0.13 184 2.7 193 19.5 309 250 4 704 3702 1.0 0.18778 7.18 0.02826 2.46 0.34 180 4.4 175 11.5 109 159 7 328 2151 1.4 0.17185 19.42 0.02761 1.28 0.07 176 2.2 161 28.9 -48 475 8 399 3155 1.2 0.19537 10.10 0.02852 1.84 0.18 181 3.3 181 16.8 180 232 9 442 2110 1.0 0.19302 12.23 0.02792 1.56 0.13 178 2.7 179 20.1 201 282 10 393 1527 0.9 0.22522 21.27 0.02907 5.94 0.28 185 10.8 206 39.7 460 457 11 390 3685 0.8 0.17208 18.75 0.02814 2.40 0.13 179 4.2 161 28.0 -92 459 12 207 2063 1.0 0.22227 16.10 0.02886 1.91 0.12 183 3.4 204 29.7 447 357 13 491 318 1.2 0.18751 17.04 0.02684 3.71 0.22 171 6.2 175 27.3 226 387 14 587 4731 1.1 0.19697 12.22 0.02867 2.78 0.23 182 5.0 183 20.4 187 278 17 596 8888 1.1 0.19587 13.15 0.02813 1.92 0.15 179 3.4 182 21.9 218 302 21 751 6720 1.0 0.19090 6.83 0.02719 1.53 0.22 173 2.6 177 11.1 238 154 24 585 9499 0.9 0.19223 5.80 0.02809 2.72 0.47 179 4.8 179 9.5 178 120
AP052904-A Zircon 1C 329 2497 0.8 0.23719 12.02 0.03017 3.32 0.28 192 6.3 216 23.4 492 256 4C 369 462 0.9 0.21124 16.79 0.02950 3.01 0.18 187 5.6 195 29.7 282 380 5C 337 4198 1.7 0.19466 14.29 0.02850 2.81 0.20 181 5.0 181 23.6 173 328 9C 540 303 0.5 0.17701 28.28 0.02927 2.37 0.08 186 4.4 165 43.2 -118 707 11C 160 314 1.8 0.24206 21.27 0.02927 3.54 0.17 186 6.5 220 42.1 603 459 12C 722 707 0.5 0.21378 12.21 0.02787 1.61 0.13 177 2.8 197 21.8 437 270 13C 416 2102 0.8 0.19161 15.45 0.02781 1.50 0.10 177 2.6 178 25.2 194 359 14C 331 4696 0.5 0.18376 18.33 0.02822 2.17 0.12 179 3.8 171 28.9 60 437 15C 168 2073 2.1 0.21815 22.80 0.02849 2.07 0.09 181 3.7 200 41.5 434 512 16C 251 2163 0.8 0.20212 19.37 0.02950 2.46 0.13 187 4.5 187 33.1 181 451 18C 513 10015 1.2 0.20896 7.99 0.02832 1.75 0.22 180 3.1 193 14.0 350 176 19C 249 490 2.2 0.16457 23.22 0.02954 2.50 0.11 188 4.6 155 33.3 -325 600 20C 262 2373 0.5 0.24066 18.45 0.02790 1.78 0.10 177 3.1 219 36.4 694 395 23T 283 1357 1.3 0.21861 10.47 0.02810 1.32 0.13 179 2.3 201 19.1 469 231 24T 2980 960 40.6 0.19708 12.52 0.02878 1.86 0.15 183 3.4 183 20.9 179 290 25T 334 248 1.2 0.14211 21.65 0.02845 3.72 0.17 181 6.6 135 27.4 -613 587 26T 394 361 0.3 0.18736 26.36 0.02557 2.60 0.10 163 4.2 174 42.3 335 604 27T 469 2352 1.0 0.19253 16.88 0.02713 2.20 0.13 173 3.7 179 27.7 262 387 60
Table DR1. U-Pb data
Isotopic ratios Apparent ages (Ma)
Sample U 206Pb U/Th 207Pb* ± (%) 206Pb* ± (%) error 206Pb* ± (Ma) 207Pb* ± (Ma) 206Pb* ± (Ma) spot (ppm) 204Pb 235U 238U corr 238U 235U 207Pb*
AP062104-A 2C 359 1381 1.3 0.23891 8.83 0.02851 2.12 0.24 181 3.8 218 17.3 631 185 3C 763 9004 2.1 0.19789 12.14 0.02808 4.40 0.36 178 7.7 183 20.4 246 261 4C 190 1971 1.2 0.19702 21.09 0.02880 2.79 0.13 183 5.0 183 35.3 177 492 5C 369 1501 1.0 0.18893 15.89 0.02715 2.65 0.17 173 4.5 176 25.7 217 365 9C 488 718 1.0 0.21249 15.85 0.02590 3.60 0.23 165 5.9 196 28.2 585 337 10C 210 3522 0.8 0.19871 18.41 0.02826 2.16 0.12 180 3.8 184 31.0 240 424 17C 638 1078 1.0 0.19988 15.65 0.02697 1.39 0.09 172 2.4 185 26.5 361 354 18C 472 5203 0.9 0.21183 11.19 0.02604 1.23 0.11 166 2.0 195 19.9 567 243 19C 416 353 1.1 0.24677 24.64 0.02710 3.14 0.13 172 5.3 224 49.6 808 519 21C 704 378 1.5 0.24037 19.56 0.02822 2.12 0.11 179 3.8 219 38.5 667 420 23C 1079 4470 1.6 0.20290 6.56 0.02685 1.10 0.17 171 1.9 188 11.2 405 145 24C 1076 4008 1.6 0.18512 4.58 0.02661 1.08 0.23 169 1.8 172 7.3 216 103 26T 888 286 0.6 0.22728 17.25 0.02655 1.94 0.11 169 3.2 208 32.4 677 369 27T 793 2819 2.0 0.21929 5.54 0.02956 1.84 0.33 188 3.4 201 10.1 363 118 30T 1340 221 2.1 0.23693 28.76 0.02379 3.68 0.13 152 5.5 216 56.0 992 592
AP060604-B 1C 329 2497 0.8 0.23719 12.02 0.03017 3.32 0.28 192 6.3 216 23.4 492 256 4C 369 462 0.9 0.21124 16.79 0.02950 3.01 0.18 187 5.6 195 29.7 282 380 5C 337 4198 1.7 0.19466 14.29 0.02850 2.81 0.20 181 5.0 181 23.6 173 328 9C 540 303 0.5 0.17701 28.28 0.02927 2.37 0.08 186 4.4 165 43.2 -118 707 11C 160 314 1.8 0.24206 21.27 0.02927 3.54 0.17 186 6.5 220 42.1 603 459 12C 722 707 0.5 0.21378 12.21 0.02787 1.61 0.13 177 2.8 197 21.8 437 270 13C 416 2102 0.8 0.19161 15.45 0.02781 1.50 0.10 177 2.6 178 25.2 194 359 14C 331 4696 0.5 0.18376 18.33 0.02822 2.17 0.12 179 3.8 171 28.9 60 437 15C 168 2073 2.1 0.21815 22.80 0.02849 2.07 0.09 181 3.7 200 41.5 434 512 16C 251 2163 0.8 0.20212 19.37 0.02950 2.46 0.13 187 4.5 187 33.1 181 451 18C 513 10015 1.2 0.20896 7.99 0.02832 1.75 0.22 180 3.1 193 14.0 350 176 19C 249 490 2.2 0.16457 23.22 0.02954 2.50 0.11 188 4.6 155 33.3 -325 600 20C 262 2373 0.5 0.24066 18.45 0.02790 1.78 0.10 177 3.1 219 36.4 694 395 23T 283 1357 1.3 0.21861 10.47 0.02810 1.32 0.13 179 2.3 201 19.1 469 231 24T 2980 960 40.6 0.19708 12.52 0.02878 1.86 0.15 183 3.4 183 20.9 179 290 25T 334 248 1.2 0.14211 21.65 0.02845 3.72 0.17 181 6.6 135 27.4 -613 587 26T 394 361 0.3 0.18736 26.36 0.02557 2.60 0.10 163 4.2 174 42.3 335 604 27T 469 2352 1.0 0.19253 16.88 0.02713 2.20 0.13 173 3.7 179 27.7 262 387 61
Table DR1. U-Pb data
Isotopic ratios Apparent ages (Ma)
Sample U 206Pb U/Th 207Pb* ± (%) 206Pb* ± (%) error 206Pb* ± (Ma) 207Pb* ± (Ma) 206Pb* ± (Ma) spot (ppm) 204Pb 235U 238U corr 238U 235U 207Pb*
AP061604-B 1C 371 6026 0.9 0.18573 8.47 0.02702 1.54 0.18 172 2.6 173 13.5 188 194 3C 413 7975 6.1 0.20725 6.89 0.02922 1.59 0.23 186 2.9 191 12.0 261 154 4C 450 5536 1.3 0.20042 13.09 0.02687 2.44 0.19 171 4.1 185 22.2 375 290 5C 257 2739 1.2 0.19393 20.29 0.02826 2.02 0.10 180 3.6 180 33.5 184 474 7C 367 6755 2.1 0.23945 9.93 0.02942 2.00 0.20 187 3.7 218 19.5 568 212 10C 499 3301 2.6 0.16691 10.68 0.02752 0.80 0.08 175 1.4 157 15.5 -111 263 11C 238 2546 1.5 0.18991 23.93 0.02874 3.56 0.15 183 6.4 177 38.8 96 567 14C 310 1551 3.5 0.19278 15.13 0.02913 3.39 0.22 185 6.2 179 24.8 99 351 17C 234 1855 2.0 0.20254 22.16 0.02698 2.06 0.09 172 3.5 187 37.9 389 501 18C 674 710 3.7 0.23463 26.13 0.02618 2.68 0.10 167 4.4 214 50.5 774 556 19T 244 1378 1.1 0.20722 14.63 0.02524 2.56 0.17 161 4.1 191 25.5 587 314 21T 243 567 2.0 0.27346 24.69 0.02716 2.49 0.10 173 4.3 245 53.9 1014 505 22T 524 3534 3.2 0.17714 15.41 0.02619 1.47 0.10 167 2.4 166 23.6 150 361 23T 156 1485 0.7 0.23349 15.53 0.02708 3.12 0.20 172 5.3 213 29.9 692 326 25T 273 3786 1.3 0.26487 12.59 0.02661 1.77 0.14 169 3.0 239 26.8 991 255 26T 496 5920 6.4 0.19788 7.75 0.02833 2.68 0.35 180 4.8 183 13.0 225 168 27T 520 8114 2.2 0.19475 8.20 0.02675 2.19 0.27 170 3.7 181 13.6 320 180
JG061504-7 2 347 5165 0.9 0.20123 7.81 0.03017 1.76 0.23 192 3.3 186 13.3 117 180 3 1167 18686 1.5 0.20916 3.12 0.02974 1.70 0.54 189 3.2 193 5.5 241 60 4 1256 25846 1.7 0.20197 3.36 0.02912 1.75 0.52 185 3.2 187 5.7 209 67 5 660 8527 1.1 0.18475 3.53 0.02811 1.15 0.33 179 2.0 172 5.6 83 79 7 1163 8317 1.1 0.20005 3.70 0.02856 0.76 0.21 182 1.4 185 6.3 232 84 9 1142 2014 1.8 0.20196 4.03 0.02862 1.10 0.27 182 2.0 187 6.9 249 89 11 1098 11644 1.5 0.20230 2.96 0.02949 1.30 0.44 187 2.4 187 5.1 183 62 14 1239 4097 1.7 0.20503 3.37 0.02944 1.16 0.34 187 2.1 189 5.8 218 73 15 1196 8106 1.7 0.20436 3.47 0.02962 1.91 0.55 188 3.5 189 6.0 196 67 17 1240 2456 1.9 0.21436 4.71 0.02974 1.75 0.37 189 3.3 197 8.4 297 100 18 1485 3635 1.6 0.19602 1.60 0.02768 0.70 0.44 176 1.2 182 2.7 257 33 19 1003 2857 1.3 0.20519 7.75 0.02864 0.82 0.11 182 1.5 190 13.4 283 176 20 1492 2653 1.6 0.20654 3.35 0.02896 0.71 0.21 184 1.3 191 5.8 273 75 22 467 1289 0.7 0.22168 4.53 0.02948 1.19 0.26 187 2.2 203 8.3 394 98 23 1466 2184 1.9 0.21214 4.93 0.02902 0.92 0.19 184 1.7 195 8.8 329 110 24 1505 3134 1.6 0.21210 2.68 0.02970 1.51 0.56 189 2.8 195 4.8 276 51 62
Table DR1. U-Pb data
Isotopic ratios Apparent ages (Ma)
Sample U 206Pb U/Th 207Pb* ± (%) 206Pb* ± (%) error 206Pb* ± (Ma) 207Pb* ± (Ma) 206Pb* ± (Ma) spot (ppm) 204Pb 235U 238U corr 238U 235U 207Pb*
PK97-6-4-1A Titanite 1 423 146 2.8 0.22957 47.78 0.02872 1.81 0.04 183 3.3 210 86.8 529 523 2 496 204 3.6 0.20024 45.61 0.02761 3.86 0.08 176 6.7 185 74.5 312 517 3 429 238 3.7 0.18681 43.48 0.02743 1.71 0.04 174 2.9 174 67.2 167 508 4 531 269 5.0 0.20325 40.66 0.02797 3.64 0.09 178 6.4 188 67.5 316 461 5 390 204 4.5 0.20796 46.18 0.02919 2.55 0.06 185 4.7 192 77.7 271 529 6 428 256 5.0 0.20349 41.81 0.02819 2.35 0.06 179 4.2 188 69.4 301 476 7 428 144 3.9 0.28401 44.86 0.02971 5.50 0.12 189 10.2 254 96.1 909 459 8 439 213 3.9 0.24096 41.98 0.02874 2.53 0.06 183 4.6 219 79.6 632 451 9 502 188 3.0 0.20119 49.01 0.02866 2.58 0.05 182 4.6 186 80.1 237 565 10 599 232 6.0 0.22083 41.61 0.02789 1.20 0.03 177 2.1 203 73.7 508 457 11 660 287 7.8 0.22097 37.87 0.02824 2.17 0.06 180 3.8 203 67.3 482 418 12 626 215 4.4 0.21520 43.11 0.02744 1.85 0.04 175 3.2 198 74.7 487 475 13 537 137 7.4 0.29358 44.73 0.02937 2.42 0.05 187 4.5 261 98.2 1000 454 14 468 194 5.1 0.23347 43.16 0.02792 4.41 0.10 178 7.7 213 79.7 627 463 15 522 206 3.4 0.20962 45.22 0.02797 3.63 0.08 178 6.4 193 76.6 386 506 16 473 203 6.9 0.26176 40.96 0.02832 4.00 0.10 180 7.1 236 82.8 839 424 17 443 237 3.9 0.22432 41.02 0.02795 2.26 0.06 178 4.0 205 73.6 537 448 18 551 251 3.7 0.20137 42.82 0.02783 1.51 0.04 177 2.6 186 70.4 306 487 19 459 206 4.3 0.24334 42.85 0.02865 2.78 0.06 182 5.0 221 81.8 660 458 20 425 153 7.3 0.26582 44.45 0.02866 4.12 0.09 182 7.4 239 90.6 846 460 21 424 209 4.3 0.25035 40.66 0.02814 3.62 0.09 179 6.4 227 79.5 759 427 22 551 154 4.4 0.21052 51.24 0.02791 4.10 0.08 177 7.2 194 86.7 401 572 23 423 209 3.1 0.23994 42.03 0.02917 2.74 0.07 185 5.0 218 79.4 591 455 24 376 221 3.7 0.20070 45.09 0.02847 2.81 0.06 181 5.0 186 73.8 246 518 25 422 121 4.2 0.30440 46.37 0.02801 7.85 0.17 178 13.8 270 104.3 1167 453 26 392 172 3.2 0.21224 46.53 0.02735 2.88 0.06 174 4.9 195 79.5 464 515 27 462 228 5.8 0.21243 42.17 0.02791 1.92 0.05 177 3.4 196 72.4 420 470 63
250 250 a) Integrated Age = 180.0 ± 0.4 Ma b) Integrated Age = 175.3 ± 0.5 Ma
200 200
150 150 Apparent Age (Ma) Apparent age (Ma) PK-97-6-4-1A Hornblende PK-97-6-4-3A Hornblende 100 100 0 20 40 60 80 100 0 20 40 60 80 100 Cumulative % 39Ar Released Cumulative % 39Ar Released
Figure DR2. Hornblende argon age and K/Ca spectra for (a) PK-97-6-4-1A and (b) PK-97-6-4-3A
180 180 a) b) 160 160
140 140 Age (Ma) Age (Ma)
120 120
100 100 Apparent Apparent PK-97-6-4-2 Muscovite PK-97-6-4-1A Biotite 80 80 0 20 40 60 80 100 0 20 40 60 80 100 Cumulative % 39Ar released Cumulative % 39Ar released
180 c) 160
140 Age (Ma)
120
100 Apparent PK-97-6-4-3A Biotite 80 0 20 40 60 80 100 Cumulative % 39Ar released
Figure DR3. 40Ar/39Ar apparent age spectra for mica from orthogneiss samples a) PK-97-6-4-2, b) PK-97-6-4-1A and c) PK-97-6-4-3A. 64
100
100 1 K/Ca 1
300 0.01 K/Ca 300 0.01 a) PK-97-6-4-1A b) PK-97-6-4-2 250 250
200 200
150
150 Apparent age (Ma) Apparent age (Ma) 100
100 Integrated Age = 149.4 ± 0.4 Ma 50 Integrated Age = 163.3 ± 0.4 Ma 0 10 20 30 40 50 60 70 80 90 100 Cumulative % 39Ar Released 50 0 10 20 30 40 50 60 70 80 90 100 Cumulative % 39Ar Released
100
1 K/Ca 300 0.01 c) PK-97-6-4-3A 250
200
150 Apparent age (Ma)
100
Integrated Age = 148.3 ± 0.3 Ma 50 0 10 20 30 40 50 60 70 80 90 100 Cumulative % 39Ar Released
Figure DR4. Feldspar age and K/Ca spectra for a) PK-97-6-4-1A, b) PK-97-6-4-2 and c) PK-97-6-4-3A. All of the samples have relatively low K/Ca values and the ca. 1% bulk K20 estimates indicate significant quartz and plagioclase contamination of the K-feldspar mineral separate (Table DR2). For the two samples that have coexisting hornblende pairs (PK-97-6-4-1A and 3A), the feldspars yield apparent ages significantly older then the hornblendes (Fig. DR2). This later observation strongly suggests excess argon contamination. 65
0.004 a) P K-97-6-4-1A feldspar b) c) P K-97-6-4-1A feldspar P K-97-6-4-1A feldspar air air S teps F,H,J ,L,N,P ,R ,T,V,X,Z,AB air S teps B-E S teps M,O,Q,S ,U,W,Y,AA,AC -AI B C Isochron age = 92 ± 4 Ma Isochron age = 123.0 ± 0.4 Ma Isochron age = 117.8 ± 0.3 Ma 0.003 D 40 36 40Ar/36Ar = 355.6 ± 1.7 40 36 E Ar/ Aro = 312.7 ± 0.8 o Ar/ Aro = 413.1 ± 0.9
r MS WD = 2.3, n = 4 MS WD = 86, n = 12 MS WD = 14, n = 15 A
0 F F 4 / r 0.002 AI H G H AI G A AJ AH P P AJ AH 6 AG J J AG 3 R R AFAE AFAE AADC AADC N N AB I AB I 0.001 Z Z AAL L AA T K T K YX VQ X V Y Q WUOSM WUOSM 0 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0 0.02 0.04 0.06 0.08 0.10 0.12 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 39Ar/40Ar 39Ar/40Ar 39Ar/40Ar
0.004 P K-97-6-4-2 feldspar (d) (e) air air P K-97-6-4-2 feldspar S teps B-G B B S teps K-AG Isochron age = 108.8 ± 1.1 Ma C C 40 36 Isochron age = 140.9 ± 0.2 Ma 0.003 Ar/ Aro = 302.0 ± 0.8 40Ar/36Ar = 319.6 ± 1.0 MS WD = 4.7, n = 6 o
r D D MS WD = 44, n = 23
A E E AI AH AI AH 0 4 / r 0.002 AJ AG F AJ AG F A H H 6 G G 3 AF AF AAED AAED AC AC AB AB 0.001 J J I I APA APA TZ TZ V NK V NK RYX L RYX L WM WM USQO USQO 0 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 39Ar/40Ar 39Ar/40Ar
0.004 P K-97-6-4-3a feldspar (f) P K-97-6-4-3a feldspar (g) P K-97-6-4-3a feldspar (h) air air air S teps B-J S teps K-T S teps U-AI Isochron age = 120.7 ± 0.5 Ma Isochron age = 130.6 ± 0.2 Ma Isochron age = 126.6 ± 0.4 Ma 0.003 B C 40Ar/36Ar = 323.0 ± 0.9 40 36 o 40Ar/36Aro = 320.1 ± 1.6 Ar/ Aro = 392.2 ± 1.6 MS WD = 59, n = 9 MS WD = 17, P rob. = 0.00, n = 10 MS WD = 88, n = 15 r A
0 D E 4
/ AI
r 0.002
A J
6 F H T 3 R AH G P AAAFAEDVC AB 0.001 AA AG Z I YX SN W U K L QOM 0 0 0.02 0.04 0.06 0.08 0.10 0.12 0 0.02 0.04 0.06 0.08 0.10 0.12 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 39Ar/40Ar 39Ar/40Ar 39Ar/40Ar
Figure DR5. Isotope correlation diagrams for selected steps from samples PK-97-6-4-1A (a-c), PK-97-6-4-2 (d,e) and PK-97-6-4-3A (f-h). 66
300 -2 (a) -3 (b) 250 Measured -4
Modeled -1 -5 200 Isochron corrected ) s 2 Modeled -6 150 -7 E=42.0 kcal/mol log(D/r 2 Log Do/ro = 4.6 /sec
Apparent Age (Ma) -8 100 -9 50 -10 0 20 40 60 80 100 6 7 8 9 10 11 12 13 14 39 Cumulative % Ar released 10000/T(K)
2.0 400 (c) (d) 350 1.5 Isochron C) o corrected
) 300 o 1.0 250 Atmosphere corrected log(r/r 0.5 200 Temperature ( 150 0.0 100 0 20 40 60 80 100 50 75 100 125 150 175 200 39 Cumulative % Ar released Age (Ma)
Figure DR6. Multiple diffusion domain modeling results for sample PK-97-6-4-1A feldspar. 67
300 -2 (a) -3 (b) 250 Measured -4 Modeled -1 -5 200 ) s Isochron corrected 2 Modeled -6 150 -7 E=35.9 kcal/mol log(D/r 2 Log Do/ro = 3.2 /sec
Apparent Age (Ma) -8 100 -9 50 -10 0 20 40 60 80 100 6 7 8 9 10 11 12 13 14 39 Cumulative % Ar released 10000/T(K)
2.0 400 (c) (d) Isochron 350 1.5 corrected C) o
) 300 o 1.0 250 log(r/r 200 0.5 Atmosphere Temperature ( corrected 150 0.0 100 0 20 40 60 80 100 50 75 100 125 150 175 200 39 Cumulative % Ar released Age (Ma)
Figure DR7. Multiple diffusion domain modeling results for sample PK-97-6-4-2 feldspar. 68
300 -2 (a) -3 (b) 250 Measured -4 Modeled -1 -5 200 ) s
Isochron corrected 2 Modeled -6
150 -7 E=46.52 kcal/mol log(D/r 2 Log Do/ro = 5.72 /sec
Apparent Age (Ma) -8 100 -9 50 -10 0 20 40 60 80 100 6 7 8 9 10 11 12 13 14 39 Cumulative % Ar released 10000/T(K)
2.0 400 (c) (d) 350 1.5 Isochron
C) corrected o
) 300 o Atmosphere 1.0 250 corrected log(r/r 0.5 200 Temperature ( 150 0.0 100 0 20 40 60 80 100 50 75 100 125 150 175 200 39 Cumulative % Ar released Age (Ma)
Figure DR8. Multiple diffusion domain modeling results for sample PK-97-6-4-3A feldspar. 69
Table DR2. NMGRL hornblende and feldspar argon isotopic data for Amdo orthogneiss samples.
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ Isochron Age (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma) (Ma)
PK-97-6-4-3A, Hornblende, 5.51 mg, J=0.007895±0.10%, D=1.004±0.001, NM-173D, Lab#=54529-01 A 800 30.78 0.6795 60.56 4.25 0.75 42.0 8.9 175.4 2.1 B 900 14.60 0.8884 12.14 1.61 0.57 75.9 12.2 151.1 2.9 C 1000 15.76 6.768 13.63 2.85 0.075 78.1 18.2 167.8 1.8 D 1030 14.74 7.546 8.709 1.87 0.068 86.9 22.1 174.4 2.6 E 1060 13.87 6.648 4.818 14.0 0.077 93.9 51.3 176.99 0.54 F 1090 13.28 6.217 3.246 15.0 0.082 96.8 82.7 174.80 0.50 G 1120 15.05 7.545 9.700 1.15 0.068 85.2 85.1 174.6 4.3 H 1170 14.30 9.200 6.625 1.36 0.055 91.8 87.9 178.8 3.6 I 1200 14.11 6.595 4.857 2.90 0.077 93.9 93.9 179.8 1.7 J 1250 14.46 6.936 4.752 2.59 0.074 94.4 99.3 185.2 2.0 K 1300 18.67 8.930 19.68 0.313 0.057 72.9 100.0 185 15 Integrated age ± 1σ n=11 47.9 K2O=0.42 % 175.13 0.54
PK-97-6-4-1A, Hornblende, 5.23 mg, J=0.007933±0.10%, D=1.004±0.001, NM-173D, Lab#=54530-01 A 800 30.71 3.280 64.78 5.74 0.16 38.5 7.3 162.1 2.0 B 900 13.29 0.8822 6.865 3.10 0.58 85.3 11.2 155.2 1.7 C 1000 14.32 3.637 6.216 7.35 0.14 89.4 20.6 174.58 0.78 D 1030 13.94 2.898 3.972 8.85 0.18 93.5 31.8 177.51 0.66 E 1060 14.01 3.381 2.923 18.9 0.15 96.0 55.9 183.09 0.42 F 1090 13.67 3.299 2.719 5.19 0.15 96.3 62.5 179.3 1.0 G 1120 14.47 3.668 4.244 3.12 0.14 93.6 66.5 184.2 1.9 H 1170 14.21 3.906 2.856 22.7 0.13 96.6 95.4 186.59 0.41 I 1200 15.94 5.345 8.952 3.66 0.095 86.4 100.0 187.3 1.5 Integrated age ± 1σ n=9 78.6 K2O=0.73 % 179.78 0.42
PK-97-6-4-3A, Feldspar, 11.53 mg, J=0.007844±0.10%, D=1.0035±0.0005, NM-173D, Lab#=54527-01 B 450 344.3 0.5212 1036.0 1.80 0.98 11.1 0.4 473 11 133 C 450 86.62 0.4010 261.9 0.751 1.3 10.7 0.5 126.4 8.5 28 D 500 30.08 0.2849 68.25 1.54 1.8 33.0 0.9 135.1 3.8 110 E 500 24.87 0.3977 56.43 1.35 1.3 33.0 1.1 112.5 3.7 92 F 550 18.10 0.4053 29.31 3.44 1.3 52.3 1.9 128.9 1.3 118 G 550 13.78 0.4777 17.76 3.34 1.1 62.1 2.6 117.0 1.5 110 H 600 19.39 0.4653 29.36 9.02 1.1 55.4 4.5 145.75 0.91 135 I 600 10.89 0.5212 7.114 6.28 0.98 81.1 5.8 120.53 0.65 118 J 650 19.40 0.7211 33.16 10.1 0.71 49.7 7.9 131.50 0.92 120 K 650 10.38 0.6195 3.429 7.85 0.82 90.7 9.6 128.23 0.45 127 L 700 10.27 0.6549 2.567 8.90 0.78 93.1 11.5 130.19 0.48 129 M 700 10.05 0.5801 1.755 7.85 0.88 95.3 13.1 130.39 0.36 130 N 750 11.56 0.6557 6.491 10.3 0.78 83.8 15.3 131.94 0.47 130 O 750 10.29 0.5256 1.917 8.29 0.97 94.9 17.0 132.84 0.40 132 P 800 14.82 0.6021 16.63 11.2 0.85 67.1 19.4 135.32 0.75 130 Q 800 10.45 0.5525 2.361 8.63 0.92 93.7 21.2 133.24 0.44 133 R 850 17.52 0.5008 25.53 12.6 1.0 57.1 23.9 136.13 0.74 128 S 850 11.89 0.4431 6.593 9.8 1.2 83.9 25.9 135.61 0.53 134 T 900 18.73 0.3893 28.37 17.2 1.3 55.4 29.5 140.91 0.61 132 U 900 11.55 0.4197 5.436 12.1 1.2 86.4 32.1 135.68 0.49 129 V 950 15.22 0.6908 16.89 13.5 0.74 67.5 34.9 139.69 0.66 118 W 950 11.83 0.4481 6.472 7.53 1.1 84.1 36.5 135.27 0.59 127 X 1000 12.30 0.4347 8.177 10.3 1.2 80.6 38.7 134.89 0.61 125 Y 1000 12.36 0.3586 8.387 10.4 1.4 80.1 40.9 134.73 0.59 124 Z 1050 13.34 0.3548 11.10 12.6 1.4 75.6 43.5 137.04 0.57 123 AA 1050 13.70 0.3339 12.47 13.0 1.5 73.3 46.3 136.48 0.51 121 AB 1100 15.06 0.3854 15.49 21.4 1.3 69.8 50.8 142.69 0.47 123 AC 1100 15.86 0.3767 18.01 20.2 1.4 66.6 55.0 143.41 0.49 121 AD 1100 15.99 0.3047 17.89 28.5 1.7 67.0 61.0 145.47 0.47 123 AE 1100 16.23 0.2875 18.44 39.3 1.8 66.5 69.3 146.45 0.62 123 AF 1100 16.69 0.2636 18.68 42.0 1.9 67.0 78.1 151.45 0.37 128 AG 1200 15.40 0.2140 13.38 48.7 2.4 74.4 88.4 155.00 0.35 138 AH 1250 21.25 0.5813 29.78 32.0 0.88 58.8 95.1 168.45 0.62 131 AI 1350 43.17 2.669 88.52 21.1 0.19 39.9 99.6 228.9 1.5 120 AK 1700 9.641 0.5044 10.10 2.04 1.0 69.4 100.0 92.1 2.1 79 Integrated age ± 1σ n=35 475.1 K2O=2.02 % 148.31 0.32 70
Table DR2 continued. NMGRL hornblende and feldspar argon isotopic data for Amdo orthogneiss samples.
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ Isochron Age (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma) (Ma)
PK-97-6-4-1A, Feldspar, 11.95 mg, J=0.007871±0.10%, D=1.0035±0.0005, NM-173D, Lab#=54528-01 B 450 711.7 0.5878 2254.1 1.77 0.87 6.4 0.4 554 18 95 C 450 218.7 0.3532 681.1 1.15 1.4 8.0 0.6 233 13 80 D 500 69.91 0.3420 200.0 1.55 1.5 15.5 1.0 147.5 5.2 102 E 500 51.20 0.3487 143.5 1.80 1.5 17.2 1.4 120.8 4.1 88 F 550 34.47 0.6782 74.62 3.99 0.75 36.1 2.2 168.7 2.2 110 G 550 18.17 1.254 33.58 3.32 0.41 45.9 2.9 114.6 1.8 H 600 33.11 1.767 62.75 7.62 0.29 44.4 4.6 197.6 1.6 149 I 600 11.99 1.683 11.41 5.18 0.30 73.0 5.7 120.02 0.81 119 J 650 17.71 1.495 27.12 8.07 0.34 55.4 7.4 134.06 0.89 113 K 650 11.11 1.058 7.554 6.82 0.48 80.7 8.9 122.79 0.57 119 L 700 12.15 0.7320 9.588 8.33 0.70 77.1 10.7 128.16 0.64 121 M 700 10.43 0.5986 4.909 6.88 0.85 86.5 12.2 123.51 0.40 116 N 750 13.44 0.6450 14.23 8.54 0.79 69.1 14.1 127.05 0.74 116 O 750 10.60 0.6648 5.374 6.41 0.77 85.5 15.4 124.07 0.62 116 P 800 18.50 0.8440 30.36 7.88 0.60 51.8 17.1 131.1 1.1 107 Q 800 10.76 0.8776 5.938 6.19 0.58 84.3 18.5 124.17 0.53 115 R 850 15.57 0.8168 20.96 7.47 0.62 60.6 20.1 129.01 0.95 112 S 850 10.46 0.6635 4.892 4.99 0.77 86.7 21.2 124.03 0.55 117 T 900 11.68 0.5678 8.114 6.75 0.90 79.8 22.6 127.57 0.72 121 U 900 10.61 0.4347 4.665 5.98 1.2 87.3 23.9 126.64 0.54 120 V 950 10.93 0.3792 5.791 8.06 1.3 84.6 25.7 126.49 0.57 122 W 950 10.80 0.3163 4.909 8.15 1.6 86.8 27.4 128.10 0.44 121 X 1000 11.48 0.2540 6.515 9.9 2.0 83.4 29.6 130.70 0.51 126 Y 1000 11.51 0.2505 6.687 11.1 2.0 83.0 32.0 130.45 0.42 121 Z 1050 13.15 0.2249 11.10 14.7 2.3 75.2 35.1 134.91 0.41 126 AA 1050 12.73 0.2026 9.784 14.0 2.5 77.4 38.2 134.39 0.49 119 AB 1100 14.56 0.2281 13.59 20.8 2.2 72.5 42.7 143.73 0.76 133 AC 1100 15.68 0.2593 17.52 17.6 2.0 67.1 46.5 143.26 0.62 116 AD 1100 16.44 0.2661 19.17 26.6 1.9 65.6 52.2 146.77 0.54 117 AE 1100 17.63 0.3083 22.44 33.5 1.7 62.5 59.5 149.78 0.41 115 AF 1100 18.88 0.3660 23.89 27.1 1.4 62.7 65.3 160.51 0.69 124 AG 1200 22.31 0.3879 33.52 46.8 1.3 55.7 75.4 168.14 0.54 117 AH 1250 27.23 0.6375 45.70 58.3 0.80 50.6 88.0 185.53 0.61 116 AI 1350 40.88 2.106 77.88 45.6 0.24 44.1 97.9 239.65 0.97 123 AJ 1700 39.71 3.132 66.02 9.8 0.16 51.5 100.0 269.7 1.4 173 Integrated age ± 1σ n=35 462.7 K2O=1.89 % 163.33 0.44 71
Table DR2 continued. NMGRL hornblende and feldspar argon isotopic data for Amdo orthogneiss samples.
40 39 37 39 36 39 39 40 39 ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ Isochron Age (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma) (Ma)
PK-97-6-4-2, Feldspar, 11.67 mg, J=0.007937±0.10%, D=1.0035±0.0005, NM-173D, Lab#=54531-01 B 450 349.1 0.4846 1134.3 2.42 1.1 4.0 0.6 190 12 92 C 450 123.8 0.3424 378.5 1.40 1.5 9.7 1.0 163.8 7.6 131 D 500 38.16 0.3123 102.0 2.00 1.6 21.0 1.5 111.4 4.3 102 E 500 30.16 0.3162 74.19 1.90 1.6 27.3 2.1 114.2 2.3 108 F 550 20.38 0.4961 40.51 3.89 1.0 41.4 3.1 116.9 1.9 113 G 550 15.66 0.4724 26.34 3.09 1.1 50.5 3.9 109.6 1.2 107 H 600 19.84 0.5960 35.84 6.87 0.86 46.8 5.7 128.2 1.1 117 I 600 11.55 0.6333 8.749 4.39 0.81 78.0 6.9 124.41 0.96 J 650 12.84 0.7015 11.51 6.36 0.73 73.9 8.6 130.75 0.76 K 650 11.33 0.6411 5.523 4.98 0.80 86.0 9.9 134.14 0.76 132 L 700 10.94 0.6672 3.947 6.52 0.76 89.8 11.7 135.21 0.54 134 M 700 10.90 0.5734 2.835 6.38 0.89 92.7 13.4 138.92 0.51 138 N 750 11.69 0.6272 5.358 10.4 0.81 86.9 16.1 139.65 0.45 138 O 750 10.64 0.5489 1.973 9.16 0.93 94.9 18.6 138.79 0.40 138 P 800 12.56 0.5869 7.857 15.0 0.87 81.9 22.6 141.34 0.41 139 Q 800 10.71 0.4889 1.795 12.7 1.0 95.4 26.0 140.38 0.33 140 R 850 11.71 0.4414 4.580 19.4 1.2 88.7 31.1 142.74 0.27 141 S 850 10.94 0.3741 1.897 17.7 1.4 95.2 35.8 142.84 0.24 142 T 900 12.50 0.2732 6.598 37.0 1.9 84.6 45.7 145.03 0.25 143 U 900 11.12 0.2623 2.086 25.0 1.9 94.6 52.4 144.36 0.22 144 V 950 12.45 0.6659 6.363 23.2 0.77 85.3 58.5 145.77 0.33 144 W 950 11.12 0.3256 2.677 13.4 1.6 93.1 62.1 142.21 0.32 141 X 1000 11.38 0.3446 4.121 12.7 1.5 89.5 65.5 140.01 0.41 139 Y 1000 11.59 0.3085 4.690 11.4 1.7 88.2 68.5 140.48 0.41 139 Z 1050 12.34 0.3775 7.048 12.1 1.4 83.3 71.8 141.24 0.55 139 AA 1050 12.89 0.3659 8.510 10.0 1.4 80.7 74.4 142.88 0.59 140 AB 1100 15.22 0.6169 16.97 8.15 0.83 67.3 76.6 140.86 0.73 135 AC 1100 16.90 0.5619 21.88 7.81 0.91 62.0 78.7 143.9 1.0 137 AD 1100 18.09 0.6334 25.89 11.5 0.81 57.9 81.7 144.04 0.85 136 AE 1100 18.85 0.5873 28.02 13.6 0.87 56.3 85.4 145.71 0.75 137 AF 1100 19.62 0.5717 29.91 13.8 0.89 55.1 89.0 148.40 0.90 139 AG 1200 29.39 0.6464 58.29 19.5 0.79 41.5 94.2 166.78 0.90 148 AH 1250 53.30 2.262 127.0 9.02 0.23 29.9 96.6 215.4 2.2 AI 1350 83.01 6.169 201.3 6.22 0.083 29.0 98.3 316.4 3.2 AJ 1700 44.36 4.325 90.11 6.49 0.12 40.8 100.0 242.7 2.0 Integrated age ± 1σ n=35 375.5 K2O=1.56 % 149.41 0.36
Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions. Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 27.84 Ma. Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties. Integrated age calculated by quadratically combining isotopic measurements of all steps. Integrated age error calculated by quadratically combining errors of isotopic measurements of all steps. Plateau age is inverse-variance-weighted mean of selected steps. Plateau age error is inverse-variance-weighted mean error (Taylor, 1982) times squareroot MSWD where MSWD>1. Plateau and integrated ages incorporate uncertainties in interfering reaction corrections and J factors (0.1%). Decay constants and isotopic abundances after Steiger and Jager (1977). 39 K20 estimated from Ar signal, sample weight and J-factor. D = 1 AMU discrimination in favor of light isotopes. Isochron age is age calculated using trapped initial 40Ar/36Ar determined from isochron analysis. 39 37 Correction factors: ( Ar/ Ar)Ca = 0.00074 ± 2e-05 36 37 ( Ar/ Ar)Ca = 0.000284 ± 6e-06 40 39 ( Ar/ Ar)K = 0.02895 ± 0.00059 72
Table DR3. UCLA mica argon isotopic data for Amdo orthogneiss samples.
Age ± 1 40 39 a 38 39 a 37 39 a 36 39 a 39 b Σ 39 40 c 40 39 d σ e Step T (°C) Ar/ Ar Ar/ Ar Ar/ Ar Ar/ Ar ArK(mol) ArK (%) Ar* (%) Ar*/ ArK ± 40/39 (Ma) 97-6-4-2 Muscovite (J=0.007344, weight = 5.8 mg) Total gas age = 166.4 ± 0.4 Ma 1 500 24.52636 0.72 0.08 0.06 0.00 0.4 26.7 6.57 0.40 85.1 ± 5.1 2 600 14.59611 0.02 0.07 0.01 0.00 0.9 76.7 11.27 0.12 143.4 ± 1.5 3 700 13.70792 0.01 0.03 0.00 0.00 2.5 92.4 12.68 0.02 160.7 ± 0.3 4 750 13.33618 0.01 0.02 0.00 0.00 5.8 95.9 12.80 0.03 162.1 ± 0.4 5 800 13.30572 0.01 0.01 0.00 0.00 8.3 96.1 12.80 0.02 162.1 ± 0.3 6 850 13.46846 0.01 0.01 0.00 0.00 12.8 96.0 12.94 0.03 163.8 ± 0.3 7 900 13.64588 0.01 0.01 0.00 0.00 22.5 95.8 13.08 0.04 165.5 ± 0.5 8 950 13.47412 0.01 0.00 0.00 0.00 40.8 98.0 13.23 0.03 167.2 ± 0.4 9 1000 13.48864 0.01 0.02 0.00 0.00 50.1 97.8 13.21 0.03 167.1 ± 0.3 10 1100 13.51966 0.01 0.01 0.00 0.00 68.9 97.7 13.22 0.03 167.1 ± 0.3 11 1200 13.50152 0.01 0.01 0.00 0.00 99.4 98.6 13.33 0.03 168.5 ± 0.4 12 1350 14.73287 0.01 0.51 0.00 0.00 100.0 90.9 13.46 0.09 170.1 ± 1.0 40 36 40 39 Measured Ar/ Aratm = 175 ± 3.34 and Ar/ Aratm = 297.5 ± 0.5; Abundance sensitivity = 5 ppm; Date irradiated = 08-16-2000; Date analyzed = 9-29-2000
97-6-4-1A Biotite (J=0.007351, weight = 5.6 mg) Total gas age = 163.6 ± 0.3 Ma 1 600 13.60089 0.42 0.15 0.02 0.00 2.5 47.3 6.45 0.04 83.5 ± 0.5 2 700 12.79718 0.03 0.07 0.01 0.00 10.5 85.1 10.91 0.02 139.1 ± 0.2 3 750 13.45496 0.02 0.02 0.00 0.00 19.9 94.2 12.69 0.03 160.9 ± 0.3 4 800 13.59582 0.02 0.01 0.00 0.00 31.9 97.0 13.20 0.01 167.1 ± 0.2 5 850 13.7659 0.03 0.01 0.00 0.00 37.8 97.6 13.46 0.02 170.2 ± 0.2 6 900 14.06217 0.03 0.02 0.00 0.00 42.0 96.1 13.53 0.07 171.1 ± 0.8 7 950 13.90634 0.03 0.04 0.00 0.00 47.0 95.0 13.23 0.03 167.4 ± 0.3 8 1000 13.95283 0.03 0.09 0.00 0.00 51.2 95.1 13.29 0.03 168.2 ± 0.3 9 1050 13.64354 0.03 0.13 0.00 0.00 59.7 96.0 13.11 0.02 166.0 ± 0.3 10 1100 13.47398 0.03 0.23 0.00 0.00 73.1 96.9 13.08 0.03 165.6 ± 0.3 11 1200 13.69293 0.04 0.29 0.00 0.00 89.3 98.3 13.48 0.04 170.4 ± 0.5 12 1350 13.87532 0.03 0.22 0.00 0.00 100.0 98.6 13.69 0.02 173.0 ± 0.2 40 36 40 39 Measured Ar/ Aratm = -36.9 ± 0.65 and Ar/ Aratm = 297.5 ± 0.5; Abundance sensitivity = 5 ppm; Date irradiated = 08-16-2000; Date analyzed = 9-29-2000
97-6-4-3A Biotite (J=0.007364, weight = 7.6 mg) Total gas age = 156.5 ± 0.4 Ma 1 500 25.96679 1.01 0.54 0.07 0.00 0.8 15.7 4.09 0.22 53.6 ± 2.8 2 600 9.390469 0.04 0.60 0.01 0.00 2.7 67.9 6.42 0.08 83.4 ± 1.0 3 700 10.49737 0.03 0.37 0.01 0.00 11.1 85.4 8.97 0.03 115.6 ± 0.4 4 780 13.13477 0.03 0.04 0.00 0.00 25.0 95.9 12.61 0.03 160.4 ± 0.4 5 850 13.14099 0.03 0.05 0.00 0.00 35.2 96.1 12.64 0.03 160.7 ± 0.4 6 900 13.35206 0.03 0.05 0.00 0.00 42.4 95.6 12.78 0.03 162.4 ± 0.3 7 950 13.48795 0.03 0.07 0.00 0.00 49.9 95.8 12.93 0.04 164.3 ± 0.5 8 1000 14.12381 0.03 0.29 0.00 0.00 54.6 93.2 13.19 0.03 167.4 ± 0.4 9 1050 14.23293 0.03 0.32 0.00 0.00 60.9 92.7 13.21 0.03 167.6 ± 0.4 10 1100 13.58004 0.04 0.51 0.00 0.00 74.6 96.0 13.05 0.03 165.7 ± 0.3 11 1200 13.21134 0.03 1.55 0.00 0.00 88.5 97.0 12.84 0.03 163.1 ± 0.4 12 1350 12.90057 0.03 0.53 0.00 0.00 100.0 96.9 12.52 0.02 159.3 ± 0.2 40 36 40 39 Measured Ar/ Aratm = 1115 ± 1430 and Ar/ Aratm = 297.5 ± 0.5; Abundance sensitivity = 5 ppm; Date irradiated = 08-16-2000; Date analyzed = 9-28-2000
Total gas age error is analytical only and 1σ 73
Table DR4. Arrhenius data for feldspar samples. Data shown in bold used for r0 determination.
PK-97-6-4-1A PK-97-6-4-2 PK-97-6-4-3A Step 10000/T(K) Log(D/r2) Step 10000/T(K) Log(D/r2) Step 10000/T(K) Log(D/r2) B 13.83 -7.82 B 13.83 -7.33 B 13.83 -7.80 C 13.83 -7.84 C 13.83 -7.44 C 13.83 -8.07 D 12.94 -7.25 D 12.94 -6.85 D 12.94 -7.33 E 12.94 -7.28 E 12.94 -7.00 E 12.94 -7.48 F 12.15 -6.49 F 12.15 -6.26 F 12.15 -6.64 G 12.15 -6.67 G 12.15 -6.48 G 12.15 -6.74 H 11.45 -5.90 H 11.45 -5.74 H 11.45 -5.87 I 11.45 -6.18 I 11.45 -6.07 I 11.45 -6.11 J 10.83 -5.62 J 10.83 -5.57 J 10.83 -5.52 K 10.83 -5.86 K 10.83 -5.85 K 10.83 -5.80 L 10.28 -5.44 L 10.28 -5.42 L 10.28 -5.40 M 10.28 -5.71 M 10.28 -5.61 M 10.28 -5.64 M 9.78 -5.30 M 9.78 -5.07 M 9.78 -5.20 O 9.78 -5.63 O 9.78 -5.31 O 9.78 -5.49 P 9.32 -5.25 P 9.32 -4.78 P 9.32 -5.06 Q 9.32 -5.59 Q 9.32 -5.03 Q 9.32 -5.38 R 8.90 -5.20 R 8.90 -4.52 R 8.90 -4.91 S 8.90 -5.60 S 8.90 -4.75 S 8.90 -5.23 T 8.53 -5.19 T 8.53 -4.09 T 8.53 -4.69 U 8.53 -5.46 U 8.53 -4.43 U 8.53 -5.05 V 8.18 -5.06 V 8.18 -4.14 V 8.18 -4.71 W 8.18 -5.27 W 8.18 -4.58 W 8.18 -5.16 X 7.86 -4.90 X 7.86 -4.30 X 7.86 -4.78 Y 7.86 -5.07 Y 7.86 -4.56 Y 7.86 -5.00 Z 7.56 -4.66 Z 7.56 -4.24 Z 7.56 -4.63 AA 7.56 -4.89 AA 7.56 -4.53 AA 7.56 -4.85 AB 7.28 -4.43 AB 7.28 -4.33 AB 7.28 -4.35 AC 7.28 -4.71 AC 7.28 -4.58 AC 7.28 -4.57 AD 7.28 -4.89 AD 7.28 -4.74 AD 7.28 -4.78 AE 7.28 -5.03 AE 7.28 -4.90 AE 7.28 -4.86 AF 7.28 -5.19 AF 7.28 -4.93 AF 7.28 -4.86 AG 6.79 -3.53 AG 6.79 -3.26 AG 6.79 -3.26 AH 6.57 -3.18 AH 6.57 -3.31 AH 6.57 -3.10 AI 6.16 -2.82 AI 6.16 -3.23 AI 6.16 -2.67 AJ 5.07 -2.05 AJ 5.07 -2.06 AJ 5.07 -2.09
E=42.0 kcal/mol E=35.9 kcal/mol E=46.5 kcal/mol 2 2 2 Log(D0/r0 ) = 4.6 /sec Log(D0/r0 ) = 3.2 /sec Log(D0/r0 ) = 5.72 /sec
2 2 2 Domain Log(D0/r ) Volume Domain Log(D0/r ) Volume Domain Log(D0/r ) Volume fraction fraction fraction 1 7.225 0.0240 1 6.481 0.0051 1 7.830 0.0604 2 6.340 0.0359 2 5.640 0.0332 2 6.589 0.0613 3 5.668 0.0399 3 2.857 0.2462 3 5.286 0.0794 4 4.944 0.0616 4 2.800 0.1658 4 4.966 0.0738 5 2.843 0.2137 5 2.699 0.1155 5 3.685 0.1203 6 1.924 0.1939 6 1.871 0.1766 6 2.522 0.6047 7 1.200 0.4310 7 0.710 0.2576 7 2.320 0.0000 74
APPENDIX B: Metamorphism and exhumation of the Amdo basement, Tibet: Implications for the Jurassic tectonics of the Bangong suture zone
Manuscript for submission to Journal of Metamorphic Geology
Jerome Guynn University of Arizona
Paul Kapp University of Arizona
George Gehrels University of Arizona
75
ABSTRACT
Gneisses and metapelites of the Amdo basement, located along the Bangong Suture
Zone (BSZ) in central Tibet, provide a record of metamorphism and magmatism not
available elsewhere along the suture. U-Pb ages of metamorphic zircon constrain the
timing of peak metamorphism to have occurred ~185-175 Ma, prior to the Early
Cretaceous collision between the Lhasa and Qiangtang terranes along the BSZ.
Thermobarometry of mafic garnet-amphibolites and thermometry of non-garnet bearing
amphibolites indicates P-T conditions of ~11 kbar and ~650-750°C across the entire 50 km2 exposure of gneisses. Small exposures of lower grade metasedimentary rocks are in fault contact with the gneisses along the southern edge of the basement.
Thermobarometry of garnet-kyanite schist provides peak conditions of 7-9 kbar and
~600°C and together with the gneisses record burial within a typical geotherm that was not affected by the coeval intrusion of Jurassic (185-170 Ma) granitoids.
The metamorphism and magmatism recorded by the Amdo basement is not seen elsewhere along the BSZ over 1200 km to the west (though it may be exposed to the east), despite the consumption of the Meso-Tethys Ocean between the Lhasa and
Qiangtang terranes. The conditions recorded by the Amdo basement are similar to those seen in the Coast Plutonic Complex in North America and indicate tectonism that resulted in the removal, possibly by underthrusting, of a continental arc along the southern edge of the Qiangtang terrane. This suggests that many major differences in structure and metamorphism seen along extinct continental arcs and collision zones may simply be a result of differential exhumation during later deformation. 76
INTRODUCTION
The Tibetan Plateau is composed of various terranes that accreted to the southern
Asian margin in the late Paleozoic and Mesozoic (Allégre et al., 1984; Dewey et al.,
1988; Yin & Harrison, 2000). Unraveling the geological history and deformation related to these accretionary events is important for deciphering the subsequent deformation due to the Indo-Asian collision during the early Tertiary and for understanding the development of this large, high plateau, the largest orogenic feature on earth. Not only could these accretionary collisional events have contributed to the crustal thickening of the plateau, but the location of sutures, the distribution of lithologies, and the tectonothermal processes related to accretion all can influence the style and nature of magmatism and deformation as a result of India’s collision with Asia. Tibet also provides a laboratory for studying the nature of accretionary processes in general and how the continental crust responds to collision. Of particular importance is the collision of the Lhasa terrane with the Qiangtang terrane during the Cretaceous because of the proximity of this collision to the Indo-Asian collision in both time and space and because the resulting suture zone has several enigmatic aspects.
The Lhasa-Qiangtang collision occurred during the Early Cretaceous and resulted in crustal shortening in the northern Lhasa and southern Qiangtang terranes from the
Cretaceous into the Tertiary (Murphy et al., 1997; Kapp et al., 2003; Kapp et al., 2005;
Guynn et al., 2006). The two terranes are separated by the Bangong suture zone (BSZ), a broad area of Jurassic flysch and mélange, discontinuous and isolated ophiolite fragments and Cretaceous and Tertiary nonmarine sedimentary rocks. The ophiolites fragments are 77
sparse, yet in both the west and east they occur in broad belts up to 200 km across strike
in the suture zone (Fig. 1). They represent dismembered sections of a true ophiolite
sequence and are involved in thrusts with Jurassic flysch and mélange, late Paleozoic
sedimentary rocks, Cretaceous volcanic rocks and Cretaceous-Tertiary redbeds
(Girardeau et al., 1984; Girardeau et al., 1985; Pearce & Deng, 1988). Their occurrence
over a wide north-south distance has been attributed to a large ophiolite nappe (Girardeau
et al., 1984; Coward et al., 1988), multiple sutures or oceanic basins (Pearce & Deng,
1988; Matte et al., 1996; Schneider et al., 2003) or ophiolite fragments within a mélange involved in thrust belt propagation (Kapp et al., 2003). The oceanic crust is Middle-Late
Jurassic based on stratigraphic evidence (Girardeau et al., 1985; Pearce & Deng, 1988)
and it was obducted prior to deposition of unconformably overlying Aptian-Albian
limestones and volcanic rocks (Girardeau et al., 1985; Matte et al., 1996), ~180-175 Ma
in the Donqiao area at the northern edge of the suture zone (Zhou et al., 1997).
Igneous rocks attributable to subduction of the Meso-Tethyan Ocean between the
Lhasa and Qiangtang terranes are virtually nonexistent (Allégre et al., 1984; Dewey et al.,
1988) with the exception of Jurassic granitoids within the Amdo basement just south of
the town of Amdo (Fig. 1) (Guynn et al., 2006). While slow subduction and minimal
basin closure have been suggested to account for the lack of magmatism (Dewey et al.,
1988; Schneider et al., 2003), both require a limited ocean between the Lhasa and
Qiangtang terranes, while a significant (thousands of kilometers wide) ocean is implied
by the paleomagnetic (Li et al., 2004), stratigraphic (Leeder et al., 1988; Yin et al., 1988)
and faunal (Metcalfe, 1996) evidence. Despite the lack of an exposed igneous arc, 78
subduction is generally thought to be northward, beneath the Qiangtang terrane, based on
southward verging Cretaceous thrusts within the suture zone and the northern Lhasa
terrane (Girardeau et al., 1984; Coward et al., 1988; Kapp et al., 2003).
Deeper structural levels are generally not exposed along the BSZ in central Tibet
(Fig. 1) and a lack of apparent deformation led Coward et al. (1988) to postulate
minimum deformation associated with the collision. However, more recent work has
documented extensive shortening in the northern Lhasa terrane that is attributed to the
collision (Murphy et al., 2000; Kapp et al., 2003; Kapp et al., 2005) and it has been
suggested that shortening within deeper levels is disconnected from upper levels
(Volkmer et al., accepted). Furthermore, the high grade metamorphism of the Amdo
basement, an approximately 50 km × 100 km exposure of amphibolite-facies gneisses, metasediments and intruding plutons within the suture zone north of Lhasa (Fig. 2), was proposed to have occurred in the Cambrian (Xu et al., 1985; Coward et al., 1988) but recent thermochronologic data presented by Guynn et al. (Guynn et al., 2006) revealed that the high-grade metamorphism was in fact Middle Jurassic (~180 Ma).
The Amdo basement is one of the few exposures of high-grade metamorphic rocks in the Tibetan Plateau (Harris et al., 1988) despite the protracted history of deformation in the region. The majority of the basement is composed of orthogneisses with a Cambrian or early Neoproterozoic protolith (Xu et al., 1985; Guynn et al., 2006) and it is the only documented pre-accretionary crystalline basement in Tibet. The basement also contains minor exposures of paragneiss, metasedimentary rocks and migmatites and it is intruded by Jurassic granitoids (~185-170 Ma; Guynn et al., 2006). It is the only exposure of 79
metamorphic rocks along the entire BSZ to the west, over 1200 km. Recent regional
geologic maps by Chinese geologists suggest other similar exposures to the east (Fig. 1;
Pan et al., 2004), although these remain undocumented in the literature. The igneous and
metamorphic rocks make the Amdo basement an ideal and unique place along the BSZ to study the tectonic history of the Lhasa-Qiangtang collision.
We present new geochronologic and thermobarometric data that confirm both the
presence of high-grade metamorphism and its occurrence in the Middle Jurassic (~185-
180 Ma). Our data reveals that the Amdo basement experienced temperatures of 650-
750°C and pressures over 11 kbar, indicative of a doubling of the continental crust.
These P-T conditions were experienced by the entire 50 km × 50 km of gneissic exposure in the study area. While some lower grade rocks are present, including garnet-kyanite schist that provides peak conditions of ~600°C at ~8 kbar, they are limited in area and only occur along the southern boundary between the basement and the unmetamorphosed sedimentary rocks. Thus the Bangong subduction zone experienced a major tectono- thermal event prior to collision, similar to suture zones in the North American Cordillera, and while the evidence for this tectonism is missing to the west, possible exposures to the east indicate this may have been a widespread event along the suture zone.
We also extensively analyzed mafic amphibolites that contain garnets with plagioclase coronas, a texture that is common in high-grade metamorphic terranes of the
Coast Plutonic Complex of the North American Cordillera. These coronas have been attributed to decompression reactions, depletion halos, or pro-grade metamorphism. We show that, at least for this case, these coronas developed at or near peak conditions of 80
temperature and pressure, but may be a result of decompression at slightly higher peak
pressures.
REGIONAL GEOLOGY AND PREVIOUS WORK
The Amdo basement occurs along the northern edge of the BSZ, next to the Lhasa-
Golmud highway between the towns of Nagqu and Amdo, about 200 km north of the city of Lhasa (Fig. 1,2). The most common rocks in the suture zone to the west, east, and south are Jurassic flysch deposits, sometimes referred to as the “Lake Area Flysch”, consisting of marine shales and turbidites (Leeder et al., 1988; Schneider et al., 2003).
To the north, in the Qiangtang terrane, Jurassic rocks are generally limestones or have shallow marine facies (Leeder et al., 1988; Schneider et al., 2003). The stratigraphy is difficult to define due to poor exposure, extensive deformation, and rapid changes in facies and thickness (Schneider et al., 2003). The Jurassic flysch occurs with a Jurassic mélange that includes deformed flysch, olistrosomes, Paleozoic sedimentary rocks and scattered outcrops of ophiolite fragments. Ophiolite exposures near the Amdo basement are located north around the town of Amdo, to the west around the town of Donqiao and to the south near the town of Nagqu (Fig. 1). The BSZ also includes Cretaceous limestones, volcanic rocks, and nonmarine sandstones, occasional Paleozoic strata that are involved in thrusts with the Jurassic mélange, and Cretaceous plutons (Girardeau et al., 1984; Girardeau et al., 1985; Coulon et al., 1986; Coward et al., 1988).
The Amdo basement consists mostly of orthogneisses with minor outcrops of paragneiss, amphibolite, marble, quartzite, and migmatite (Harris et al., 1988) as well as 81
marbles, quartzites, and metapelites of a lower grade along the edges (Fig. 2). The
basement is intruded by granitoids and hypabyssal dikes that account for about 30% of
the total area. Two U-Pb zircon ages have been reported for the orthogneisses, a
Cambrian (~530) age (Xu et al., 1985) and a Neoproterozoic (~850 Ma) age (Guynn et
al., 2006). Xu et al. (1985) also obtained a slightly discordant U-Pb titanite age of ~171
Ma from the orthogneiss, but attributed this to low-grade metamorphism during the
Lhasa-Qiangtang collision. However, U-Pb titanite and zircon and 40Ar/39Ar hornblende analyses by Guynn et al. (2006) documented high-grade metamorphism at ~185-180 Ma, followed by cooling to ~300°C at ~165 Ma, and than resumed cooling to upper crustal levels (< 150°C) between ~130-115 Ma. They attributed the metamorphism and initial cooling to subduction related tectonism, possibly a minor collision, and the second period of cooling to the Lhasa-Qiangtang collision, consistent with other estimates of collision timing (e.g. Dewey et al., 1988; Kapp et al., 2003). The Cretaceous collision resulted in the basement being thrust over Jurassic and Cretaceous sedimentary rocks (Coward et al.,
1988). Early Tertiary thrusting along the BSZ is undocumented but suspected (Coward et al., 1988) and in the late Tertiary to the present eastward extrusion of central Tibet has resulted in minor strike-slip and normal faults within the BSZ (Taylor et al., 2003), some of which border the Amdo basement (see Fig. 2; Kidd & Molnar, 1988).
The only previous thermobarometry of the Amdo basement comes from a sillimanite paragneiss that did not contain garnet (Harris et al., 1988). Harris et al. (1988) obtained a minimum temperature of ~570°C based on muscovite-plagioclase equilibria, but the presence of granitic leucosomes, K-feldspar, sillimanite, and muscovite indicate anatexis 82 at temperatures over 680°C. Guynn et al. (2006) also reported sillimanite in the gneisses and a kyanite-bearing schist along the southwestern contact with the sedimentary rocks.
Basement rocks
Orthogneisses compose the majority of the Amdo basement and probably represent over 90% of the metamorphic rocks exposed. They generally have well-developed foliation and banding due to segregation of mafic and felsic minerals, flattening of quartz, and alignment of biotite, but they also occur as lightly foliated granitoids. Lineations are less common and are typically a result of stretched quartz, aligned and stretched feldspars, and aligned amphibole. The orthogneisses are intermediate to felsic in composition, with granite and granodiorite being the most common protoliths, and they contain biotite, often amphibole and more rarely muscovite. Migmatites are occasionally found within the orthogneisses and may have paragneiss protoliths. The orthogneisses do not contain appropriate assemblages for geobarometry but the granodioritic gneisses contain amphibole and plagioclase that can be used for geothermometry.
Paragneisses are uncommon within the Amdo basement and were only observed in the northern part of the region, just west of the Lhasa-Golmud highway and just east of the Nyainrong road, where they occur as small (several km to < 1 km) exposures within the orthogneiss. Sillimanite and garnet were found in some of them, though rarely together, and all the samples collected show retrograde reactions.
Mafic amphibolites are ubiquitous within the orthogneisses and are composed of amphibole + plagioclase + quartz + ilmenite ± garnet ± biotite. They occur as small (< 1 83
m) pods (Fig. 3) and are usually lacking a tectonic fabric. The garnets range in size from
0.1 mm up to 2 mm and often show plagioclase coronas, from thin, discontinuous halos
to fully developed coronas (Fig. 9a). Garnets typically occur in the cores of large pods and the garnet is absent or smaller near the edges. Mineral composition and major element analysis from one sample indicate a probable gabbroic protolith (Table 1), possibly as dikes within the granitic basement that have been stretched into boudins during gneissic development. In addition, several mafic amphibolites without garnet were found in the metasedimentary rocks in the northeast field area just east of
Nyainrong. JG062204-2 appears to be a mafic dike within marble, with a foliation that matches the marble, and JG062204-4 is a foliated and lineated amphibolite that occurs within a marble and schist sequence and may represent a basalt layer within the original sedimentary protoliths.
Metasedimentary rocks that occur along the edges of the Amdo orthogneiss are predominantly marbles but also include quartzites and metapelites. The majority of the metapelites are monotonous schists composed of quartz + mica + plagioclase ± chlorite, but along the southern contact garnet-kyanite and garnet-staurolite schist were discovered. Both are a lower metamorphic grade than the orthogneiss. A small outcrop of marble and quartzite were also found within the Amdo gneiss, near an exposure of paragneiss, just south of Nyainrong. Detrital zircon data from two quartzites demonstrate maximum depositional ages in the Cambro-Ordovician (unpublished data) and together with the marbles and metapelites suggest the protoliths are Paleozoic continental shelf 84
rocks. Combined with the felsic to intermediate composition of the orthogneisses, this
establishes that the basement is continental and not an island-arc complex.
The Amdo gneisses are intruded by large, Jurassic granitoid plutons that are coeval
with the high-grade metamorphism (Guynn et al., 2006). Despite crystallization ages
similar to timing of metamorphism, the granitoids are undeformed except for narrow
mylonitic and proto-mylonitic shear zones. The bulk of the intrusions are granite, but
they also include quart monzonite, granodiorite, tonalite, and quartz-syenite. Biotite and
amphibole often show replacement by chlorite, particularly in the K-feldspar porphyritic
Jgr1.
THERMOBAROMETRY
Pressure and temperatures for rocks containing the assemblage garnet + amphibole + plagioclase + quartz + ilmenite ± biotite were determined using the amphibole- plagioclase thermometer (AP; Blundy & Holland, 1990; Holland & Blundy, 1994), the garnet-amphibole thermometer (GA; Graham & Powell, 1984), and the garnet- amphibole-plagioclase geobarometer (GAP; Kohn & Spear, 1990). Metapelites
containing garnet + biotite + aluminosilicate + plagioclase + quartz ± muscovite were
analyzed using the garnet-biotite thermometer (GP; Bhattacharya et al., 1992) and the
garnet-aluminosilicate-silica-plagioclase geobarometer (GASP; Ghent, 1976) with the
garnet activity model of Ganguly and Saxena (1984), the plagioclase activity model of
Holland and Powell (1992) and the data set calibrations of Holland and Powell (1985). A few samples that lacked garnet or aluminosilicate (e.g. amphibole & plagioclase bearing 85
orthogneiss) were analyzed for peak temperatures only using the aforementioned thermometers. All the mineral compositions are within the recommended guidelines for each geothermometer (e.g. sufficient Ca content in garnet, moderate An content of plagioclase, etc.).
The GA thermometer results in lower temperatures than the AP thermometer by 20-
120°C, or an average of 80°C (Fig. 4). Given the lack of pyroxene in the rocks and the more realistic temperatures for inclusions, we feel the GA thermometer may be closer to the actual metamorphic conditions. However, since we can use the AP thermometer in non-garnet bearing rocks, we report both sets of temperatures and use the AP thermometer to compare relative temperatures between garnet bearing and garnet absent rocks. The AP thermometer of Blundy and Holland (1990) uses two different calibrations, one based on edenite-tremolite and one based on edenite-richterite, and the latter generally results in slightly higher temperatures. As we have no reason to favor one over the other, an average of the two is reported.
While most of the garnets in the samples show some degree of growth or diffusion zoning, the other minerals generally lack any significant zonation of major elements.
However, extensive microprobe analysis shows that elemental compositions among the minerals vary within samples enough to give pressure and temperature variations up to
±10%. These variations are not consistent with edges, cores, or other differences in analysis location (see sample JG060404-1 below for an extensive discussion) and they are probably a result of variable closure temperature of minerals to diffusion, possibly enhanced by short duration at peak metamorphic conditions or minor retrograde reactions 86
below peak conditions. Nonetheless, the individual P-T analyses cluster in groups that are consistent with the mineralogy of the rock. Thus, we conducted multiple analyses for each sample, averaged the results, and report a 2-σ standard deviation based on the variance in these analyses. In addition, calculations are made using the average values of mineral compositions in the same area of the thin section. In the case of garnet zoning, locations for garnet analysis were chosen based on the specific garnet profile and generally paired with other nearby matrix mineral analyses. Only a few representative analyses and the averages are reported in the data tables.
Microprobe analyses were conducted at the University of Arizona’s Lunar and
Planetary Laboratory (LPL) on a Cameca SX50 electron microprobe equipped with 5
LiF, PET, and TAP spectrometers. Counting times for each element were 20 s with a 10 s background at an accelerating potential of 15 kV and a beam current of 10-20 nA.
Garnet-kyanite schist thermobarometry
JG060904-13 and JG061204-2 are schists composed of garnet + kyanite + biotite + muscovite + plagioclase + quartz + graphite that occur next to the orthogneiss in the southwest area of the Amdo basement (Fig. 5). The two sample outcrops are about 1 km from each other. The kyanite in JG060904-13 is generally too small to see in hand sample, but the crystals in JG061204-2 are several mm long. In addition, kyanite crystals several cm long, associated with massive quartz, were found in float around the outcrop for JG061204-2. The garnets in each are subhedral to euhedral, 0.5 – 2 mm in diameter, and generally lack inclusions aside from some quartz. Biotite and muscovite wrap 87
around the garnets and quartz and plagioclase occur as pressure shadows next to the
garnets. Schistosity is well-developed due to aligned biotite and muscovite. Biotite
occurs both in thin layers and as large porphyroblasts, while muscovite is generally
interlayered with the biotite. Quartz and plagioclase occur as small (< 50 µm), disseminated grains, with the exception of ~0.5 mm quartz veinlets in JG060904-13.
Graphite occurs as very thin layers between the micas.
In sample JG060904-13 garnets are almandine rich and show strong growth zoning
(Fig. 5a, 6a), indicated by the bell-shaped Mn profile and bowl-shaped Fe and Mg curves.
The outer ~100 µm rims of the garnets often have strong diffusion profiles, with a large rise in Mn and a drop in Fe and Mg (Fig. 6a). The “shoulders” of the garnet element curves were chosen for thermobarometric analyses as an approximation of peak conditions. The retrograde diffusion of Fe and Mg from the garnet rims should not greatly affect the biotite composition, as the schists are biotite rich (Bi > Grt) and the Fe and Mg has been evenly distributed within the biotite as indicated by the lack of biotite zoning, even next to the garnet. Kohn and Spear (2000) suggest that a Mn kick-up is a sign of a net-transfer reaction involving the breakdown of garnet as opposed to a simple net-exchange of cations since only garnet contains significant Mn. Such a reaction will result in abnormally high temperatures for the garnet-biotite thermometer. We do not think this is the case for these samples due to the euhedral shape of the garnets analyzed, which lack any reaction textures along their edges, and because the resulting P-T conditions are consistent with the mineralogy of the rocks. The Mn for the rim kick-up could have come from the biotite, which contains some Mn and is volumetrically much 88
greater than the thin diffusion rims on the garnet. Plagioclase is unzoned and andesine in
composition (An39±2).
Matrix plagioclase in JG061204-2 is An39±2, while plagioclase next to the garnet is generally more calcic (as high as An43), though it can also be less (as low as An36).
Biotite is slightly iron rich in composition, Ann52±2, with no consistent variation in composition relative to garnet. A garnet from one thin section displays growth zoning with no diffusion on the edges (Fig. 5c, 6b), while garnet from another thin section has only slight growth zoning and a thin diffusion profile (Fig. 5d, 6c). In the former case, analyses at the edge of the garnet were used in the P-T calculations, while in the latter case, the values at the start of the diffusion profile were chosen.
The garnet measurements were paired with biotite and plagioclase adjacent to the garnet when possible, but in many cases the biotite and plagioclase grains were not in direct contact with the garnet. However the biotite and plagioclase do not show any significant zoning or compositional variation related to proximity to the garnets. The resulting cluster of 25 analyses for five different garnets in JG060904-13 record P-T conditions of 7.8 ± 0.8 kbar and 610 ± 40°C, in the lower amphibolite facies and consistent with the observed mineralogy. The results for JG061204-2 (13 analyses, 5 garnets) are 7.1 ± 1.0 kbar and 560 ± 50°C, similar to JG060904-13.
Mafic amphibolite thermobarometry
Sample JG060904-11 is located near the southwest edge of the Amdo basement near the Lhasa-Golmud highway. It is a coarse grained amphibolite composed of garnet 89
(~35%) + amphibole (~55%) + plagioclase (~5%) + quartz (~5%) + ilmenite (< 1%) with garnet crystals 0.5-1.5 mm across. Few of the garnets have complete coronas and the rims that are present are very thin. Garnets without any type of corona texture were chosen for thermobarometry (Fig. 7). Plagioclase in this sample shows a considerable range of compositions from An15 to An45, with most analyses in the range An20-An40
(oligoclase-andesine). The compositions show no systematic variation based on location in the sample, such as proximity to garnet or edges versus cores, with the exception that the few plagioclase inclusions tend to have more calcic compositions (~An40).
Amphibole is ferroan tschermakite-pargasite and shows minor compositional variations as well, particularly with Mg and Fe where an increase in Mg is accompanied by a decrease in Fe. Variations are generally on the order of a couple mole percent and do not coincide with grain boundaries with other minerals. This sample does not contain any crystals of pyroxene, but in performing traverses across amphibole with the microprobe, several analyses adjacent to quartz (< 20 µm from the edge) had orthopyroxene compositions (Table 2), suggesting incipient granulite facies metamorphism.
Some of the garnets show minor element variations that appear to be growth zoning
(Fig. 8a,c). Typical elemental growth profiles in garnet include a bell-shape for Mn, a bowl-shape for Fe and Mg and a bowl-shaped core with maxima towards the edges for
Ca (e.g. Chakrabory and Ganguly, 1991; Spear, 1995). Mn, Mg, and Ca concentrations for these garnets fit that profile, but Fe typically has a bell-shaped curve like Mn. Other garnets have a change in composition from one end of the garnet to the other (Fig. 8a,b), though the Mn in these cases still shows a bell-shaped growth zoning profile. The 90
departure from the average composition seen in Figure 8a appears to be due to the
proximity of an inclusion in the garnet. The garnets contain many inclusions which are
usually quartz, but a few were found that contain coexisting amphibole and plagioclase.
The garnets all have significant diffusion profiles at their edges (Fig. 8) that show
much greater variation than generally observed in the cores. As seen in Figures 8a & 8b,
the amphiboles do not have any zoning in major elements and only the one on the right
side of garnet “C” shows any sign of being perturbed by the diffusion of elements from
the garnet rims. The lack of reaction textures at the edges of the garnets, the subhedral
shape of the garnets, and the lack of zoning in the amphiboles suggests that the profiles at
the edges of the garnet are in fact due to diffusion and not net-transfer reactions involving
the breakdown of garnet. These garnets are Mn poor (XMn ~ 0.04) and the amphibole does contain some Mn so that it could provide the Mn for the increase in the thin (25-50
µm) diffusion rim of the garnets. Given the thin diffusion rims of the garnet and the large quantity of amphibole in the sample, the redistribution of elements from the garnet will not have significantly changed the composition of the matrix amphibole. Thus, for thermobarometric calculations, average values for the cores of the garnets were combined with average values from matrix amphibole and plagioclase in the vicinity of each garnet analyzed. Note that in this context, the core of the garnet is defined as everything inside the diffusion rims. While the garnets show some growth zoning, the elemental variation within individual cores is on the same order as differences between different garnets and dividing the garnets into inner and outer cores does not result in significantly different average values for those zones. All the garnet cores fall within the composition 91
Alm54±2Prp20±4Grs22±4Sps4±1. Inclusions were analyzed using the garnet compositions immediately next to the plagioclase and amphibole. Garnet near the inclusions has slightly higher Fe, Mg, and Mn and lower Ca (Alm58±2Prp22Grs15±1Sps5±1).
The pressures and temperatures calculated using the garnet cores group together around 11 ± 0.3 kbar and 688 ± 76°C (AP: 764 ± 15°C) and indicate uppermost amphibolite facies for this rock, consistent with the mineralogy. The four inclusions record lower pressures and slightly lower temperatures that are similar to the garnet- kyanite schist: 8.6 ± 1.2 kbar and 633 ± 51°C (AP: 823 ± 41°C). These AP temperatures are anomalously high and further support the GA thermometer as being more accurate.
Sample JG060404-1 is also a garnet-amphibolite, composed of garnet + amphibole + plagioclase + quartz + ilmenite + biotite, and is from a road cut beside the Lhasa-Golmud highway at the southern edge of the Amdo basement (Fig. 1). The rock is similar to
JG060904-11 with approximate minerals abundances: 25% garnet, 55% amphibole, 10% plagioclase, 8% quartz and 2% ilmenite (biotite is minor). The sample came from the center of a ~1/2 m wide amphibolite pod within a granitic orthogneiss (Fig. 3). Toward the edges of the pod, garnet is absent and the amphibolite is foliated, with the foliation bent towards the gneissic layering.
The garnets in this sample are a similar size as those in JG060904-11 (0.5 – 1.5 mm), but have more developed coronas composed of plagioclase with minor quartz and amphibole (Fig. 9a). In some cases, the coronas are very thin (< 50 µm), while in others the garnet is almost completely absent (Fig. 9d). The plagioclase in the coronas consists of distinct, small crystal domains visible under crossed-polars, unlike the larger 92
plagioclase grains in the matrix. On average, the plagioclase in the coronas tends to be
more calcic than the matrix (An40 vs. An35), especially immediately adjacent to garnet.
The difference is small, though, and there is significant variation in all the plagioclase, such that compositions between the corona and matrix overlap. In several traverses across the coronas around garnet “A” (Fig. 9a,c), plagioclase sometimes showed a zoning pattern where it became less calcic away from the garnet but than increased towards the amphibole, but in other traverses there was no zoning (Fig. 10b). Amphibole in the coronas consists of individual, euhedral crystals or “peninsulas” from surrounding amphibole. In the latter case, the amphibole in the corona does not appear distinct from that of the matrix. Both forms of amphibole in the corona often occur in a bladed habit roughly perpendicular to the perimeter of the garnet (Fig. 9c). The amphibole in the corona shows no difference in composition from the matrix amphibole and the bladed forms show no zoning across the corona; the overall composition of the amphibole is ferroan tschermakite-pargasite. Similar to amphibole, garnet occasionally occurs in the
coronas as either individual grains or “peninsulas” still attached to the main garnet (Fig.
9a,b). Both the garnet and the amphibole display embayed grain boundaries. These
textures suggest that the garnet was larger and has been replaced by the plagioclase
coronas. Amphibole-amphibole contacts in the matrix are typically straight but quartz
and plagioclase in the matrix have rounded boundaries.
Growth zoning is not particularly well-displayed in the garnet cores, although there
are some compositional variations and the Mn displays a shallow, bell shaped curve. The garnets have typical diffusion profiles at their edges. Figure 10a and 10b show elemental 93
profiles across a garnet with an amphibole-plagioclase inclusion. In general, the core of
the garnet is flat, but the element concentrations are disturbed in the vicinity of the
inclusion, with an increase in Fe and Mg and a decrease in Ca. Part of the profile of
garnet “A” crosses garnet that protrudes into the corona (Fig. 9a,b). This profile shows that the composition of the core of this peninsula is similar to the garnet core, indicating that the diffusion profile developed after the garnet was resorbed. The diffusion profile varies around the garnet. The average core composition of the garnet is
Alm57±1Prp11±2Grs29±2Sps3±1., similar to JG060904-11 but with Prp content about 10 mol
% lower and Grs correspondingly higher. These basic patterns are the same even for a very thick corona around a small garnet, as seen in Figure 10d. The core of this garnet has the same composition as the larger garnets and the same type of diffusion profile.
Plagioclase generally shows a slight increase from An36-38 near amphibole to An42 next to the garnet, but also deviates from this trend (Fig. 10b).
Matching average garnet cores with average matrix amphibole and plagioclase results in a cluster of P-T conditions around 10.8 ± 0.6 kbar and 688 ± 29°C (AP: 746 ± 15°C).
This is very close to the P-T conditions recorded by sample JG060904-11. The nine inclusions result in an average pressure and temperature of 9.4 ± 0.7 kbar and 692 ± 72°C
(AP: 775 ± 64°C), the same temperature but slightly lower pressure than the core-matrix pairs. Temperatures are also calculated using amphibole-plagioclase pairs within the coronas and those in the matrix. The resulting temperatures are essentially the same, with the matrix minerals resulting in 754 ± 62°C (n=30) and the corona minerals in 758 ±
50°C (n=22). 94
At the edge of the amphibolite pod, garnet is absent, biotite is more abundant
(~15%) and there are small spots of plagioclase, amphibole and biotite that are similar to the coronas. However, these spots are a little smaller than the garnet (0.5-1.0 mm) and contain more amphibole and biotite, particularly in the center of the spots. Some of the amphibole is dark green to light brown, but much of it is also light-green and blue-green, suggesting it formed at a lower grade, though there is no epidote or chlorite. All the amphibole has a skeletal appearance and is formed into bands between plagioclase and quartz rich layers.
Thermometry of non-garnet bearing amphibolites
The two samples of amphibolite from east of Nyainrong were analyzed using amphibole-plagioclase thermometry. Compositions between cores and edges of grains are not consistently different and the results for the two were averaged. We report the temperatures at 10 kbar assuming conditions similar to those of the garnet-amphibolites.
The exact choice of pressure is not critical since equilibrium lines for the amphibole- plagioclase thermometer are relatively shallow and a ±5 kbar difference in pressure results in only about ± 50°C for the ed-tr thermometer and about ± 20°C for the ed-ri thermometer.
A total of 14 analyses, a core and an edge pair for 7 different grains, were performed for JG062204-2 (Fig. 11a) and result in an average temperature of 785 ± 45°C, consistent with that recorded by the garnet-amphibolites. For JG062204-4, an average of 12 95
analyses, 5 cores and 7 edges, yielded a temperature of 760 ± 60°C, similar to JG062204-
2.
Samples without thermobarometry
Several samples had distinctive mineral assemblages indicative of the metamorphic grade but were not analyzed for thermobarometry. They are included on the map in
Figure 2. Samples JG062904-2 and JG060604-1 are garnet-amphibolites from within the orthogneiss which show signs of major retrograde reactions, including mostly resorbed garnets and recrystalized amphibole. However, their mineralogy is consistent with the other garnet-amphibolites and is suggestive of similar P-T conditions. JG062505-2,
JG062005-4 and JG062005-7 are paragneiss samples that contain sillimanite; none of them contain garnet, but the last does have biotite + sillimanite + plagioclase + quartz pseudomorphs after garnet (Fig. 11b). The presence of sillimanite in these samples indicates relatively high temperatures. Other samples located in the same area as the other paragneisses contain garnet but no aluminosilicates. Sample JG052804-1 is a garnet-staurolite schist (Fig. 11c) from the metapelites along the southern boundary of the
Amdo basement, east of the garnet-kyanite schist. It contains garnet + staurolite + biotite
+ muscovite + plagioclase + quartz, mineralogy indicative of lower amphibolite facies, similar to the garnet-kyanite schist farther to the west.
Sample JG061704-3 is a paragneiss containing the mineral assemblage garnet + biotite + muscovite + sillimanite + plagioclase + K-feldspar + quartz (Fig. 11d) from the northwest area of the basement near the Lhasa-Golmud highway. The amount of 96
muscovite is very minor and interlayered with biotite. The sillimanite is fibrolitic and
also occurs with biotite and the two define the foliation within the gneiss. The garnets are
generally small, usually surrounded by biotite and fibrolite that wrap around them, and
display a variety of morphologies including atoll garnets (Fig. 11e). The presence of sillimanite and K-feldspar together with the sparse muscovite is indicative of an upper- amphibolite grade rock that has undergone some degree of partial melting due to the muscovite dehydration reaction:
muscovite + quartz → K-feldspar + Sillimanite + H2O
As seen in Figure 12, major element zoning within the garnets is complex. Fe shows a bowl shaped profile and Ca shows a roughly bell-shaped profile with a depressed center, both of which are typical of growth zoning. However, instead of the expected bell-shaped Mn and bowl-shaped Mg curves, both are essentially flat, indicating that the elements re-equilibrated at peak temperatures, eliminating the previous growth zoning.
The remaining Fe and Ca zoning suggests that the rock did not remain at peak metamorphic conditions long enough to re-equilibrate these elements. In addition, the atoll garnets and small diffusion profiles at the edges of some garnets are suggestive of retrograde reactions. Furthermore, unlike the garnet-kyanite schist, this sample contains relatively little biotite so that diffusion of elements from the garnet into the matrix biotite could greatly affect the biotite composition. All these factors suggest that the paragneiss minerals are not in equilibrium and this is supported by the P-T results. Pressures and 97
temperatures calculated using the edges of garnets in contact with biotite and plagioclase are in the sillimanite field but result in temperatures too low for the presence of K-
Feldspar. Pairing cores of the garnets with biotite and plagioclase in the matrix still results in temperatures below the muscovite dehydration curve and gives pressures well within the kyanite stability field.
U-Pb METAMORPHIC GEOCHRONOLOGY
U-Pb dating of zircons in the orthogneisses has revealed that metamorphic zircon growth occurred during the Jurassic metamorphism (Guynn et al., 2006). The metamorphic growth observed in the previous study is too thin to date using the laser system at the University of Arizona, but further dating of orthogneisses revealed thicker zircon growth that allows analysis uncontaminated by older cores. The geochronology of the Amdo orthogneisses will be presented in a separate paper and we only concentrate on the Jurassic zircon growth here. We only provide an outline of the U-Pb dating method; for a complete description, see Guynn et al. (2006). All analyses were conducted at the
University of Arizona Laserchron Center using the Laser-Ablation Multicollector
Inductively Coupled Plasma Mass Spectrometer (LA-MC-ICPMS).
Apparent ages of individual zircon analyses are reported at the 1σ level in the tables, but mean ages are reported with 2σ uncertainty (95% confidence limits). The low concentration of 235U relative to 238U for younger zircons results in higher uncertainties for 207Pb*/235U and 206Pb*/207Pb* ages relative to 206Pb*/238U ages, so the Jurassic ages are reported using the 206Pb*/238U age. The stated uncertainties on the assigned 98
crystallization ages are absolute values and include contributions from all known random
and systematic errors. Random errors are included in the data tables. Systematic errors
(2σ) are as follows: JG061604-1, 1.15%; JG061504-4, 1.50%; JG063004-1, 1.34%. All
U-Pb weighted average calculations and plots and probability density plots were made using Isoplot 3.00 (Ludwig, 2003).
Metamorphic zircon growth can usually be distinguished from lead loss by high U/Th concentrations (Mojzsis & Harrison, 2002). Uranium can form water soluble compounds (Faure, 1986) so that zircon which precipitates from metamorphic fluids has
U concentrations similar to igneous zircon while Th concentrations are uniformly low
(Fig. 13). A plot of U/Th versus 206Pb*/238U age (Fig. 14) for all the analyses from the three Amdo orthogneisses in this study and the Jurassic granitoids from Guynn et al.
(2006) shows that igneous zircons, including those from inherited grains, generally have
U/Th values less than 5, while the Jurassic zircons from the gneisses, including tips on older cores, typically have a range of U/Th values from 5 to 80. Orthogneiss zircons with
apparent ages less than the crystallization age but older than Jurassic and with
approximately the same U/Th ratio as older zircons are interpreted as magmatic zircon
that suffered lead loss during metamorphism.
Sample JG061604-1 is a Cambro-Ordovician orthogneiss that contains Jurassic zircon crystals as well as older zircons that appear to have suffered lead loss. The younger zircons yielded an average 206Pb*/238U age of 182 ± 4 Ma (Fig. 15a), the majority of which are from zircon cores. 99
Sample JG061505-4 is a paragneiss from the central part of the Amdo basement.
Many of the zircons in this metasedimentary rock have euhedral shapes with very pointed tips, but cathode-luminescence (CL) SEM images reveals that they generally have bright, rounded, or semi-angular cores that represent the original detrital zircon, while the tips are Jurassic overgrowths (Fig. 16). The tips are often large enough to shoot with a 25 or
35 µm laser diameter. Most of these tip analyses yielded mid-Jurassic ages with U/Th ratios in the range ~10-18, while the cores of grains were always older and typically have
U/Th ratios ~0.5-3 (though 8 have U/Th ratios from 3-10). The average age of all
Jurassic tip analyses is 175.2 ± 3.4 Ma (Fig. 15b).
Granodiorite orthogneiss JG063004-1 yielded very few zircons from ~3 kg of rock and the zircons were small and subhedral. All the ages for this gneiss are Jurassic with the exception of one Precambrian zircon. The weighted average for 19 well-defined, concordant analyses is 181 ± 3.3 Ma (Fig. 15c), the only Jurassic age we determined for an orthogneiss. CL-SEM images of the zircons from this sample show some complex zoning of the zircons, but analysis of different zones did not reveal any difference in ages with the exception of the one Precambrian core. However, the U/Th ratios for the zircons are very high, typically in the range of 15-80, which in almost all other cases in the Amdo basement is a sign of metamorphic zircon (Fig. 14). This suggests that these zircons are metamorphic in origin. Given that the other Jurassic granitoids show no signs of deformation except for narrow mylonitic zones and that several other orthogneisses near this sample yielded early Paleozoic ages, we tentatively regard this sample as an older orthogneiss and the Jurassic zircon as metamorphic in origin. This interpretation requires 100
that any previous zircon was thermally reset, which is unlikely, was entirely
recrystallized, or was completely dissolved within metamorphic fluids prior to new zircon
growth. There is some evidence that zircon in high-grade garnet or hornblende bearing
metamorphic rocks is not as stable as previously thought (Fraser et al., 1997).
A relative probability plot, which sums the zircon ages as normal distributions based on their determined age and uncertainty, is shown in Figure 15d and the combined curve reveals a broad peak centered ~178 Ma. Sample JG061604-1 has two peaks, but the relatively few analyses make it statistically untenable to say that these represent two periods of zircon growth. Sample JG061504-4 has a younger age than the other two samples, but it is located adjacent to JG061604-1 (< 1 km distance), and since the ages overlap at the 2σ level, the age given by the combined probability density plot may be the most accurate. We assign an age of ~178 ± 8 Ma as a robust estimate of peak metamorphism as recorded by the zircon growth.
DISCUSSION
Thermobarometric results
Plagioclase coronas around garnets have been attributed to retrograde decompression reactions (Misch & Onyeagocha, 1976; Mengel & Rivers, 1991; Valley et al., 2003), isothermal decompression (Whitney, 1992; Jamieson et al., 1995; Wernicke & Getty,
1997), incomplete prograde reactions due to a decrease in temperature (Stowell & Stein,
2005), and prograde depletion halos (Winter, 2001). A singular explanation may not 101
suffice given the differences between the coronas. For instant, in Misch and Onyeagocha
(1976), the coronas are dominantly plagioclase but still contain more amphibole and biotite than in this study and their corona plagioclase shows a distinct increase in Ca content, from ~An35 to ~An55. In Stowell and Stein (2005), amphibole is absent and the garnets are very Fe rich.
At first glance, a retrograde reaction involving the breakdown of garnet to plagioclase appears to be the simplest explanation, especially given the circular shape of the coronas and the presence of garnet within them. However, the lack of minerals other than plagioclase within the coronas provides a significant elemental balance problem involving Fe and Mg disposal and Na and Si introduction:
(Fe,Mg,Ca)3Al2Si3O12 + (Si, Na) → CaAl2Si2O8 + NaAlSi3O8 + (Fe, Mg)
garnet anorthite albite
Modeling by Misch and Onyeagocha (1976) demonstrates that even with more
amphibole and biotite present than the coronas in this study, there is an element
imbalance. Furthermore, the corona plagioclase does not show a substantial increase in
An content as might be expected and the plagioclase and amphibole in the corona, adjacent to the corona, and in the matrix all have essentially the same composition with no zoning and result in the same temperatures with the amphibole-plagioclase thermometer. This suggests that while there was a reaction from garnet to plagioclase, the reaction involved the diffusion of elements between the corona and matrix minerals, 102
which in turn would require relatively high temperatures. This also suggests that the minerals are in equilibrium, with the exception of the thin diffusion rims on the garnet,
and this is supported by the lack of reaction textures, other than the coronas, within the
rock. The occurrence of a diffusion profile across garnet within the corona (Fig. 9b,10a)
shows that the diffusion profiles, which develop as the garnet cools from peak
temperatures, formed after the creation of the coronas. Thus the pairing of the relatively
undisturbed garnet cores with amphibole and plagioclase in the matrix is providing an
estimate of peak P-T conditions when the rock formed, though it is possible the pressure
is less than the highest achieved.
This conclusion is supported by the P-T conditions recorded by garnets with coronas
in JG060404-1 that are equivalent to those without coronas in JG060904-11.
Interestingly, the mineralogy and garnet and plagioclase composition of JG060904-11 are
very similar to metadiabase dikes in the Grenville Front Tectonic Zone of Ontario which
yield almost exactly the same results (~750°C at ~11 kbar; Jamieson et al., 1995). Some
of the rocks in that study contain relict metamorphic clinopyroxene and garnets with
plagioclase and quartz coronas. The temperatures recorded by the amphibole-plagioclase
thermometer in other, non-garnet bearing rocks in the Amdo area are also consistent with
the garnet-amphibolite temperatures. This leaves the question, though, of what caused
the reaction from garnet to plagioclase. In some cases, the garnet is thought to have
existed prior to metamorphism (e.g. Wernicke & Getty, 1997), but this seems unlikely for
a gabbroic protolith presumably within the upper crust. The reaction could be the result
of isothermal decompression where the high temperature allowed the minerals in the rock 103
to equilibrate at the lower pressure. In the case of the Grenville Front Tectonic Zone,
additional thermobarometry on other lithologies gave convincing evidence for isothermal decompression (Jamieson et al., 1995). This would imply pressures greater than 11 kbar, which approaches the eclogite facies, but even if the rocks achieved those pressures a short residence time might not allow eclogitization (Jamieson et al., 1995) and equilibration at higher temperatures would erase any small eclogite domains.
Furthermore, the garnet-amphibolites record pressures in the kyanite stability field, yet the paragneisses contain sillimanite. Together with the breakdown of garnet to produce sillimanite, biotite, and plagioclase in the paragneisses, this indicates decompression at relatively high temperatures. It is unknown why one garnet-amphibolite would have more developed coronas than another, though it may have to do with subtle differences in rock composition.
A summary of the P-T data is presented in Figure 17. The observed mineralogy and rock types combined with the thermobarometry calculations reveals that the entire Amdo basement was subject to upper-amphibolite facies metamorphism. Plagioclase- amphibole thermometry indicates temperatures in the 700-800°C range. The garnet- amphibole thermometer suggests a lower temperature, perhaps ~650-750°C, which may be more reasonable given the lack of pyroxene in the mafic amphibolites, but the migmatites, K-Feldspar-sillimanite-muscovite paragneiss and pyroxene microprobe analyses from JG060409-11 also demonstrate conditions right at the amphibolite- granulite transition. The lack of epidote in the garnet-amphibolites also indicates temperatures greater than ~650-700°C. Pressures are less-constrained across the region 104
due to a lack of appropriate mineralogies for geobarometry, but the two garnet-
amphibolites analyzed record similar values of ~11 kbar. The common grade across the
gneisses requires a minimal vertical offset within the Amdo basement, approximately less
than 5 km.
The conditions recorded by the garnet-kyanite schist are ~8 kbar and ~600°C. Both this and the gneiss P-T conditions are consistent with a normal crustal geotherm (Fig. 17) and crustal burial, suggesting that the intruding Jurassic granitoids did not supply extra heat for the metamorphism. The difference in P-T between the two samples implies ~10 km of vertical offset between the two. Garnet-amphibolite sample JG060604-1 and garnet-kyanite schist sample JG060904-11 are located ~2 km from each other across the contact between the gneiss and metasedimentary rocks, so this requires a significant fault or shear zone between the two samples.
Tectonic implications
The Amdo basement underwent burial to over 30 km depth in the Middle Jurassic during closure of the Meso-Tethys Ocean between the Lhasa and Qiangtang terranes, resulting in peak temperatures over 700°C. These conditions are similar to those experienced by high-grade metamorphic rocks of the Coast Plutonic Complex along the western margin of North America, a long (> 1500 km) belt of rocks related to a
Cretaceous-early Tertiary arc. The Swakane Gneiss and Napeequa Complex, located within the Chelan and Wenatchee blocks of the North Cascades, record pressures of 9-12 kbar and temperatures of 640-740°C along a clockwise P-T path (Valley et al., 2003), 105
while the Cascade River unit of the Chelan block reached 8-9 kbar and ~650°C (Miller et al., 1993). The Skagit Gneiss Complex of the Chelan block was buried and heated to ~9
kbar and 700-800°C (Whitney, 1992; Wernicke & Getty, 1997) and than underwent
isothermal decompression to ~4 kbar and 650-700°C (Whitney et al., 1992). Further
north, the Central Gneiss Complex of the Coast Mountains, British Columbia, is a large
region of gneiss and amphibolite that underwent amphibolite and granulite facies metamorphism (Hollister & Andronicos, 2000). Mineral assemblages and thermobarometry have shown metamorphic conditions of 8-9 kbar and over 700°C
(Hollister, 1982; Hollister & Andronicos, 2000; Rusmore et al., 2005) for the Central
Gneiss Complex and, similar to the Amdo basement, these conditions are uniform over a
large (~50 km × 200 km) area. The region also underwent isothermal decompression
(Rusmore et al., 2005 and references therein).
The mechanism for burial and exhumation of these rocks is debated, but in general they are thought to be related to thickening of the arc crust coeval with magmatism. In some cases, the deformation may be related to the collision of outboard terranes accreted to the arc, and in other cases it may be due to crustal thickening along a compressive margin like the central Andes today. Similar to the Amdo basement, most of these areas are intruded by plutons, with the exception of the Swakane Gneiss. Exhumation in the
Cascades had a two-stage exhumation history similar to the Amdo basement (Wernicke
& Getty, 1997; Paterson et al., 2004). In the case of the Coast Plutonic Complex, the intruding plutons are the same age as large batholiths and volcanic sequences that are obvious expressions of the continental arc. The similarity of the P-T-t paths and the 106
magmatic history between the Amdo basement and the high-grade rocks of the Coast
Plutonic Complex is suggestive of metamorphism in an arc setting for the Amdo
basement.
While it is unknown how long it took to bury the basement or how long it resided at peak conditions, thermochronologic data presented by Guynn et al. (2006) and in this study reveal relatively rapid cooling and exhumation, > 20°C/Myr. Metamorphic zircon growth indicates peak conditions ~178 Ma, similar to the 40Ar/39Ar hornblende age of
~180 Ma of Guynn et al. (2006). The similarity between the ages suggests that either 1) the orthogneiss cooled very quickly, as the closure temperature for hornblende (~500°C) is much less than zircon (> 700°C), or 2) the zircon grew at a temperature less than its closure temperature, which would imply it may have grown at temperatures below peak conditions (Fraser et al., 1997). Mica 40Ar/39Ar data indicates the Amdo basement was at a mid-crustal level by ~165 Ma (Guynn et al., 2006). Whether erosion, compression
(thrust faults) or extension (normal faults) exhumed the basement to mid-crustal depths is difficult to ascertain given the overprint of Cretaceous and Tertiary deformation and the cover of Neogene basins. However, comparable metamorphic grade across the basement suggests a vertical ascent similar to a dome or diapir and the presence of sedimentary rocks just north of the Amdo basement points to a normal fault contact. The Amdo basement has many components of “gneiss domes” in the broad sense, including a core of high-grade orthogneisses and migmatites intruded by coeval granitoids, lower-grade metasedimentary rocks adjacent to the gneiss (“cover rocks”), and an oval shape in map view (Whitney et al., 2004). However, our mapping has not revealed any consistent 107
foliation in the orthogneiss dipping away from its center or any sign of normal faults
bordering the basement, though the bordering metasedimentary rocks do have foliations
paralleling their contact with the gneiss. An alternate explanation could be that the Amdo
basement is a structural culmination exhumed by duplexes or underthrusting in the lower
crust.
The lack of arc-related rocks along the BSZ remains enigmatic. Guynn et al. (2006) proposed that the arc is buried beneath the northern Qiangtang terrane, either by underthrusting or burial by younger sediments. The metamorphism in the Amdo basement may suggest the former. Alternately, the collision of an island-arc system, suggested by geochemical signatures of some of the ophiolite fragments (Pearce & Deng,
1988), might have led to the burial of the Amdo basement as well as the arc. Either case would be a remarkable case of subduction erosion given that there is no sign of this process to the west. The possible basement exposures to the east, however, may be an indication that the tectonics revealed by the Amdo basement are common to the BSZ and were only exposed in certain regions where the Cretaceous fold and thrust belt exhumed deeper portions of the suture zone. This idea is also supported by similar ages and styles of metamorphism and magmatism in the Rushan-Pschart suture of the Pamirs, which has been proposed to be an extension of the BSZ across the Karakorum fault to the west
(Schwab et al., 2004). This would suggest that some variations seen in the structure of collisional orogens and continental arcs, such as differences between the Sierra Nevada and the North Cascades, is a result of varying levels of exhumation rather than greatly different tectonic histories. 108
ACKNOWLEDGMENTS
The authors thank Ken Dominik of the University of Arizona Lunar and Planetary
Laboratory for indispensable help with the electron microprobe and Alex Pullen and Ross
Waldrip for invaluable field assistance. This work benefited from conversations with
Jibamitra Ganguly, Mihai Ducea, Dave Pearson, and Steve Kidder. This work was supported by NSF grant EAR-0309844 to P. Kapp and GSA, ChevronTexaco,
ExxonMobil and Tucson Gem and Mineral Show student grants to J. Guynn.
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FIGURE CAPTIONS
Figure 1. General geological map of central and southern Tibet showing the Bangong suture zone, defined by ophiolites and Jurassic mélange. “A.B.” is the Amdo Basement.
Based on Pan et al. (2004).
Figure 2. Generalized geological map of the central and western Amdo basement. Based on our mapping, satellite photos, Kidd et al. (1988) and Pan et al. (2004).
Figure 3. Mafic amphibolite pods in a felsic orthogneiss of the Amdo basement in a road-cut along the Lhasa-Golmud Highway. 117
Figure 4. Difference between calculated temperatures using the amphibole-plagioclase
(Blundy and Holland, 1990; Holland and Blundy, 1994) and the garnet-amphibole
(Graham and Powell, 1984) thermometers from garnet-amphibolites. Filled circles are sample JG060404-1 and open triangles are sample JG060904-11. Dashed line represents a 1:1 correlation.
Figure 5. Photomicrographs of garnet-kyanite schist. (a) JG061204-2 thin section #1,
showing major mineralogy. (b) Garnet “A” from JG060904-13. Line X-X’ is the
microprobe traverse in Figure 6a. (c) Garnet “GK1” from JG061204-2 thin section #1.
Line X-X’ is the microprobe traverse in Figure 6b. (c) Garnet “A” from JG061204-2
thin section #2. Line X-X’ is the microprobe traverse in Figure 6c. Dark area in the
lower left corner is from the marker used to number the thin section. Bt = biotite; Gr =
graphite; Grt = garnet; Ky = kyanite; Ms = muscovite; Pl = plagioclase; Qtz = quartz.
Figure 6. Profiles of element mole fractions and Fe/(Fe+Mg) across garnet from the
garnet-kyanite schist. Note the different scales for (Mg, Ca, Mn) and (Fe, Fe/(Fe+Mg)).
(a) Garnet “A” from JG060904-13. “A7g29” is a microprobe spot used in
thermobarometry calculations - see Table 2. (b) Garnet “GK1” from JG061204-2 thin
section #1. “gr_top.15” – see Table 2. (c) Garnet “A” from JG061204-2 thin section #2.
“Ag006” – see Table 2.
118
Figure 7. Photomicrographs of garnet-amphibolite JG060904-11. (a) Garnets “E1”
(top) and “E2” (bottom). Line X-X’ is a microprobe traverse across the two garnets,
intervening amphibole and amphibole next to “E2” (Fig. 8a). (b) Garnet “C”; X-X’ is a microprobe traverse from center of garnet into neighboring amphibole (Fig. 8b). (c)
Garnet “I”; X-X’ is a traverse across the garnet (Fig. 8c). Am = amphibole; Grt = garnet;
Ill = ilmenite; Pl = plagioclase; Qtz = quartz.
Figure 8. Profiles of element mole fractions and Fe/(Fe+Mg) across garnet from garnet-
amphibolite JG060904-11. Note the different scales for (Mg, Ca, Mn, Na) and (Fe,
Fe/(Fe+Mg)). (a) Garnet “E1” and “E2” and associated amphibole. (b) Garnet “C”. (c)
Garnet “I”. Amph = amphibole.
Figure 9. Photomicrographs and BSE images of garnet-amphibolite JG060404-4. (a)
Garnet “A”. Insets are BSE images in (b) and (c). X-X’ and Y-Y’ are microprobe
traverses shown in Figure 10a and 10c, respectively. Lines across coronas are locations
of microprobe traverses in plagioclase, Figure 10b. (b) Peninsular garnet in plagioclase corona. Line is the first part of X-X’. (c) Amphibole in the plagioclase corona. Line across corona is a microprobe traverse shown in Figure 10b. (d) BSE image of garnet
“M” with a thick plagioclase corona. X-X’ and Y-Y’ are microprobe traverses across plagioclase and garnet shown in Figure 10d. Symbols are the same as in Figures 5 and 7.
119
Figure 10. Profiles of element mole fractions and Fe/(Fe+Mg) across garnet from garnet- amphibolite JG060404-1. Note the different scales for (Mg, Ca, Mn) and (Fe,
Fe/(Fe+Mg)). (a) Garnet “A”, traverse X-X’ (b) Plagioclase anorthite (An) content across corona. (c) Garnet “A”, traverse Y-Y’. (d) Garnet “M”, traverses X-X’ and Y-
Y’. The profiles are matched up at the center of the garnet.
Figure 11. Photomicrographs of Amdo basement rocks without thermobarometry. (a)
Mafic amphibolite JG062204-2. (b) Garnet pseudomorph in paragneiss JG062005-7.
Garnet is replaced with biotite, sillimanite, plagioclase and quartz. (c) Garnet-staurolite
schist JG052804-1. (d) Garnet-sillimanite paragneiss JG061704-3 under crossed-polars to show the tartan twinning in K-Feldspar. Sillimanite is interlayered with biotite; muscovite is also interlayered with biotite but is too minor to show up in the image. (e)
Atoll garnets in JG061704-3. Symbols as in Figure 5 and 7, plus: Kfs = K-Feldspar; Sil
= Sillimanite; St = staurolite.
Figure 12. Profiles of element mole fractions and Fe/(Fe+Mg) across a garnet from the garnet-sillimanite paragneiss JG061704-3. Note the different scales for (Mg, Ca, Mn)
and (Fe, Fe/(Fe+Mg)).
Fig 13. U and Th concentrations (ppm) and resulting U/Th ratios for zircons from
orthogneiss JG061604-1. U and Th concentrations are on the left y-axis, U/Th ratio on 120
the right y-axis. Zircon age is based on 206Pb*/238U. Young (~185 Ma) ages are
metamorphic zircon.
Figure 14. Zircon ages and associated U/Th ratios from three gneisses and several
Jurassic granitoids within the Amdo basement. JG061504-4 is a paragneiss so it has a range of older ages; the range in older ages for orthogneiss JG061604-1 is interpreted as due to lead loss. Note the log-scale on the y-axis.
Figure 15. Weighted mean age diagrams for metamorphic (Jurassic) zircons in (a) orthogneiss JG061604-1, (b) paragneiss JG061504-4 and (c) orthogneiss JG063004-1.
(d) Probability density plots for these samples. The combined curve is all three added
together and has been arbitrarily scaled to be slightly larger than the other three for
readability.
Figure 16. Cathode-luminescence SEM images of zircons from JG061504-4. The
zircons show older, rounded or broken cores with younger (Jurassic) tips. The spot size
in all cases is 35 µm.
Figure 17. Pressure-temperature diagram summarizing the thermobarometry results.
Error bars and squares represent a 2σ uncertainty based on multiple analyses; also shown
as open symbols are P-T points based on the representative analyses reported in Table 2.
The gray shaded area represents normal geotherms; the thick, dashed gray line is the 121
approximate equilibrium line for the muscovite dehydration reaction: Muscovite +
Plagioclase + Quartz → K-Feldspar + Sillimanite + melt. Dashed, dark line is a
possible P-T path; high pressure limit could be higher if mineral assemblages equilibrated after significant decompression. Filled hexagon represents the temperature recorded by
40Ar/39Ar in biotite. Ages are based on geo- and thermochronology.
122
FIGURES
Figure 1. Guynn et al.
123
Figure 2. Guynn et al.
124
Figure 3. Guynn et al.
Figure 4. Guynn et al.
125
Figure 5. Guynn et al.
126
Figure 6. Guynn et al.
127
Figure 7. Guynn et al.
-
128
Figure 8. Guynn et al.
129
Figure 9. Guynn et al.
130
Figure 10.
131
Figure 11. Guynn et al.
132
Figure 12. Guynn et al.
Figure 13. Guynn et al.
133
Figure 14. Guynn et al.
134
Figure 15. Guynn et al.
135
Figure 16. Guynn et al.
136
Figure 17. Guynn et al.
137
TABLES
Table 1. Major element geochemistry of mafic amphibolite JG060404-1.
Oxide Raw Normalized
SiO2 48.24 48.62
TiO2 2.28 2.30
Al2O3 14.16 14.27 FeO 14.38 14.49 MnO 0.24 0.24 MgO 6.54 6.59 CaO 9.94 10.02
Na2O 2.48 2.50
K2O 0.68 0.69
P2O5 0.28 0.28 Total 99.22 100.00
Determined by XRF analysis at the Washington State University Geoanalytical Laboratory.
Table 2. Representative and average garnet analyses. 138 Spot ID Ag208 A ave. Bg9 B ave. Dg2a Eg087 E ave. Ig154 I ave. Gg131 gr_top.15 Ag006 A8g53 Ag295 Ag003 A7g29 Rock type grt amph grt amph grt amph grt amph grt amph grt amph grt amph grt amph grt amph grt amph g-k schist g-k schist g-k schist g-k schist g-k schist g-k schist Sample 60441 60441 60441 60441 60441 6094B 6094B 6094B 6094B 6094B 61242_1 61242_2 6094D 60441 61242_2 6094D Spot Loc. core core core core incl. core core core core inc. edge shoulder shoulder shoulder shoulder shoulder
SiO2 37.84 37.50 36.97 38.18 38.51 37.24 37.27 36.85 37.72 37.89 36.90 37.41
TiO2 0.16 0.15 0.02 0.03 0.16 0.04 0.12 0.08 0.03 0.17 0.10 0.09
Al2O3 21.25 21.11 21.36 21.60 21.31 21.56 21.07 21.49 21.20 21.03 21.38 21.09
Cr2O3 n.a. n.a. 0.01 0.00 0.00 0.00 0.04 n.a. 0.03 0.00 n.a. 0.02
Fe2O3 1.50 1.21 2.97 1.65 0.87 2.62 0.77 1.04 0.13 1.51 0.74 0.00 FeO 24.78 25.07 25.34 23.48 24.31 23.76 30.15 27.89 30.77 24.99 28.27 30.71 MnO 1.14 2.49 1.48 2.24 1.40 2.76 2.69 3.63 2.29 1.43 4.14 1.93 MgO 3.21 2.43 4.40 5.19 5.67 5.80 2.41 2.25 3.17 2.66 2.49 3.29 CaO 10.61 10.08 7.32 8.34 7.94 5.95 5.86 6.50 4.84 10.71 5.67 4.67
Na2O 0.03 0.03 0.03 0.00 0.04 0.01 0.00 0.05 0.04 0.12 0.01 0.02
K2O n.a. n.a. 0.01 0.00 0.00 0.00 n.a. n.a. 0.00 0.00 n.a. n.a. Total 100.52 100.07 99.91 100.71 100.21 99.74 100.38 99.77 100.22 100.51 99.70 99.23
Cations (12 oxygen basis) Si 2.967 2.966 2.971 2.987 2.919 2.962 2.976 2.992 2.981 2.923 2.976 2.953 3.002 2.980 2.961 3.002 Ti 0.009 0.010 0.009 0.010 0.001 0.002 0.008 0.009 0.008 0.002 0.007 0.005 0.002 0.010 0.006 0.005 Al 1.964 1.962 1.972 1.947 1.988 1.976 1.959 1.952 1.970 1.995 1.984 2.030 1.989 1.950 2.023 1.995 Cr 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.002 0.000 0.000 0.001 Fe3+ 0.088 0.094 0.072 0.075 0.177 0.096 0.075 0.051 0.056 0.155 0.047 0.062 0.008 0.089 0.045 0.000 Fe2+ 1.625 1.633 1.661 1.650 1.673 1.523 1.533 1.580 1.541 1.560 2.014 1.869 2.048 1.644 1.897 2.061 Mn 0.076 0.083 0.167 0.100 0.099 0.147 0.116 0.092 0.089 0.184 0.182 0.246 0.154 0.095 0.281 0.131 Mg 0.375 0.347 0.287 0.354 0.518 0.600 0.627 0.657 0.663 0.679 0.286 0.269 0.376 0.312 0.298 0.394 Ca 0.891 0.897 0.856 0.866 0.619 0.694 0.701 0.661 0.686 0.500 0.502 0.558 0.413 0.902 0.488 0.402 Na 0.005 0.009 0.005 0.005 0.005 0.000 0.004 0.006 0.005 0.002 0.000 0.008 0.006 0.018 0.002 0.003 K 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Sum 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0
The letter in a microprobe spot ID refers to a particular garnet; for non-garnet analyses, these spots are located in the vicinity of that garnet. Average values are reported in cations only. Rock type abbreviations: g-a amph = garnet-amphibolite; g-k schist = garnet-kyanite schist; amph = amphibolite. Sample abbreviations: 60441 = JG060404-1; 6094B = JG060409-11; 61242_1 = JG061204-2, thin section #1; 61242_2 = JG061204-2, thin section #2; 6094D = JG060904-13; 62242 = JG062204-2; 62244 = JG062204-4. Other abbreviations: incl. = inclusion; n.a. = not analyzed. Certain elements were not analyzed only after multiple analyses on that specific thin section demonstrated negligible (< 0.05 %) quantities. Core measurements refer to garnets without significant growth zoning and are located beyond any diffusion rims. Edge refers to an analysis on the very edge (within 10 µm) of a mineral. Shoulder refers to garnets with growth zoning and diffusion rims and is the point on the element profiles between the two. See Figure 6 for examples. Matrix analyses are typically at
least several 100 µm away from garnet or corona edges. Microprobe measured only total FeO; FeO and Fe2O3 determined according to the program WinAX by T.J.B. Holland (http://www.esc.cam.ac.uk/astaff/holland/ax.html).
Table 2. (Cont’d). Representative and average amphibole analyses. 139 Spot ID Ah269 Ah226 A ave. Bh7 B ave. Dh2a Eh074 E ave. Ih172 I ave. Gh135 Eh_c Eh_e Qh_c Qh_e Rock type grt amph grt amph grt amph grt amph grt amph grt amph grt amph grt amph grt amph grt amph grt amph amph amph amph amph Sample 60441 60441 60441 60441 60441 60441 6094B 6094B 6094B 6094B 6094B 62242 62242 62244 62244 Spot Loc. matrix matrix matrix matrix matrix incl. matrix matrix matrix matrix incl. core edge core edge
SiO2 39.93 41.61 41.70 40.00 41.89 42.57 42.78 42.79 42.19 42.87 43.43 45.37 44.33
TiO2 1.78 1.73 1.86 2.16 1.80 1.54 1.65 1.67 1.19 1.30 1.28 0.66 0.68
Al2O3 12.83 12.69 11.73 13.23 13.25 12.71 12.47 12.50 13.05 9.35 9.24 9.20 9.91
Cr2O3 n.a. n.a. 0.02 0.00 n.a. n.a. n.a. n.a. 0.00 n.a. n.a. n.a. n.a.
Fe2O3 4.94 2.99 2.76 3.69 7.15 6.58 5.79 6.06 6.76 4.80 4.39 6.50 5.88 FeO 13.83 14.84 14.96 13.61 9.92 10.25 10.36 10.14 9.50 12.25 12.39 10.22 10.52 MnO 0.170.18 0.19 0.130.190.210.170.180.240.290.330.340.34 MgO 9.46 9.65 9.60 9.79 11.59 11.66 11.98 11.92 11.84 11.19 11.26 12.04 11.76 CaO 11.07 11.18 11.07 11.20 10.65 10.65 10.81 10.59 11.09 11.55 11.43 11.43 11.29
Na2O 1.941.84 1.66 1.781.961.922.021.901.911.391.391.101.44
K2O 1.141.13 1.08 1.260.620.580.520.560.620.890.840.190.20 Total 97.10 97.84 96.63 96.85 99.02 98.68 98.55 98.30 98.40 95.88 95.98 97.05 96.35
Cations (23 oxygen basis) Si 6.079 6.252 6.215 6.343 6.306 6.076 6.120 6.239 6.261 6.270 6.188 6.525 6.588 6.708 6.617 Ti 0.204 0.196 0.195 0.213 0.187 0.247 0.198 0.172 0.182 0.184 0.131 0.149 0.146 0.073 0.076 Al 2.303 2.248 2.217 2.103 2.127 2.369 2.282 2.181 2.151 2.159 2.257 1.678 1.652 1.604 1.744 Cr 0.000 0.000 0.000 0.002 0.004 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Fe3+ 0.566 0.338 0.492 0.316 0.434 0.422 0.786 0.726 0.637 0.668 0.746 0.550 0.501 0.723 0.661 Fe2+ 1.761 1.865 1.788 1.902 1.803 1.728 1.212 1.245 1.268 1.244 1.166 1.559 1.572 1.264 1.313 Mn 0.022 0.023 0.023 0.024 0.023 0.017 0.024 0.025 0.021 0.022 0.030 0.037 0.042 0.043 0.043 Mg 2.146 2.161 2.165 2.176 2.204 2.216 2.524 2.557 2.613 2.602 2.588 2.538 2.545 2.653 2.616 Ca 1.806 1.800 1.768 1.804 1.789 1.823 1.667 1.664 1.695 1.663 1.743 1.884 1.858 1.811 1.806 Na 0.573 0.536 0.539 0.490 0.498 0.524 0.555 0.544 0.573 0.539 0.543 0.410 0.409 0.315 0.417 K 0.222 0.217 0.201 0.210 0.200 0.244 0.116 0.108 0.097 0.105 0.116 0.173 0.163 0.036 0.038 Sum 15.7 15.6 15.6 15.6 15.6 15.7 15.5 15.5 15.5 15.5 15.5 15.5 15.5 15.2 15.3
Table 2. (Cont’d). Representative and average plagioclase analyses for garnet amphibolites. 140 Spot ID Ap249 Ap256 Bp5 A,B ave. Dp2b Ep084 E ave. Ip188 I ave. Gp134 Rock type grt amph grt amph grt amph grt amph grt amph grt amph grt amph grt amph grt amph grt amph Sample 60441 60441 60441 60441 60441 6094B 6094B 6094B 6094B 6094B Spot Loc. matrix matrix matrix matrix incl. matrix matrix matrix matrix incl.
SiO2 61.00 57.64 58.11 56.36 59.20 60.02 56.87
TiO2 0.04 0.04 0.03 0.00 0.01 0.04 0.04
Al2O3 24.77 27.08 25.41 26.49 26.16 25.75 28.02
Cr2O3 n.a. n.a. 0.01 0.00 n.a. n.a. n.a.
Fe2O3 0.21 0.19 0.19 0.66 0.23 0.27 0.59 FeO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MnO 0.00 0.00 0.01 0.01 0.00 0.00 0.02 MgO 0.00 0.01 0.00 0.00 0.00 0.00 0.00 CaO 5.75 7.96 6.87 8.28 6.93 6.26 9.13
Na2O 8.43 7.18 7.60 6.78 7.72 8.06 6.76
K2O 0.45 0.16 0.32 0.29 0.16 0.16 0.11 Total 100.65 100.26 98.55 98.87 100.41 100.56 101.54
Cations (8 oxygen basis) Si 2.700 2.575 2.636 2.608 2.563 2.632 2.635 2.659 2.659 2.521 Ti 0.001 0.001 0.001 0.001 0.000 0.000 0.000 0.001 0.001 0.001 Al 1.293 1.426 1.359 1.387 1.420 1.371 1.361 1.345 1.345 1.465 Cr 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Fe3+ 0.007 0.006 0.006 0.010 0.022 0.008 0.009 0.009 0.009 0.020 Fe2+ 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Mn 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 Mg 0.000 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 Ca 0.273 0.381 0.334 0.364 0.403 0.330 0.318 0.297 0.297 0.434 Na 0.724 0.622 0.668 0.629 0.598 0.665 0.690 0.692 0.692 0.581 K 0.025 0.009 0.019 0.017 0.017 0.009 0.019 0.009 0.009 0.006 Sum 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 Table 2. (Cont’d). Representative and average plagioclase analyses for amphibolites and schists. 141 Spot ID Ep_c Ep_e Qp_c Qp_e GK1.7 Ap057 A2p46 A2p01 Rock type amph amph amph amph g-k schist g-k schist g-k schist g-k schist Sample 62242 62242 62244 62244 61242_1 61242_2 6094D 6094D Spot Loc. core edge core edge matrix near near near
SiO2 57.49 57.44 58.09 58.39 57.74 54.78 58.08 58.07
TiO2 0.04 0.02 0.00 0.03 0.00 0.08 0.03 0.05
Al2O3 25.99 26.74 25.92 26.16 26.82 27.37 26.43 27.18
Cr2O3 n.a. n.a. n.a. n.a. 0.04 n.a. 0.00 0.02
Fe2O3 0.23 0.32 0.23 0.29 0.14 0.17 0.11 0.18 FeO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MnO 0.02 0.00 0.01 0.01 0.01 0.00 0.00 0.03 MgO 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.01 CaO 7.26 7.99 7.38 7.38 8.48 8.87 7.74 7.78
Na2O 7.51 7.21 7.58 7.72 7.16 6.89 7.13 6.98
K2O 0.14 0.08 0.07 0.08 0.09 0.05 0.16 0.20 Total 98.68 99.80 99.29 100.07 100.45 98.21 99.68 100.50
Cations (8 oxygen basis) Si 2.607 2.579 2.617 2.611 2.578 2.512 2.605 2.584 Ti 0.001 0.001 0.000 0.001 0.000 0.003 0.001 0.002 Al 1.389 1.416 1.376 1.379 1.412 1.479 1.397 1.426 Cr 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.001 Fe3+ 0.008 0.011 0.008 0.010 0.005 0.006 0.004 0.006 Fe2+ 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Mn 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.001 Mg 0.000 0.000 0.001 0.001 0.000 0.000 0.000 0.001 Ca 0.353 0.384 0.356 0.354 0.406 0.436 0.372 0.371 Na 0.660 0.628 0.662 0.669 0.619 0.613 0.620 0.602 K 0.008 0.005 0.004 0.005 0.005 0.003 0.009 0.011 Sum 5.0 5.0 5.0 5.0 5.0 5.1 5.0 5.0
Table 2. (Cont’d). Representative and average mica analyses. 142 Spot ID GK1.8 Ab061 A2b42 A2b04 gr2tr.45 Am060 A2m45 A2m47 Rock type g-k schist g-k schist g-k schist g-k schist g-k schist g-k schist g-k schist g-k schist Sample 61242_1 61242_2 6094D 6094D 61242_1 61242_2 6094D 6094D Spot Loc. matrix near near near near near near near
SiO2 35.17 33.95 35.51 34.99 0.84 44.90 46.50 45.99
TiO2 2.42 2.14 2.07 2.41 9.97 0.74 0.53 0.48
Al2O3 19.10 19.39 19.77 20.12 46.23 37.07 37.93 37.82
Cr2O3 0.03 n.a. 0.01 0.03 0.84 0.00 0.05 0.05
Fe2O3 0.00 0.73 0.00 0.00 36.47 0.96 0.60 0.24 FeO 18.52 18.02 17.17 17.52 0.05 0.37 0.61 0.79 MnO 0.15 0.13 0.15 0.19 0.00 0.02 0.00 0.00 MgO 9.57 10.06 9.52 9.25 0.00 0.68 0.75 0.64 CaO 0.00 0.03 0.08 0.00 1.05 0.03 0.04 0.00
Na2O 0.18 0.15 0.16 0.10 0.02 0.80 0.74 0.78
K2O 9.29 8.66 8.77 9.52 0.69 9.66 10.08 10.02 Total 94.43 93.26 93.22 94.14 96.16 95.25 97.84 96.81
Cations (11 oxygen basis) Si 2.691 2.628 2.720 2.673 3.038 2.978 3.001 2.999 Ti 0.139 0.125 0.119 0.138 0.034 0.037 0.026 0.024 Al 1.723 1.769 1.785 1.812 2.825 2.898 2.886 2.908 Cr 0.002 0.000 0.001 0.002 0.001 0.000 0.003 0.003 Fe3+ 0.000 0.042 0.000 0.000 0.000 0.048 0.029 0.012 Fe2+ 1.185 1.166 1.100 1.119 0.058 0.021 0.033 0.043 Mn 0.010 0.009 0.010 0.012 0.000 0.001 0.000 0.000 Mg 1.091 1.161 1.087 1.053 0.082 0.067 0.072 0.062 Ca 0.000 0.002 0.007 0.000 0.003 0.002 0.003 0.000 Na 0.027 0.023 0.024 0.015 0.107 0.103 0.093 0.099 K 0.908 0.856 0.858 0.929 0.837 0.818 0.831 0.834 Sum 7.8 7.8 7.7 7.8 7.0 7.0 7.0 7.0
Table 3. U-Pb geochronologic analyses for Amdo metamorphic zircon determined by Laser-Ablation Multicollector ICP 143 Mass Spectrometry
Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ±206Pb*± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma) Orthogneiss JG061604-1 15t 581 11864 14.4 0.1866 4.2 0.0272 1.1 0.27 172.9 1.9 173.7 6.7 184.2 93.7 172.9 1.9 06 311 2984 35.3 0.1837 8.3 0.0275 2.6 0.31 174.7 4.4 171.3 13.0 124.4 185.1 174.7 4.4 09 511 7751 39.2 0.1819 4.9 0.0275 0.9 0.19 175.0 1.6 169.7 7.7 96.3 115.0 175.0 1.6 04t 553 5646 49.9 0.1884 2.8 0.0275 0.9 0.31 175.1 1.5 175.2 4.6 177.5 63.3 175.1 1.5 16 251 5906 49.9 0.1893 10.2 0.0282 1.7 0.16 179.2 2.9 176.0 16.5 133.8 236.7 179.2 2.9 07 429 5706 14.8 0.1857 7.9 0.0283 2.7 0.34 180.0 4.8 173.0 12.6 78.6 177.9 180.0 4.8 17 775 14382 61.2 0.1936 5.0 0.0285 3.7 0.74 181.1 6.6 179.7 8.2 161.5 78.5 181.1 6.6 08 830 10912 53.2 0.1872 4.4 0.0286 1.2 0.27 181.5 2.1 174.3 7.0 77.2 99.7 181.5 2.1 25 571 14545 56.5 0.1937 4.1 0.0286 2.0 0.50 182.0 3.7 179.8 6.8 150.8 83.5 182.0 3.7 04c 327 4212 54.3 0.1819 7.8 0.0287 1.4 0.18 182.1 2.6 169.7 12.1 -1.0 184.2 182.1 2.6 26c 282 10382 59.9 0.1978 5.3 0.0287 0.7 0.13 182.2 1.3 183.3 8.9 198.0 122.8 182.2 1.3 29 344 10588 52.7 0.1935 6.1 0.0288 0.8 0.13 182.8 1.4 179.6 10.0 137.8 141.4 182.8 1.4 19 468 11114 15.0 0.1900 4.7 0.0291 0.8 0.17 184.8 1.4 176.6 7.6 68.1 110.2 184.8 1.4 11t 427 10364 35.8 0.1919 5.9 0.0292 0.7 0.12 185.7 1.3 178.3 9.6 81.2 139.0 185.7 1.3 20 210 4845 49.8 0.2018 7.9 0.0301 0.7 0.09 191.1 1.4 186.6 13.4 130.2 185.0 191.1 1.4 Paragneiss JG061504-4 b50t1 613 9499 17.9 0.1816 2.4 0.0266 1.7 0.71 169.4 2.8 169.5 3.7 170.2 39.0 169.4 2.8 b50c 36 18037 1.2 3.1426 2.5 0.2482 1.6 0.63 1429.3 20.3 1443.2 19.4 1463.7 37.2 1463.7 37.2 b57t1 382 8634 16.4 0.1741 3.0 0.0268 2.1 0.69 170.4 3.5 163.0 4.6 55.9 52.7 170.4 3.5 b57c 327 41664 0.9 1.4269 1.5 0.1442 1.0 0.68 868.5 8.1 900.2 8.8 978.9 22.0 868.5 8.1 42t1 854 22889 14.8 0.2000 4.2 0.0269 3.0 0.70 171.0 5.0 185.1 7.2 369.6 68.8 171.0 5.0 b41t1 680 13003 16.6 0.1826 2.5 0.0270 1.7 0.70 171.7 2.9 170.3 3.9 150.0 41.8 171.7 2.9 b41c 477 27747 2.6 8.3877 3.5 0.3725 3.2 0.91 2041.2 56.7 2273.8 32.2 2490.2 24.4 2490.2 24.4 39t1 651 26161 11.7 0.1986 7.0 0.0271 3.2 0.46 172.2 5.4 183.9 11.7 337.2 140.3 172.2 5.4 b58t1 480 11872 15.0 0.1774 3.0 0.0271 2.4 0.80 172.3 4.1 165.8 4.6 74.4 42.6 172.3 4.1 05t1 492 21967 12.3 0.1980 7.3 0.0273 2.8 0.38 173.6 4.8 183.5 12.3 312.6 154.1 173.6 4.8 34t1 525 11365 16.0 0.2084 7.5 0.0273 3.3 0.44 173.9 5.6 192.3 13.1 424.0 149.9 173.9 5.6 31t2 750 7826 10.2 0.1927 3.4 0.0274 1.5 0.43 174.3 2.5 178.9 5.6 239.7 71.4 174.3 2.5 35t1 800 30468 11.6 0.1952 4.7 0.0275 2.0 0.42 174.7 3.4 181.0 7.8 264.2 97.9 174.7 3.4 43t1 565 35817 14.5 0.1899 3.9 0.0276 1.8 0.48 175.6 3.2 176.5 6.2 189.1 78.9 175.6 3.2 38t1 811 34025 11.9 0.1952 3.3 0.0280 2.0 0.63 178.3 3.6 181.0 5.4 217.2 58.5 178.3 3.6 16t1 615 7843 10.3 0.1813 4.0 0.0281 2.5 0.62 178.8 4.3 169.2 6.2 36.4 74.6 178.8 4.3 31t1 1051 7405 9.9 0.1888 5.7 0.0283 1.5 0.26 179.7 2.6 175.6 9.2 121.2 130.2 179.7 2.6 35t2 812 53797 10.9 0.2018 2.9 0.0283 1.6 0.54 179.8 2.8 186.7 4.9 273.9 55.8 179.8 2.8 10t1 808 86708 13.8 0.2054 3.8 0.0284 2.0 0.52 180.8 3.5 189.7 6.6 301.9 73.8 180.8 3.5 Table 3. (Cont’d). U-Pb geochronologic analyses for Amdo metamorphic zircon determined by Laser-Ablation 144 Multicollector ICP Mass Spectrometry
Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma) Orthogneiss JG063004-1 01C 1043 14734 12.8 0.1934 6.1 0.0283 1.9 0.30 180.0 3.3 179.5 10.0 172.7 135.8 180.0 3.3 01R 893 11938 19.4 0.1958 8.7 0.0290 0.9 0.10 184.5 1.7 181.6 14.4 143.1 202.9 184.5 1.7 03 365 5190 35.8 0.1789 13.4 0.0281 1.9 0.14 178.9 3.3 167.1 20.6 2.0 320.1 178.9 3.3 05 3714 9099 77.9 0.1920 3.7 0.0282 2.2 0.60 179.1 3.9 178.3 6.0 168.3 68.7 179.1 3.9 06T 387 4393 24.9 0.1891 19.7 0.0266 2.0 0.10 169.5 3.3 175.9 31.9 262.3 454.6 169.5 3.3 07 53 267 57.6 0.0824 87.3 0.0270 7.9 0.09 172.0 13.4 80.4 67.6 -2220.0 1277.7 172.0 13.4 08 822 6752 17.2 0.1852 4.8 0.0275 1.6 0.32 175.0 2.7 172.5 7.6 138.4 106.3 175.0 2.7 09 254 1075 16.8 0.2865 15.1 0.0421 4.4 0.29 265.6 11.5 255.8 34.1 167.4 337.8 265.6 11.5 11 49 469 75.7 0.2486 69.8 0.0267 9.5 0.14 169.7 15.9 225.5 142.0 856.5 1662.5 169.7 15.9 12 138 1631 32.2 0.2229 24.9 0.0320 2.7 0.11 203.3 5.5 204.3 46.1 216.5 580.0 203.3 5.5 13 925 6520 15.7 0.1817 8.9 0.0277 1.5 0.17 175.9 2.5 169.5 13.9 81.4 208.3 175.9 2.5 14 799 5430 19.3 0.1889 7.8 0.0280 1.0 0.13 177.9 1.8 175.7 12.5 145.2 180.8 177.9 1.8 15 1345 23033 28.8 0.1907 4.1 0.0287 2.3 0.57 182.2 4.2 177.3 6.7 111.7 80.6 182.2 4.2 16 1029 10544 22.5 0.1837 5.7 0.0284 1.9 0.32 180.7 3.3 171.2 9.0 42.1 129.1 180.7 3.3 17R 815 11477 19.9 0.2067 5.0 0.0296 1.6 0.32 188.2 3.0 190.8 8.7 222.2 109.2 188.2 3.0 18 61 1267 20.3 0.2927 43.1 0.0290 7.0 0.16 184.0 12.7 260.6 99.4 1022.0 902.2 184.0 12.7 19 1391 39990 39.6 0.1887 4.8 0.0278 2.6 0.53 176.8 4.5 175.5 7.8 158.6 96.0 176.8 4.5 22 1188 8972 30.0 0.1942 9.6 0.0277 1.8 0.19 176.4 3.2 180.2 15.9 231.1 219.1 176.4 3.2 2C 47 388 67.3 0.3270 88.7 0.0271 7.5 0.08 172.7 12.7 287.3 225.6 1368.5 8.4 172.7 12.7 B1 1227 19568 45.6 0.2018 2.6 0.0295 2.1 0.81 187.5 3.9 186.7 4.5 175.8 36.3 187.5 3.9 B2 1386 34576 29.2 0.1922 2.7 0.0283 2.3 0.83 179.6 4.1 178.5 4.5 164.2 35.5 179.6 4.1 B3 946 13632 25.1 0.2047 1.9 0.0295 1.5 0.76 187.2 2.7 189.1 3.3 212.6 29.0 187.2 2.7 B4 1281 30474 27.8 0.1967 1.9 0.0287 1.3 0.71 182.2 2.4 182.3 3.1 183.7 30.5 182.2 2.4 B5 763 34610 3.5 1.4639 4.5 0.1341 4.1 0.92 811.3 31.6 915.6 27.3 1176.2 35.8 811.3 31.6 B6 38 1470 61.6 0.1923 18.5 0.0276 9.0 0.48 175.3 15.5 178.6 30.4 221.9 377.8 175.3 15.5 B6T 185 1185 37.4 0.2151 13.5 0.0275 3.2 0.24 175.2 5.5 197.8 24.3 477.1 290.8 175.2 5.5 B7 564 25235 15.5 0.1916 2.7 0.0276 1.7 0.65 175.3 3.0 178.0 4.4 213.4 47.8 175.3 3.0 B9 28 3387 56.5 0.2482 27.3 0.0297 8.4 0.31 188.7 15.6 225.1 55.2 625.2 569.2 188.7 15.6
145
APPENDIX C: Jurassic and Cretaceous tectonic evolution of the Bangong suture zone near Amdo, central Tibet
Manuscript for submittal to GSA Bulletin
Jerome Guynn University of Arizona
Paul Kapp University of Arizona
George Gehrels University of Arizona
Lin Ding Institute of Tibetan Plateau Research and Institute of Geology and Geophysics
146
Jurassic and Cretaceous Tectonic Evolution of the Bangong
Suture Zone near Amdo, Central Tibet
Jerome Guynn
Paul Kapp,
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 100029 China
147
ABSTRACT
Unraveling the tectonic history of the mid-late Mesozoic Bangong suture zone, central
Tibet, is fundamentally important for understanding the development of the Tibetan
Plateau as it represents the last accretionary event in southern Asia prior to the Indo-
Asian collision. The Amdo basement provides a unique place to study the evolution of the suture zone due to the exposure of high-grade metamorphic rocks and granitoids not present elsewhere along the suture and to its location in the hinterland of Cretaceous fold and thrust belts in the northern Lhasa terrane. We present new geochronology, stratigraphic data and structural analysis that shed new light on the Jurassic and Early
Cretaceous history of the suture zone. We suggest that the lack of a magmatic arc along the southern Qiangtang terrane is due to subduction erosion, subduction beneath island arcs in a supra-subduction zone and burial by sediments. The occurrence of ophiolite fragments in a deformed Jurassic flysch is more consistent with an accretionary wedge setting than a single ophiolite thrust sheet, particularly in light of obduction occurring prior to continental collision between the Lhasa and Qiangtang terranes. Early
Cretaceous collision resulted in the development of a south-verging basement-cored thrust belt and corresponding foreland basin that was active at least between ~130-100
Ma. Total shortening likely exceeded 100 km. Aptian-Albian deposition of coarse clastic sediments in front of the thrust belt was simultaneous with deposition of shallow marine limestones to the south. Southward propagation of the northern Lhasa thrust belt in the Late Cretaceous – Early Tertiary was coeval with northward propagation of a 148
Gangdese retro-arc thrust belt in the southern Lhasa terrane, contributing to crustal thickening and decreased erosion prior to the Indo-Asian collision.
149
INTRODUCTION
The Bangong suture zone in central Tibet is the site of the late Mesozoic collision
between the Lhasa and Qiangtang terranes, the last of several Paleozoic-Mesozoic
accretionary events that built the southern Asian margin prior to India’s collision during
the early Tertiary (Allégre et al., 1984; Dewey et al., 1988). The suture zone is a broad
and diffuse zone defined by ophiolite fragments, Jurassic flysch and thrust related
deformation occurring over a north-south distance up to 200 km (Coward et al., 1988;
Girardeau et al., 1984; Girardeau et al., 1985; Kidd et al., 1988). The suture lacks a magmatic arc despite the evidence for a significant ocean (the Meso-Tethys) between the
Lhasa and Qiangtang terranes. Thrust belts within the suture zone generally involve
Mesozoic sedimentary rocks and only minor Paleozoic, mostly Carboniferous and
Permian, strata (Kapp et al., 2003a; Kapp et al., 2005; Kidd et al., 1988; Pan et al., 2004), part of the reason initial studies suggested the collision resulted in only minor deformation (Coward et al., 1988). More recent work, however, has documented the presence of significant crustal shortening in the northern Lhasa terrane during the
Cretaceous (Kapp et al., 2003a; Kapp et al., 2005; Murphy et al., 1997; Volkmer et al., accepted). Furthermore, the only exposure of metamorphic rocks along the suture zone, the Amdo basement, was originally thought to have undergone high-grade metamorphism in the Cambrian (Harris et al., 1988a; Xu et al., 1985), but new studies reveal Middle
Jurassic high-grade metamorphism, during oceanic subduction but prior to collision
(Guynn et al., 2006). We present new data on the timing and nature of thrust belt 150
development in the vicinity of the Amdo basement and, combined with other recent work at Amdo, present a comprehensive model for the initial development of the suture zone in this area. We also examine possible models for the pre-collisional history of the suture zone as revealed by the Amdo basement.
The question of the amount of Cretaceous shortening, and resultant crustal thickening, is important for determining the role of the Indo-Asian collision in creating the Tibetan Plateau. Models have always assumed Tibet was at sea level at the start of the Indio-Asian collision (e.g. Beaumont et al., 2001; Royden et al., 1997), but the existence of initial elevation would greatly change the results of those models. The occurrence of Aptian-Albian (100-120 Ma) limestones in the northern Lhasa terrane indicates that portions of the terrane were below sea-level at that time (Leeder et al.,
1988; Yin et al., 1988), but thrusts that involve the limestones indicate shortening in the
Late Cretaceous (Murphy et al., 1997; Volkmer et al., accepted). Shortening in the Early
Cretaceous is more controversial and the depositional environment of the limestones, and
associated redbeds, has been proposed to be a foreland basin in response to
compressional stresses (Kapp et al., 2005; Murphy et al., 1997) or a back-arc basin as a
result of extensional tectonics (Zhang, 2004; Zhang et al., 2004). Our mapping and
observations, together with the thermochronology of Guynn et al. (2006) and
geochronology presented here, indicates hinterland thrust belt development along the
suture zone in the Early Cretaceous and the presence of a foreland basin composed of
nonmarine clastic sedimentary rocks rather than carbonates.
151
REGIONAL GEOLOGY
The northernmost Tibetan terranes are the Qilan-Qaidam terranes which collided with
Asia during the mid-Paleozoic (Sobel and Arnaud, 1999; Yin and Harrison, 2000).
Between these and the Qiangtang terrane lies the Songpan Ganzi flysch complex, a thick accumulation of deformed Triassic and early Jurassic deep marine sediments (Yin and
Harrison, 2000). The southern boundary of the Songpan-Ganzi terrane is the Late
Triassic-Early Jurassic Jinsha suture, which represents the collision with the Qiangtang terrane to the south (Roger et al., 2003; Yin and Harrison, 2000). Surface geology of the
Qiangtang terrane is dominated by upper Paleozoic (Carboniferous and Permian) shallow
marine strata and Jurassic-Triassic shallow marine carbonates, fluvial deposits and
volcanic sequences, with the exception of exposures of metamorphic rocks, including
blueschists, in the central interior (Kapp et al., 2003b; Kapp et al., 2000; Yin and
Harrison, 2000). The southernmost Tibetan terrane is Lhasa, bounded by the Bangong
suture zone to the north and the Indus suture to the south. Rocks exposed in the Lhasa
terrane consist of upper Paleozoic shelf strata, Mesozoic marine and fluvial rocks, and,
especially to the south, Cretaceous-Tertiary volcanic rocks and plutons (Yin and
Harrison, 2000). The voluminous upper Cretaceous-lower Tertiary volcanic and plutonic
rocks of the southern Lhasa terrane are related to the subduction of the Neo-Tethys Ocean
beneath Tibet as India moved northward. The Indus suture is the sight of the early
Tertiary collision of India with Asia and the Himalayan mountain chain marks the
southern boundary of Tibet. 152
Paleomagnetic data for the Qiangtang and Lhasa terranes are sparse, partly due to a lack of study and partly due to Mesozoic and Cenozoic remagnetizations, but the available data are consistent with the timing of rifting events and collisions. A summary by Li et al. (2004) indicates the end of northward Qiangtang movement and separation of
Lhasa from Gondwana during the Late Triassic and a coeval latitude for Lhasa and
Qiangtang by the end of the Early Cretaceous, with a maximum separation of ~40° latitude during the Late Triassic. Thus approximately 4000 km of ocean crust had to be subducted between the Lhasa and Qiantang terranes in the Mesozoic.
The Bangong suture zone is represented by a wide region of Jurassic-Cretaceous marine flysch, scattered and discontinuous Jurassic ophiolite fragments and Cretaceous-
Tertiary nonmarine clastic rocks (Pan et al., 2004; Yin et al., 1988). Arc-related igneous rocks are largely absent along the suture (Allégre et al., 1984; Dewey et al., 1988) but subduction of the Meso-Tethys is thought to have been northward based on south- directed thrusts within the suture zone (Coward et al., 1988; Girardeau et al., 1984; Kapp et al., 2003a). Timing of ophiolite obduction is Middle-Late Jurassic based on fossil assemblages in overlying sedimentary rocks (Girardeau et al., 1984; Girardeau et al.,
1985) and 40Ar/39Ar dating of a metamorphic sole (Zhou et al., 1997). In western Tibet, two separate occurrences of ophiolites and associated mélange ~70 km apart have been interpreted as either the result of underthrusting beneath the Lhasa terrane (Kapp et al.,
2003a) or of two collision zones (Matte et al., 1996). Ophiolite fragments in the central part of the Bangong suture are restricted to the southern edge of the Qiangtang terrane, but in the eastern section between Siling Lake and the Lhasa-Golmud highway, ophiolites 153
again occur across a wide region, up to ~200 km south of the presumed southern edge of
the Qiangtang terrane (Kidd et al., 1988; Pan et al., 2004). Here the scattered exposures have been explained by a southward obducted, originally continuous ophiolite thrust sheet that has been tectonically dissected and partially eroded (Coward et al., 1988;
Girardeau et al., 1984), a transtensional rift basin that did not evolve to a full ocean
(Schneider et al., 2003) or again, as multiple suture zones, possibly involving an island arc and back-arc basin (Pearce and Deng, 1988; Pearce and Mei, 1988). Another enigmatic aspect of the Bangong suture is the apparent lack of subduction-related magmatism along almost its entire length, as indicated by the lack of Triassic-Jurassic
igneous rocks (Fig. 1), with the only documented exception being Jurassic granitoids that
intrude the Amdo basement (Guynn et al., 2006). Guynn et al. (2006) interpret the mid-
Jurassic granitoids to be related to a magmatic arc which has been buried, either tectonically or depositionally, beneath the southern Qiangtang margin.
The Amdo basement, located southeast of the town of Amdo (Fig. 1), is a large exposure of amphibolite-grade orthogneisses, with subordinate metasedimentary rocks, amphibolites and migmatites, that have been intruded by extensive Jurassic granitoids
(Coward et al., 1988; Guynn et al., 2006; Harris et al., 1988a; Schärer et al., 1986). It is generally considered part of the Lhasa terrane due to its occurrence south of ophiolite exposures at the town of Amdo (Coward et al., 1988), but if during the mid-Jurassic the southern Qiangtang margin was active and the northern Lhasa margin passive, the Middle
Jurassic timing of metamorphism implies a Qiangtang affinity. Ophiolite fragments are located north of the basement, in the town of Amdo, and south of the basement near 154
Nagqu (Girardeau et al., 1984; Pearce and Deng, 1988). The western and northern
boundaries of the basement are recently active normal and sinistral strike-slip faults,
respectively (Kidd and Molnar, 1988). Contacts between the basement and rocks on the
opposite sides of the faults are obscured by Neogene basin fill and alluvium. To the
south, the basement appears to be thrust over unmetamorphosed sedimentary rocks that
have been interpreted to be Jurassic and Paleozoic (Coward et al., 1988; Kidd et al.,
1988), Triassic (Pan et al., 2004) or Jurassic and Cretaceous (Guynn et al., 2006). The eastern exposure of the Amdo basement was not investigated in this study because of a lack of access, but regional geologic maps indicate a fault contact with Jurassic mélange
(Pan et al., 2004).
The Lhasa-Qiangtang collision is considered to be diachronous (Yin and Harrison,
2000), occurring during the Late Jurassic in the east around Amdo (Dewey et al., 1988;
Girardeau et al., 1984) and during the Early Cretaceous to the west around Bangong Lake
(Kapp et al., 2003a; Matte et al., 1996), though recent thermochronology of the Amdo
basement suggests an Early Cretaceous collision in the east as well (Guynn et al., 2006).
Coward et al. (1988) suggested minimal deformation due to the collision, but more recent
work has documented extensive Cretaceous shortening in the northern Lhasa terrane
(Kapp et al., 2003a; Murphy et al., 1997; Volkmer et al., accepted). Despite the presence of Cretaceous thrusts, Zhang et al (2004) argue that the northern Lhasa terrane was a back-arc basin of the Gangdese arc in the Aptian-Albian based on sedimentalogical provenance and geochemistry of the clastic sedimentary rocks.
155
GEOCHRONOLOGY
In order to provide provenance and age constraints on sedimentary units and cross- cutting igneous rocks, we analyzed U-Pb ages of detrital zircons from several different
sedimentary rocks and of zircons from several Cretaceous granitoids and hypabyssal
intrusions, some of which provide age constraints on faulting. We only provide an
outline of the U-Pb dating method here; for a complete description, see Guynn et al.
(2006). All analyses were conducted at the University of Arizona LaserChron Center
using the Laser-Ablation Multicollector Inductively Coupled Plasma Mass Spectrometer
(LA-MC-ICPMS).
Typically, ~25 analyses (1 analysis per zircon grain) were conducted for igneous
zircons and ~100 for detrital samples. Multiple igneous analyses allow the resulting
useable ages to be averaged to provide a higher accuracy than a single LA-MC-ICPMS
zircon analysis. At least 60 useable ages are desirable for detrital samples to adequately
characterize major peaks of the age spectrum (Dodson et al., 1988) and the additional
analyses compensate for ages that are unusable due to low U, high common Pb, high
discordance or large age uncertainty. Detrital zircon analyses are rejected if they have
more than 10% uncertainty, 30% discordance or 5% reverse discordance. The low
concentration of 235U relative to 238U results in higher uncertainties for 207Pb*/235U and
206Pb*/207Pb* ages relative to 206Pb*/238U ages for younger samples. Therefore ages less than ~1000 Ma are reported using the 206Pb*/238U age and older ages are reported using
206Pb*/207Pb* ages. 156
All analyses are plotted with 2σ or 95% confidence limits, while apparent ages of individual zircon analyses are reported at the 1σ level (Table 2). Maximum depositional ages are based on at least three ages that overlap at the 2σ level and are reported at the 2σ level. Means for igneous samples are weighted by the uncertainty but means for maximum depositional ages are not. The stated uncertainties (2σ) on the assigned crystallization ages are absolute values and include contributions from all known random and systematic errors. Random errors are included in the data tables. Systematic errors
(2σ) are as follows: JG052704-1, 0.84%; AP060504-B, 1.33%; AP062004-B, 0.96%;
JG062504-1, 1.00%; JG061805-1, 1.38%; JG061805-2, 1.90%; JG062305-1, 1.57%;
JG063005-1, 1.27%; AP052504-B, 1.15%; JG070605-1, 2.28%. Systematic errors are not factored in the detrital zircon age spectra but are included in calculations of maximum depositional age. Each samples location and age data is summarized in Table 1 and all zircon U-Pb analyses are reported in Table 2. All U-Pb plots, weighted average calculations and discordia regressions were made using Isoplot 3.00 (Ludwig, 2003).
Igneous Zircon Analysis
Sample JG052704-1 is a biotite bearing quartz-monzonite with K-Feldspar porphyroclasts to 1 cm across that intrudes the Jurassic mélange and low-grade metasedimentary rocks at the southern edge of the Amdo basement. A cluster of 15 concordant zircons gives an average age of 117.4 ± 2.2 Ma (Fig. 2). A quartz-diorite,
AP062004-B, intrudes the Jgr1 granite (JG062004-1, 170.7 ± 3.2 Ma; (Guynn et al.,
2006)) in the central part of the Amdo basement and a cluster of 25 concordant zircons 157
results in an age of 111.6 ± 1.6 Ma (Fig. 2). Small hypabyssal intrusions occur
throughout the Amdo basement as well as in some of the sedimentary rocks that surround
it. AP060504-B intrudes metapelites at the central-southern edge of the Amdo basement
and 16 zircons yield an age of 116.2 ± 2.0 Ma (Fig. 2). At the southern edge of the Amdo basement on the west side of the highway, a series of hypabyssal dikes (JG062504-1) cuts both Jurassic marine shale and overlying marble and a cluster of 12 zircons produced an average age of 105.9 ± 2.2 Ma (Fig 2).
Detrital Zircon Analysis
Concordia diagrams for six detrital zircon samples are shown in Figures 3 and 4, probability density functions (PDFs) are shown in Figure 5, and PDFs for the minimum age groups are shown in Figure 6. The PDFs are only plotted for ages less than 1200 Ma because there are fewer older ages and they do not typically occur in distinct groups, so that the peaks are negligible. Included with the PDFs is an age spectrum based on our U-
Pb dating of rocks within the Amdo basement, including orthogneisses (Guynn et al., unpublished), Jurassic granitoids (Guynn et al., 2006) and Cretaceous intrusions (this study). The broad spread of ages around the Cambrian and earliest Neoproterozoic peaks is a result of lead loss and young metamorphic growth in the upper-amphibolite facies orthogneisses, which also results in the subdued peaks compared to those from the younger igneous rocks. The maximum depositional ages and details of the age spectra are presented here; lithology and stratigraphic significance will be discussed in a later section. 158
Jurassic flysch
Sample JG061805-2, a sandstone from a sequence of thin turbidites (Fig. 9e,f), yielded only a few very small zircons. Only 73 zircons were analyzed, 8 of which were poor analyses and not included in the PDF; of the remaining 65, 26 were analyzed with a
25 µm laser spot diameter and the other 39 had to be analyzed with a 15 µm spot size.
The youngest are a group of 3 zircons that yield a mean age of 171.2 ± 14.5 (Fig. 6).
Another flysch sample, JG062305-1, comes from a strongly cleaved sequence of sandstone and mudstone to the east along the road from Nagqu to Chamdo (just beyond the mapped area). The youngest zircons in this sandstone are a group ~150 Ma; the youngest 8 yielded a mean age of 148.5 ± 10.6 Ma (Fig. 6), substantially younger than the other flysch sample. Both samples have a large group of zircons less than 300 Ma and a large spread of ages between 300 Ma and 900 Ma that do not define specific peaks.
Red arenites and shale
Sample JG061805-1 is a red arenite dominated by a large, ~500 Ma peak which encompasses 77% of the zircon ages. There is a small group at ~800 Ma and a group of three zircons defines the maximum depositional age as 178.0 ± 16.2 Ma (Fig. 6). There are two younger ages, ~154 and ~79 Ma, but these are single zircon ages and may be affected by lead loss. Red arenite sample JG063005-1 has a much broader, subdued group of ages from 400-520 Ma, but two sharp peaks at ~165 Ma and ~115 Ma; there is also a small group of Triassic ages. The younger peak, represented by seven zircon ages, 159
provides a maximum age estimate of 115.7 ± 9.0 Ma (Fig. 6), similar to the ages
determined for the Cretaceous igneous intrusions. There is a single zircon with an age of
~11 Ma, but this zircon is small and has a U concentration over 3000 ppm, which suggests that the young age is a result of lead loss due to radiation damage in the crystal.
This sandstone also contains a greater percentage of older zircons (43% over 900 Ma) than the other samples, with groups around 850-1250 Ma and 1800-2000 Ma and a ~2500
Ma peak. Sample AP052504-B has many similarities with both other red arenite samples. It has a large Cambro-Ordovician peak and a significant ~175 Ma peak. It also contains a small group ~800 Ma, a few Permo-Triassic ages and three zircon ages from
~105-115 Ma. The last group yields a maximum depositional age estimate of 108.4 ± 6.2
Ma (Fig. 6), also coeval with the Cretaceous magmatism. In general the age spectrum is comparable to JG063005-1.
Two samples of thin sandstone interbeded with thicker shales were also collected, but one did not yield any zircons and the second (JG070605-1) only yielded 26 small grains.
Two analyses resulted in young, overlapping ages, ~21 and ~24 Ma. However, both of these grains have high U concentrations (1200 and 1680 ppm) and combined with their small size (~30-35 µm) they are likely to have suffered radiation damage that resulted in lead loss. The next two youngest ages are ~106 and ~116 Ma and when averaged provide a maximum depositional age of 111 ± 8 Ma, but given the low number of analyses, this is a very tentative constraint. The sample also contains two ages ~150 Ma, a few Permo-
Triassic ages, a ~480 Ma peak and a broad, ~750 Ma peak. There are two mid-Jurassic ages, but these also have high U concentrations. In general, the age spectrum is similar to 160 that of the Cretaceous redbeds and suggests a similar source provenance and depositional timing, though there are two few analyses for a definitive conclusion.
GEOLOGY OF THE AMDO AREA
Rock Types and Ages
The oldest rocks in the Amdo area are the gneisses and metasedimentary units of the
Amdo basement. The orthogneisses are Cambrian or older (Guynn et al., 2006; Xu et al.,
1985) and have Proterozoic Nd model ages (Harris et al., 1988b). The majority of the basement consists of amphibolite facies orthogneiss with minor outcrops of paragneiss, mafic garnet-amphibolite and migmatite (Fig. 8a), but there are also lower grade metasedimentary units along the edges, which in the area mapped consist of marble, amphibolite, schist, and quartzite. U-Pb analysis of detrital zircons from two quartzites
(AP061304-1 & JG062305-3) and a paragneiss (JG061504-4) indicates a Paleozoic
(maximum Ordovician) depositional age for the former and a possible Neoproterozoic age for the latter (Guynn et al., unpublished data). The association of carbonates clastic and pelitic rocks, together with the maximum defined ages, suggests that the protoliths of the metasedimentary rocks are Paleozoic shelf deposits that are common throughout the
Lhasa and Qiangtang terranes and not Triassic-Jurassic forearc assemblages. It is difficult to estimate their thickness given the high degree of folding and deformation, some of which probably occurred during Middle Jurassic burial and exhumation.
The Amdo gneisses are intruded by extensive granitoids that are ~185-170 Ma
(Guynn et al., 2006) and make up about 30% of the Amdo basement exposure (Fig. 7). 161
In addition, we have mapped small granitoid and hypabyssal intrusions within the
basement, some of which intrude the Jurassic granitoids (Fig. 7), and several of these we
dated as mid-Cretaceous. The hypabyssal intrusions are widespread but small, with individual outcrops no more than several square meters in size. They consist of a fine- grained groundmass with phenocrysts of plagioclase, hornblende and biotite, indicating a
shallow level of emplacement, and vary in composition from rhyolite to andesite. All the
hypabyssal dikes in the area are presumed to be mid-Cretaceous in age based on our
dating.
The Middle-Late Jurassic ophiolites exposed in the vicinities of the towns of Amdo and Nagqu are assumed to be slices of the oceanic “basement” on which much of the suture zone’s Jurassic flysch and mélange was deposited (Pearce and Deng, 1988). The outcrops are only small pieces of an ophiolite sequence, generally associated with the
Jurassic flysch, and geochemical analysis suggests they represent different oceanic environments, including a supra-subduction zone (Pearce and Deng, 1988). The only outcrop examined by us consisted of completely serpentinized ultramafic rock.
The mélange and flysch in the map area consist of turbidites, alternating layers of
dark shale and fine-grained sandstone (Fig. 9e,f), or massive, dark shale. In some cases,
the sedimentary layering is well-preserved, but in general the rocks are transposed (Fig.
9g), strongly cleaved and poorly exposed, making it difficult to assign a thickness to the
unit. Other studies have indicated the flysch may be as thick as 5-6 km in places, but the
thickness and lithology are highly variable (Schneider et al., 2003; Yin et al., 1988).
These and similar units throughout northern Lhasa have sometimes been lumped together 162
as the “J-K”, indicating a poorly defined age that includes Jurassic and possibly
Cretaceous sedimentary rocks (Kidd et al., 1988; Schneider et al., 2003). Recent Chinese
regional geological maps have divided these rocks into multiple units of Triassic-Jurassic
age, including a Triassic-Jurassic and Jurassic mélange south of the Amdo basement
exposure (Pan et al., 2004). Our detrital zircon analysis of two samples indicates a
maximum Middle-Late Jurassic depositional age (~149 Ma and ~171 Ma) and suggests
they are not as young as Cretaceous nor as old as Triassic. These are younger than a “J-
K” maximum depositional age of ~270 Ma from the Lunpola basin just to the west
(Leier, 2005) but older than an Early Cretaceous maximum depositional age of ~125 Ma
for a “J-K” marine turbidite from ~300 km to the west near Nima (Kapp et al.,
unpublished). The PDFs for these two latter samples are shown with the samples from
the Amdo area in Fig. 10.
The 200-300 Ma ages suggest a provenance to the north, as Leier (2005) proposed
for the Lunpola unit based on provenance and southward paleocurrent data. Granitoids of
these ages are found in the Kunlun batholith (Harris et al., 1988b), along the Jinsha suture
(Roger et al., 2003), and in the Qiangtang terrane itself (Kapp et al., 2003b). Sandstones of the Songpan-Ganzi flysch complex also contain abundant detrital zircons with 200-300
Ma ages (Weislogel et al., 2006). The cluster of ages around 170 Ma for JG061805-2 is consistent with the ages of the Jurassic granitoids intruding the Amdo basement. There are a few ages similar to those of the Jurassic granitoids in JG062505-1 & the Lunpola sample, but not a distinct peak, even though the maximum depositional age for the former
(~149 Ma) is younger than JG061805-2. Igneous rocks with ages ~150 Ma are reported 163 from the central Qiangtang terrane (Kapp et al., 2003a; Kapp et al., 2005) and central
Lhasa terrane (Murphy et al., 1997; Volkmer et al., accepted), though the latter is precluded as a source if the marine sandstones were deposited on the Qiangtang margin during the Jurassic. The group of Cretaceous ages (~125 ma) in the Nima sample indicates depositional and structural variations along strike in the Bangong suture zone.
The abundance of older zircons in the marine sandstones and the many peaks (a result of only a few zircons per peak) may indicate significant recycling of sedimentary rocks as a source for the Jurassic marine rocks. In a more detailed study of the Jurassic flysch by
Schneider et al. (2003), they concluded that the Middle-Late Jurassic Qiangtang margin was steep, with olistrosomes and braided rivers feeding deltas on top of carbonate platforms, while the Lhasa margin had a shallow continental slope.
A wide variety of conglomerates, red sandstones and multi-colored shales crop out on the south side of the Amdo basement. It is difficult to put these in a stratigraphic context due to the poor exposure and limited extent of each unit. No complete, undeformed section is exposed within the mapped area. Detrital zircon analysis of several different samples reveals that the units are no older than mid-Cretaceous, but the data are not sufficient to define the relative ages of the units.
Clast-supported conglomerates are common along the southern edge of the basement exposure (Fig. 7) and we divided these into two separate units. One, Kc1, is composed of rounded to sub-rounded clasts of limestone, marble and carbonate breccia, typically pebble to cobble sized, intercalated with thin beds of coarse, pebbly sandstone. The sandstone and conglomerate matrix contains coarse grains of quartz. Another section of 164
this unit also contains quartzite and occasional red arenite and granitoid clasts. These
clasts are also rounded to sub-rounded, typically cobble-sized, but also with pebbles and
a few small boulders. The second unit, Kc2, is well-exposed in a stream-cut just east of
the Lhasa-Golmud highway (Fig. 9a). It consists of alternating meter thick layers of
conglomerate with rounded to sub-rounded, cobble and boulder sized clasts and several
centimeter thick sandstone layers with sub-angular and very-coarse grains. Clast
composition consists of almost every rock type in the Amdo area: marble, limestone,
quartzite, gneiss, mafic amphibolite, granitoid, volcanic-hypabyssal rock, red arenite and
dark shale. Distinctly missing are mafic or ultramafic clasts of ophiolite origin. Well- defined clast imbrication indicates a southward flow direction, assuming the beds are not overturned (Fig. 9b). While we have no direct age control, the presence of clasts of hypabyssal intrusive rocks and red arenites suggests a maximum age of ~115-105 Ma for
Kc2.
One particular section of note, located just west of the road to Nyainrong, is a hill with four different, thin layers of north-dipping limestone interbedded with Kc2 (Fig.
11a). The limestone forms ~2-5 m thick layers, whose thickness varies along strike
(Fig.11b), and is separated by conglomerate layers ~200 m thick. The entire outcrop is
~2.5 km wide. The limestone is cracked and brecciated and does not appear to contain
any fossils. The association of the limestone with a coarse conglomerate suggests a
landslide deposit, but the continuity and thin nature of the beds does not support that
hypothesis. Instead, we cautiously interpret these conglomerates as alluvial fan deposits
at the edge of a warm sea, where changes in eustatic sea level, subsidence or depositional 165
rate resulted in occasional transgressions that deposited thin carbonate beds. The
irregular thickness of the units could be due to a change in water depth between fan lobes
(Fig. 11b). In particular, given that the detrital zircon ages from the red sandstones suggest deposition in the mid-Cretaceous, we propose that these carbonates are the northern-most expression of the Aptian-Albian limestones that occur as much thicker deposits further south and west of Amdo and are conspicuously missing in this area.
Thus, they may mark the northern part of a foreland basin formed by the advancing fold and thrust belt resulting from the Lhasa-Qiangtang collision.
The red sandstones crop out widely across the southern region of the map area west of the road to Nyainrong (Fig. 7). Some exposures consist of red or occasionally tan, medium to coarse-grained sandstone interbedded with red, brown and grey shale and siltstone. In some cases, the sandstone beds are thin (< 10 cm) and intercalated with thicker (to 20 cm) mudstone beds, but in general sandstone is dominant (Fig. 9c). The sandstone includes small-scale crossbeds, channel fills and fining upwards sequences.
There are also pebbly layers with sub-angular to rounded, matrix-supported clasts with lithologies that include granite, gneiss, red sandstone and quartzite. More common exposures are red and occasionally tan, thickly bedded (10-50 cm), medium-coarse grained and pebbly sandstone with only occasional, thin siltstone beds. The pebbles are matrix supported, sub-angular to sub-rounded, and include granite, gneiss, marble and quartzite. Crossbeds are very common (Fig. 9d). While the coarser, sandier lithologies may represent a different time of deposition, mapping indicates the two occur in close proximity, and without any control on relative stratigraphy, we group them together as 166
unit Kr. Without any control on the relative stratigraphy of the different sandstone and
siltstone exposures, we group them all together as unit Kr.
Adjacent to the best exposure of Kc2 is a thin section of alternating red sandstone and red siltstone (Fig. 9c). The beds are subvertical to steeply south-dipping and crossbeds, channels, fining upwards sequences and mudcracks all indicate stratigraphic up to the north. Sample JG061805-1 from this outcrop was analyzed for detrital zircon ages. A single zircon yielded a ~154 Ma age, but a more robust maximum depositional age is
~177 Ma based on three grains. This sample has a predominant ~500 Ma peak indicative of the Cambrian Amdo orthogneisses, which suggests they were exposed at the time of deposition. A sample of red sandstone (AP052504-B) collected along strike to the east from a cross-bedded section without substantial mudstones also has the ~500 Ma peak and a well-constrained minimum age of ~108 Ma. Just south of the Lhasa-Golmud highway, another outcrop of red sandstone consists of medium-coarse grained, occasionally pebbly and K-feldspar rich, layers contain cross-beds up to ~1/2 m in height.
The beds are south-dipping but overturned and up-section (to the north) they grade into
coarse-grained sandstone and sub-angular pebble conglomerates and than to
conglomerates with sub-rounded, cobble (3-6 cm) size clasts. Clast composition is
limestone, quartzite, marble and red sandstone. Sample JG063005-1 from this section
yielded a robust maximum depositional age of ~115 Ma. It does not have a well-defined
peak at ~500 Ma, though there are some zircons around that age.
In the southwestern part of the mapped area, visible just north of the Lhasa-Golmud
highway, there is a section of light-colored shales (Ks). The shales are predominantly 167
white and light-tan, though there are also red, purple and green beds. In the lower part of
the section, the shale layers (~1/2 – 2 m thick) are interrupted by thin (few cms), tabular
beds of fine-grained gray sandstone. The number, thickness and coarseness of the
sandstone layers increases up-section and near the top there are matrix-supported
conglomerates with pebble to cobble sized clasts filling in channels. Clast composition
includes limestone, marble, quartzite, red sandstone and conglomerate. From a distance
the section appears homogeneously tilted to the north, but closer inspection reveals tight, recumbent folds within the unit (Fig. 9h). Detrital zircon analysis did not provide a reliable maximum depositional age for the unit, but suggests a similar age as the red sandstones, which is also supported by the comparable clast composition in the channel fills.
The coarse nature of the clastic sedimentary rocks, including conglomerates indicative of an alluvial fan, and the terrestrial deposition suggests a foreland basin setting for their deposition. In that context, the simplest interpretation of the stratigraphy is a coarsening upward sequence, from the shales and siltstones interbedded with sandstones, to the coarse-grained redbeds and than to the conglomerates. The different conglomerates could than be interpreted as representing an unroofing sequence of the
Amdo basement, Kc1 as the oldest and Kc2 as the youngest, with the Kc1 unit containing quartzite, granitoid and red arenite clasts representing a transition between the two.
However, it is unlikely the carbonate clasts of Kc1 indicate the Amdo gneiss was regionally unexposed, since granite and gneiss pebbles appear in the red arenites. The overall lithology of these Cretaceous sedimentary rocks is quite similar to that of the 168
Early Cretaceous rocks of the Duba formation ~200 km west-southwest. The Duba
formation contains abundant red sandstone interbedded with subordinate colored shale
and also has abundant conglomerates with clasts of limestone, quartzite, granite and
volcanic rocks (Schneider et al., 2003). The Duba formation fines upward, with the
number and thickness of shale beds increasing, and the upper part consists of dark shales that may signal the beginning of the Aptian-Albian sea transgression (Schneider et al.,
2003).
There are two tufa deposits associated with active hot springs just south of the Amdo basement (Fig. 7) that indicate active faults, though no clear sign of recent faulting was observed. North of Nyainrong, a thin (< 10 m) carbonate conglomerate rests on top of a
Jurassic granitoid and probably represents Neogene basin fill in a small graben created by a south-dipping normal fault at the northern edge of the Amdo basement.
Structure of the Amdo Basement Region
The entire Amdo orthogneiss exposure has experienced upper-amphibolite facies metamorphism with regional temperatures of ~700°C and pressures ~9-11 kbar (Guynn et al., Appendix B). Thermochronologic data along the Lhasa-Golmud highway reveal that peak metamorphism occurred ~180-185 Ma and that the entire basement had cooled to ~300°C at ~165 Ma (Guynn et al., 2006). Along the southwest contact, garnet-kyanite schist adjacent to the gneiss provides P-T conditions of 7-9 kbar at ~600°C and garnet- staurolite-biotite-muscovite schist farther to the east suggests a similar grade (Guynn et al., Appendix B). The abrupt change in grade across the contact indicates a fault between 169
the gneiss and the metapelites. Furthermore, the limited outcrop of metasedimentary rocks, representing slivers of lower-amphibolite or upper-greenschist facies rather than a full Barrovian sequence, requires that much of the stratigraphic section has been tectonically thinned or cut out.
The gneisses in the Amdo basement display a widely varying degree and orientation
of foliation and lineation development. Most often they have a well-defined foliation and
banding due to separate felsic and mafic mineral assemblages, but occasionally are only
slightly foliated. S-tectonites are most common. In general, the strike of the foliations
and the trend of the lineations are approximately northward, but there is not a consistent
pattern and they can change considerably over short distances. Detailed measurements of
foliations and small-scale fold axes in two different areas in the interior ( stereonets (a)
and (b) in Fig. 7) show that the D1 flattening fabric has been folded in a phase of D2
deformation. In the western outcrop (a), the dip of the foliation is generally steep and the
calculated fold-axis plunges slightly to the north. At the eastern outcrop (b) the
orthogneiss is highly folded and the fold-axes plunge moderately to the south and a calculated fold-axis also plunges 45° to the south. At an outcrop south of (a), the calculated fold axis trends to the east of north with a moderate plunge (stereonet (c), Fig.
7), but here foliations may be affected by the contact with the marble. The short-distance variability in fold-axes, foliations and lineations suggests a third phase, D3, of ductile deformation.
Near the southern boundaries with the metamorphic and sedimentary rocks,
particularly in the southwest, the foliation strike tends to align itself with the contact. 170
The foliation in both the gneiss and metasedimentary rocks at the contact is typically
steep and fold axes plunge moderately to the northwest (stereonet (d), Fig. 7). The
garnet-kyanite schist dips steeply to the northeast and contains microfabrics indicating
top to the southeast sense-of-shear (Fig. 12e-g). At an outcrop along the Lhasa-Golmud
highway (stereonet (e), Fig. 7), foliation dip and lineation trend is generally to the
northwest, as is the fault dip and striation trend of small faults within the orthogneiss (Fig
8b,c). Small-scale asymmetric folds and offset dikes indicate top to the southeast.
Further to the east, the orthogneiss is in contact with quart-mica-chlorite schist along an
E-W striking fault that dips 62° to the north with mineral slip fibers indicating top to the south motion. In the next drainage valley to the east, folded schist layers define a fold axis with a shallow plunge to the east (stereonet (i) in Fig. 7; 76°→33°) and mica folding and cleavage in the schist indicate a top to the north sense of shear (Fig. 12h). Schist and marble along the northeast contact with the Jurassic granite also parallel the contact and define a northwest plunging fold axis (stereonet (h) in Fig. 7; 305°→30°). Overall the fabric and structures indicate southward directed thrust movement along north dipping faults (Fig. 11d).
The intrusion of the granitoids into the gneisses was coeval with the high-grade metamorphism of the gneisses. However, the granitoids do not display any gneissic fabric or foliations with the exception of narrow shear zones. This indicates that either the high-temperature metamorphism and fabric development began some time before the granitoids intruded the gneisses or that much of the gneissic fabric was developed prior to the Jurassic metamorphism, possibly during Cambro-Ordovician orogenesis associated 171 with magma emplacement (Coward et al., 1988). In the southeast corner of the study area, the Jgr1 granitoid varies from undeformed to mylonitic zones 100s of meters wide.
S-C fabrics are not well-developed and no sense of shear was obtained. Further north along the road to Nyainrong, a zone several tens of meters wide in the same Jgr1 granitoid has a proto-mylonitic development with flattened and stretched quartz grains and bookshelf-faulted K-feldspar phenocrysts that indicate a top-to-the-north sense of shear (Fig.12a-c). In addition, an otherwise undeformed Jurassic granitoid near the shear zone contains recrystallized quartz grains (Fig. 12d).
The conglomerates and sandstones at the southern edge of the Amdo basement generally dip northward at moderately steep angles. Redbeds farther south in the vicinity of the Jurassic (?) limestone dip steeply to the south and are overturned. Folds in the shales and sandstones west of the Lhasa-Golmud highway have east-west fold axes with shallow plunges (stereonet (j); Fig. 7), indicating north-south compression. With the exception of the southern orthogneiss outcrop by the Lhasa-Golmud highway, fault surfaces were not observed due to the lack of exposure, but the relationships suggest that gneisses and metasediments are thrust southward over the Jurassic and Cretaceous sedimentary rocks. Marble is structurally above sedimentary rocks along the southern area and windows of dark shale, probably the Jurassic mélange, occur beneath the marble in the western mapped area. To the west of the Lhasa-Golmud highway, Cretaceous hypabyssal intrusions (JG062504-1) cut through the shale and the overlying marble, indicating the thrust was inactive by ~106 Ma. Along the road to Nyainrong, marble overlies a Cretaceous granitoid (~117 Ma; JG052704-1) and we interpret this as a fault 172
contact based on 1) the flat nature of the contact (Fig. 11c), 2) the lack of a metamorphic
aureole in the limestone, and 3) the occurrence of brecciated marble float near the contact
between the two. There is also brecciated and chloritized orthogneiss along the contact
between the granitoid and the orthogneiss in the same area. Between the Lhasa-Golmud
highway and the road to Nyainrong, Jurassic (?) limestone sits above the red arenites along flat or south-dipping contacts and at one locality a conglomerate composed of subangular-subrounded limestone and limestone breccia clasts cropped out between the two. We interpret the contact between the two units as a thrust fault.
DISCUSSION
Following Guynn et al. (2006), we regard the age of Lhasa-Qiangtang collision as
Early Cretaceous and not related to the Middle-Jurassic metamorphism. This conclusion is based on 1) the Early Cretaceous cooling history, 2) the development of Cretaceous thrust belts in northern Lhasa, including Amdo, as a proxy for collision, 3) the continuation of marine sedimentation during the Late Jurassic and even Early Cretaceous near Nima, 4) the change to nonmarine sedimentation in the Early Cretaceous, and 5) the paleomagnetic data which indicate Lhasa traveled northward during the Late Jurassic and
Early Cretaceous. Obduction for some of the ophiolite exposures is Middle-Late
Jurassic, but obduction often precedes collision (e.g. the Troodos and Oman ophiolites which have preceded African-Eurasian collision).
We also adopt the view of Coward et al. (1988) that subduction of the Meso-Tethys was northward based on southward directing thrusting, though this is more tenuous. 173
Furthermore, there may have been additional arcs and back-arc basins that had
subduction zones with southward subduction. Given northward subduction at the
southern Qiangtang margin, we also regard the Amdo basement as part of the Qiangtang terrane prior to Lhasa-Qiangtang collision since it was metamorphosed at the Bangong subduction zone in the Middle Jurassic.
Jurassic Tectonic Evolution of the Bangong Suture at Amdo
The presence of transposed and highly deformed turbidites and deep marine sedimentary rocks and their association with olistrosomes and small, dismembered slices of ophiolites in a “Cordilleran” style of obduction (e.g. Moores, 1982; Moores et al.,
2000) are indicative of an accretionary wedge complex, possibly in a back-arc setting, that involved offscraping or underplating of oceanic crust and its incorporation into the
wedge complex (Gray and Foster, 1998; Spaggiari et al., 2004). This precludes a single,
ophiolite thrust sheet (e.g. Girardeau et al., 1984), a conclusion reinforced by the lack of
any mafic or ultramafic rocks in the Cretaceous clastic deposits. Some of the Jurassic
mélange and ophiolite fragments show the development of oxidized and silicified
horizons followed by marine transgressions, demonstrating that they were above sea- level in the Late Jurassic and than subsided again in the latest Jurassic (Girardeau et al.,
1984; Leeder et al., 1988), indicating sufficient shortening for the accretionary wedge to become subaerial at times. The ophiolite fragments themselves have a geochemistry suggestive of a supra-subduction zone (SSZ) setting (Pearce and Deng, 1988), a young, 174
hot environment that would also explain the presence of a metamorphic sole and the lack of an accreted island arc (Hawkins, 2003; Zhou et al., 1997).
The specific tectonic event that led to the burial, metamorphism and initial exhumation of the Amdo basement is difficult to assess given the overprinting of later tectonics and basin development. One possibility is that the Amdo basement was a piece of the Qiangtang terrane that rifted off, creating a back-arc basin, and then collided with the Qiangtang margin, possibly as a result of an island arc arriving at the trench, closing the back-arc basin and burying the Amdo basement (Fig. 13a). This scenario provides a supra-subduction zone setting for the generation of the different types of ophiolite within the Bangong suture zone (Pearce and Deng, 1988). The island arc complex itself would have been underthrust and tectonically eroded or buried by subsequent marine sedimentation, as would much of the Amdo basement and any continental arc that developed on the basement (Fig. 13b). This collision also would have resulted in ophiolite “obduction” and incorporation into the Jurassic mélange and overthrusting of the mélange onto the Amdo basement and Qiangtang terrane. Regional geologic maps indicate large areas of Triassic sedimentary rocks just north of the Bangong suture (Pan et al., 2004) which may represent areas of elevated topography and erosion on the southern
Qiangtang margin due to this collision. Subsequent exhumation, possibly by extension, would have thinned the thickened crust and dropped unmetamorphosed sedimentary rocks, including the mélange and ophiolites, against the Amdo basement (Fig. 13c). A new convergent boundary and accretionary wedge would than be re-established (Fig.
13d). During the Early Cretaceous, continental collision with the Lhasa terrane resulted 175
in the Jurassic and Early Cretaceous marine rocks being thrust onto the Lhasa terrane’s
passive margin (Fig. 13e). The Late Jurassic – Early Cretaceous Meso-Tethys Ocean also may have contained additional small island arc complexes, rifted continental fragments, and back-arc basins that could account for the widespread Jurassic flysch and ophiolite fragments to the south of the Amdo basement, the rapid changes in thickness
and lithology of the Jurassic flysch (Schneider et al., 2003), the continuation of marine
sedimentation into the Cretaceous near Nima, and the lack of Late Jurassic arc
magmatism. The closure of the Meso-Tethys in this region may be analogous, on a
smaller scale, to final closure of the Mediterranean Sea, resulting in multiple sutures,
ophiolites of different ages and formational settings, and a variety of sedimentary basins
within a wide “suture” zone.
Cretaceous Tectonic Evolution of the Bangong Suture at Amdo
A detailed cross-section across the area is difficult to construct due to the limited
exposure, poor age constraints on the sedimentary units involved, and late Tertiary strike-
slip and normal faulting. In addition the change from flat to steeply dipping contacts and
different lithologies along strike indicates corresponding changes in structure. However,
a reasonable regional model can be developed by taking into account the following
constraints:
1) Regional ophiolite obduction during the Middle-Late Jurassic, the occurrence of
ophiolite fragments to the north and south of the Amdo basement, and continued marine
deposition in the Late Jurassic. 176
2) Exhumation of the Amdo basement to upper crustal levels (< 4 km) from ~130 Ma to ~115 Ma.
3) Undeformed Jurassic granitoids except for shear zones that indicate thrusting at mid-crustal levels.
4) The presence of high-grade rocks (> 600°C) across the entire Amdo basement.
5) Deformed red sandstones as young as ~108 Ma and the presence of deformed alluvial fan conglomerates approximately the same age.
6) A ~117 Ma granite cut by a southern fault and a southern fault cut by ~106 Ma intrusions.
7) The following contact relationships from south to north: Metamorphic rocks thrust over unmetamorphosed sedimentary rocks; upper amphibolite facies gneisses thrust over lower-middle amphibolite facies metapelites; 50 km width of the gneisses; unmetamorphosed sedimentary rocks only a few kilometers north of the gneisses (and possibly in contact with the gneisses in an unmapped region of the Amdo basement).
8) Unmetamorphosed Paleozoic sedimentary rocks are not involved in the thrust belt south of the Amdo basement.
9) The Amdo basement is thrust over Cretaceous nonmarine sediments in the west, but to the east these rocks are “missing” and the gneisses are in contact with Jurassic marine rocks.
In Figure 14a, we start with an idealized stratigraphy composed of basement, a thin cover of metamorphosed sedimentary rocks, a thin section of Paleozoic and Triassic sedimentary rocks and a thick section of Jurassic flysch and mélange. The northern end 177
of this undeformed cross-section could be significantly different but not affect the
resulting model, as long as the basement is buried to > 10 km. Also, we do not include
the initial thickening of the basement due to thrust faulting as indicated by the granitoid
mylonites. The initiation of the thrust belt places a long section of basement on top of the
metasedimentary rocks in a flat-on-flat relationship (Fig. 14b). The second step involves
thrusting the basement over the rest of the section and on top of the Jurassic strata, taking
a horse of the metasedimentary rocks with it (Fig. 14c). We also show the development
of a foreland basin, the erosion of the frontal part of the thrust sheet and the start of the
thrust sheet overriding the foreland basin. This step of thrust belt development occurs
~130-115 Ma based on the K-feldspar 40Ar/39Ar cooling data for the Amdo basement and establishes the long flat of basement rocks. The initiation of a thrust ramp within the weak Jurassic unit results in the northward tilting of the gneiss-metasediment flat-on-flat
thrust, thrusting of the Jurassic flysch over the Cretaceous clastic sedimentary rocks of
the foreland basin and folding and thrusting within the Cretaceous rocks (Fig. 14d). This
is essentially the relationship seen in the eastern part of the mapped area (Fig. 7).
Another ramp in the Jurassic could result in a fault-bend anticline that structurally
elevates the Jurassic-over-Cretaceous fault, exposing the Cretaceous sedimentary rocks
(Fig. 14e), as seen in the eastern part of the mapped area (Fig. 7).
The total amount of shortening in this model is ~115 km. Assuming a thrust belt
development from ~130 Ma to ~100 Ma, this is a shortening rate of ~3.8 km/Myr, or 3.8
mm/yr, a typical rate for thrust belts. This model does not take into account intra-
basement thickening or additional folds and thrusts within the Jurassic and Cretaceous 178
sedimentary units, which would increase the amount of shortening. The shortening
calculation is only an approximate value based on a simplified model of the Amdo
basement area, but does suggest a typical fold and thrust belt setting for exhumation of
the Amdo basement with a reasonable amount of deformation.
The Amdo basement region provides a record of continuous Early Cretaceous shortening in the Bangong suture zone. The basement cooling, thrust faulting, and folding of nonmarine sedimentary rocks in the Aptian-Albian does not support an extensional setting for the Bangong suture zone at this time as proposed by Zhang et al.
(2004) and Zhang (2004). We view the basement, which is located at the northernmost edge of the suture zone, as a hinterland to the northern Lhasa fold and thrust belt which developed in the Late Cretaceous and Early Tertiary farther south (Murphy et al., 1997;
Volkmer et al., accepted). The formation of a foreland basin south of the thrust belt
(e.g. Kapp et al., 2005; Murphy et al., 1997) resulted in a mid-Cretaceous seaway and the deposition of Aptian-Albian limestone in deeper areas and nonmarine clastic sediments in the north near the leading edge of the thrust belt. The continued development of the thrust belt eventually incorporated the early foreland basin, including the Aptian-Albian limestone. Thus, shortening in the Lhasa terrane occurred throughout the Cretaceous, leading to significant underthrusting of the Lhasa terrane beneath the Qiangtang terrane
(Kapp et al., 2003a).
The Lhasa terrane appears to have experienced simultaneous foreland basin development in the north and south during the mid-Cretacous, the latter due to the Lhasa-
Qiangtang collision and the former due to a Gangdese retroarc fold and thrust belt (Kapp 179
et al., submitted; Leier et al., in press). Two facing mountain belts that “met” in the middle of the Lhasa terrane in the Late Cretaceous would create a significant area of thickened crust with a decreased topographic gradient towards the center, which would reduce the erosion rate and provide an intermontane setting to accumulate sediment, resulting in an elevated, low relief region prior to the Indo-Asian collision. The lack of denudation after Cretaceous crustal thickening is supported by the Early Cretaceous age of the K-Feldspar 40Ar/39Ar data (Guynn et al., 2006). This scenario for the Lhasa terrane is similar to that proposed by Tapponnier et al. (2001) for the Tibetan Plateau, except that they have the Lhasa terrane rising in the Eocene, after the Indo-Asian collision. Our study contributes to the recent work promoting substantial crustal thickness in Tibet prior to the Indo-Asian collision and underthrusting of the Lhasa terrane beneath the Qiangtang terrane.
CONCLUSIONS
The Bangong suture zone is the sight of protracted orogenesis and deformation throughout the mid-late Mesozoic as revealed by the Amdo basement and associated sedimentary rocks. The lack of an arc due to Meso-Tethys subduction is likely a result of tectonic underthrusting and subduction erosion, intra-oceanic island arcs and sediment burial prior to the Lhasa-Qiangtang collision. Accretionary wedges and mélange complexes probably developed in different settings at different times, resulting in widespread, extended and varied Jurassic deposition, ophiolite obduction, and 180
deformation. Marine deposition in the Amdo area continued into the latest Jurassic as
revealed by detrital zircon age groups of ~171 Ma and ~149 Ma in marine sandstones.
The Cretaceous collision between the Lhasa and Qiangtang terranes exhumed the
Amdo basement to upper crustal levels along southward verging thrust faults. Initial
shortening in the Early Cretaceous thickened the basement itself as revealed by mylonitic
and proto-mylonitic shear zones, though the displacements were not substantial enough to
juxtapose distinctly different metamorphic grades within the basement. Nonmarine
deposition of coarse clastic sediments occurred in the mid-Cretaceous as revealed by
detrital zircon populations of 105-115 Ma, contemporaneous with marine limestone
deposition to the south and southwest. Thrust faulting at the southern edge of the Amdo
basement continued until at least the latest Early Cretaceous based on a ~117 Ma
granitoid cut by a thrust fault and the deformation of the mid-Cretaceous sedimentary
rocks. A simplified regional cross-sections indicates moderate shortening on the order of
~100 km over ~30 Myr; this suggests ~100 km of the lower crust of the Lhasa terrane
being thrust beneath the Qiangtang terrane. The lack of denudation of the Amdo
basement after mid-Cretaceous shortening indicates that most of the crustal thickening
generated during the Early Cretaceous has persisted until the present day.
ACKNOWLEDGMENTS
This work was supported by NSF grant EAR-0309844 to P. Kapp and student grants
to J. Guynn sponsored by The Geological Society of America, ChevronTexaco,
ExxonMobil and the Peter J. Coney Fellowship. We would like to thank Alex Pullen and 181
Ross Waldrip for assistance in the field and Jen Fox for lab work. The manuscript benefited from conversations with Pete DeCelles, Ross Waldrip, Alex Pullen and John
Volkmer.
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rocks from central Tibet (China) indicate a tectonic transition from continental
collision to back-arc rifting: Earth and Planetary Science Letters, v. 229, p. 73-89. 188
Zhang, K.-J., Xia, B.-D., Wang, G.-M., Li, Y.-T., and Ye, H.-F., 2004, Early Cretaceous
stratigraphy, depositional environments, sandstone provenance, and tectonic
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116, p. 1202-1222.
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Science, v. 34, p. 59-65.
FIGURE CAPTIONS
Figure 1. Regional geologic map of southern and central Tibet based on Kapp et al.
(2005). Inset shows location of map relative to India and China. Thrusts shown in red are related to the Lhasa-Qiangtang (L-Q) collision; those in blue are part of the Gangdese retro-arc fold and thrust belt. GCT = Great Counter Thrust.
Figure 2. Pb/U concordia plots, weighted averages and probability density plots (PDFs) for Amdo Cretaceous igneous samples. Ellipses in gray on the concordia plot are not
used in the calculation of average ages. Rel. Prob. = Relative Probability. The PDFs are
scaled to all have the same height. The concordia for AP062004-B does not show spot
20C (very high uncertainty) or 16C (slightly discordant & Ordovician) and the concordia
for JG062504-1 does not show spot 1 (discordant Paleoproterozoic) or 4C (discordant
Paleozoic).
189
Figure 3. Pb/U concordia plot of detrital zircon analyses of the three red sandstone
samples. Ages less than ~1000 Ma are based on 206Pb*/238U apparent ages, those older than ~1000 Ma are 206Pb*/207Pb* apparent ages.
Figure 4. Pb/U concordia plot of detrital zircon analyses of the two Jurassic flysch
samples and a sandstone from the Ks shale unit. Ages less than ~1000 Ma are based on
206Pb*/238U apparent ages, those older are 206Pb*/207Pb* apparent ages.
Figure 5. Probability density for all the Amdo detrital zircon samples. The curves are only shown to 1200 Ma; older ages are sparse and generally do not occur in groups so that the peaks are negligible compared to the younger ages. “n” is the number of zircons used in the curves; in parenthesis is the total number of good analyses. The curves have been normalized so that each has the same area.
Figure 6. Probability density functions for the zircon analyses of each sedimentary sample used in the calculation of the maximum depositional age. Sample JG070605-1 is not shown because the number of ages is too small for a robust determination. Average ages and uncertainty are not weighted.
Figure 7. Geologic map of the Amdo basement and southern sedimentary rocks.
Structural measurements are plotted on lower hemisphere equal-area stereonets. Areas outside of the mapping limit are based on observations from a distance, satellite photos 190
and the geological map of Pan et al. (2004). Sample numbers are those mentioned in the text; for locations of dated Jurassic granitoids, see Guynn et al. (2006).
Figure 8. a) Layered Amdo orthogneiss at a road cut on the Lhasa-Golmud highway
(location of samples PK97-6-4-1A and PK97-6-4-1B). Dark rocks within the orthogneiss
are garnet-amphibolite pods. A hammer for scale is located between the two lower
amphibolite pods. b) Small faults within the orthogneiss at the same location; see
stereonet plot on map (stereonet (e), Fig. 7). c) Close-up of a fault surface.
Figure 9. (a) Kc2 conglomerate near sample location JG061805-1. Beds are dipping 55°
to the north (left in the photo). Hammer circled for scale. (b) Imbrication in the Kc2
conglomerate at the same location. Photo is taken at an angle such that the top and
bottom edges are parallel to bedding; “D” is down and “N” is north (horizontal). Flow
direction is to the right which is south after removing tilt and assuming the beds are not
overturned. (c) Sequence of sandstone and siltstone just south of the conglomerate;
location for detrital zircon sample JG061805-1. North, toward the conglomerate, is left.
(d) Cross-bedding in red sandstone just north of the Lhasa-Golmud highway. (e)
Sequence of Jurassic turbidites next to the Lhasa-Golmud highway; location of detrital
zircon sample JG061805-2. (f) Close-up of the thinly bedded sandstone and shale. (g)
Transposed bedding in the Jurassic flysch. (h) Tight fold in the shale unit Ks.
191
Figure 10. Probability density plot for Jurassic flysch samples from Amdo, the Lunpola basin, and Nima. The Lunpola curve is sample LNPLA from Leier (2005) and the Nima curve is sample 7-11-05-2 from Kapp et al. (unpublished). “n” is the number of zircons used in the curves. The curves have been normalized so that each has the same area.
Figure 11. (a) A view of Kc2 conglomerate just west of the road to Nyainrong showing the thin beds of limestone located within the conglomerate. View is toward the northeast and beds are dipping north ~64°. Houses and yaks in lower left for scale. Pm = marble;
Ps = schist and other metasedimentary rock. (b) Close-up view of one of the limestone beds. Line shows the curved boundary between the limestone and the conglomerate.
Knobs at the top of the hill in the background are also limestone. Alex Pullen to left in foreground for scale. (c) View looking south at marble (Pm) overlying Cretaceous granite (Kgr) along a flat fault contact. Location is just west of the road to Nyainrong.
(d) Orthogneiss (gn) thrust south over schist (Ps) along the southern basement boundary.
View is to the northwest. Picture is taken standing on Jurassic flysch (Jm). Location is
~10 km west of the edge of the map in Fig. X along the road east from Nagchu (road is in foreground).
Figure 12. (a, b & c) Bookshelf faulting of K-feldspar in the Jgr1 granitoid in a shear zone along the road to Nyainrong (see Fig. 7). South is to the right. (d) Crossed-polars photomicrograph of quartz recrystallization in an otherwise undeformed granodiorite next to the shear zone. (e) Pressure shadows around garnet in a crossed-polars 192
photomicrograph of garnet-kyanite schist JG060904-13 indicating top to right sense of shear. Dip direction (northeast) is to the left. (f) Plain light photomicrograph of another garnet in JG060904-13; same orientation. (g) Plain light photomicrograph of microfolds in JG061904-13 outlined by graphite. (h) Plain light photomicrograph of quartz- muscovite schist from metasedimentary rocks on the southeast border of the gneiss.
Microfolds are outlined by quartz and muscovite and indicate top to the right sense of shear. Dip direction (north) is to the left.
Figure 13. One possibility for the evolution of the southern Qiangtang margin during the
Mesozoic. Not drawn to scale. (a) As the Meso-Tethys ocean develops during the
Triassic to Early Jurassic, rifting of the Amdo basement from southern Qiangtang creates a back-arc ocean basin. Another back-arc basin to the south of Amdo involves a small island arc and the creation of oceanic crust in a supra-subduction zone (SSZ). Fore-arc flysch is deposited on the slopes of the island arc and the Amdo basement. (b) During the Middle Jurassic, compressive stresses cause the Amdo basement to collide with the
Qiangtang terrane. Compressive stresses may be a result of island arc collision or shallowing of the downgoing oceanic slab, which may also enhance tectonic erosion.
The Amdo basement and the associated continental arc are tectonically buried and metamorphosed. Jurassic flysch, including mélange with ophiolite fragments, is obducted onto the margin of Qiangtang and the Amdo basement. (c) The Amdo basement is exhumed during the Late Jurassic, coeval with magma emplacement. In this scenario, exhumation is accomplished via normal faulting across the newly accreted 193
Qiangtang margin, possibly in response to a thickened crust and a steepening slab. A
normal fault drops down accreted mélange and Qiangtang supracrustal assemblages
against the Amdo basement. The accretionary wedge at the convergent boundary is re- established. (d) During the Late Jurassic – Early Cretaceous, the remaining Meso-
Tethys Ocean is consumed. Additional small island arcs, back-arc basins or small terranes (not shown) may exist and contribute to the additional ophiolite mélange and the lack of arc magmatism south of the Amdo area. (e) During the Early Cretaceous, the
Lhasa terrane collides with and is underthrust beneath the southern Qiangtang margin, resulting in obduction of Jurassic flysch and mélange onto the northern Lhasa terrane.
AB = Amdo basement; LT = Lhasa Terrane; QT = Qiangtang terrane; SSZ = Supra- subduction zone.
Figure 14. Generalized cross-section of the Amdo basement and the Cretaceous thrust
belt based on the major features of the geologic map and on the available data. No
vertical exaggeration. Jurassic mélange may have been thickened prior to Lhasa-
Qiangtang collision as part of an accretionary wedge and incorporates Jurassic and
possibly Paleozoic sedimentary rocks and ophiolite fragments. The basement involved in
the thrust was also probably thickened by thrusting prior to emplacement to upper crustal
levels. The cross-section assumes a tectonic contact between the high-grade metapelites
and low-grade Paleozoic sedimentary layers. (a) Initial layers. Unit thicknesses are:
basement thrust sheet, 6 km; metamorphosed sedimentary rocks, 2 km; Paleozoic
sedimentary rocks, 2 km; Jurassic flysch and mélange, 6 km. (b) Initial fault places a 194
long sheet of basement on metapelites in a flat-on-flat relationship. (c) Subsequent
faulting cuts a horse of metapelites and creates a long flat-on-flat thrust between the
metamorphic rocks and the Jurassic sedimentary rocks. Syn-orogenic (foreland basin) sediments start to be overthrust. Note that erosion is assumed to have removed the leading part of the thrust sheet of Jurassic and Paleozoic sedimentary rocks. (d) A long thrust forms in the weak Jurassic units, which are then thrust over the foreland basin sedimentary rocks. The resulting ramp in the Jurassic rocks exhumes the metamorphic and basement rocks to surface levels. The northernmost edge of the mid-Cretaceous
Aptian-Albian seaway laps against the syn-orogenic conglomerates at the front of the
thrust belt. The erosion level depicts the geology of the eastern part of the geologic map,
where foreland basin sedimentary rocks are not exposed. (e) Another ramp and thrust in
the Jurassic units creates a fault-bend fold that forms an anticline in the overlying thrust
sheet and exposes the overthrust foreland basin sedimentary rocks, as seen in the western
part of the geologic map. A normal fault to the north drops unmetamorphosed
sedimentary rocks against the high-grade basement; this fault was likely active during
step (d) as well. Total shortening is 115 km. 195
FIGURES
Fig. 1 Guynn et al.
196
Figure 2. Guynn et al.
197
Figure 3. Guynn et al.
198
Figure 4. Guynn et al.
199
Figure 5. Guynn et al.
200
Figure 6. Guynn et al.
201
Q 91°40' E Amdo 92°00' E 92°20' E Tcg ?
Tcg ? Tcg ?
Jgr Tcg ? Q Q Q Q Q gn Tcg ? Jgr Tcg ? Tcg ?
Q Pm Q Pq ? Q Tcg ? Tcg ? Jgr gn Pq ? ? 77 Jgr Pm/Ps Q 47 5 45 Jgr ? 25 ? Pq Q Poles to Foliation 68 75 63 70 42 Jgr 66 Fold Axis Tcg 85 305 → 30 Q Tcg 28 Pm gn gn 48 ? 75 Pm ? Jgr ? 58 65 Jgr gn 56 Jgr 80 Pm/Ps Q Q Jgr 70 Nyainrong PK97-6-4-3A (h) 30 39 gn 18 Jgr 21 Jgr Pm 21 33 Jgr ? 44 ? 40 Jgr ? Jgr ? 5 35 Q gn Jgr Q gn gn
Nag gn Lake Poles to Foliation gn 68 57 Pq 46 Jgr Jgr1 ? PK97-6-4-2 56 Pm Poles to Foliation Lineation Q (a) Jgr ? Fold Axis Jgr ? Q Jgr1 gn 355 → 10 (c) Jgr
Jgr1
gn Jgr Fold Axis 45 → 40 Poles to Foliation Fold Axes 20 33 32°00' N gn gn 40 gn gn gn Jgr1
gn gn 48 47 64 gn (b) 24 83 gn 36 Q 16 Fold Axis gn 191 → 43 46 31 31 gn Q Jgr1 74 gn Kgr Jgr1 gr gn 75 Jgr ? 75 57 PK97-6-4-1A Gray: Faults Striations Q Black: Foliations Lineations Q gn Poles to Foliation Q 40 Fold Axes AP062004-B 41 gn gn 48 60 Pm (d) gn Pm gn Q 36 gn 9 36 Fold Axis 60 Jgr ? gn 31 78 325 → 45 48 43 gn 43 31 gn Q Pm gn (e) gn Jgr1 65 34 Jgr ? 61 5 gn gn gn 60 Pm gn gn 33 20 44 Jgr ? Q Q 31 gn 87 gn gn 80 Q 64 48 76 83 65 gn 62 71 27 Pm 84 gn 21 Pm 29 gn Ps 30 84 Fold Axes 27 70 36 26 29 35 85 ? 40 gn Ps Jm ? 85 62 14 22 Q (f) Q 78 Kgr 74 5 37 16 43 JG062504-1 40 Ps 12 Pm 40 83 38 39 54 32 ? Kgr 40 34 45 52 Pq 45 43 Jgr1 33 Pm 64 Poles to Foliation Pm Ps Q Pm gn Ps 25 Kgr 34 56 54 Ps Jm ? Kgr ? ? 31 33 Pm ? Q 72 60 34 40 ? 14 87 (g) 27 Kc2 75 Pm Jm Jm Pm 45 Ps 87 Kc2 Pm Qt Jm ? Jm 55 Ps 60 Pm 25 Pm Kc2 Jm ? 72 Fold Axis 70 Kc1 ? Kc2 30 JG052704-1 330 → 43 Kr 44 Pm 62 60 AP060504-B Kr Kc2 Kgr ? Kc1 62 45 Kl Kr Poles to Foliation Q Jm Pm 46 JG061805-1 45 Kr Kr ? Kc2 Pm Q 64 Ps 60 Kl Kr ? Jm 5 Kl Kl Kc1 ? Poles to Foliation Kr Kl Kl Kc1 Kr Q Jm JG070605-1 Kl Jm ? Kl ? Kc1 AP052504-B (j) Kc1 42 56 Kc1 16 Ks 24 30 Qt Fold Axis 25 Ks 76 → 33 Jl Qt 55 (i) Fold Axis Kr Kr Kr Kr 45 44 21 72 Kr 265 → 20 Kr Jl 59 82 53 Jl Kr 44 32°00' N Jm 0 1 2 3 4 5 10 km Jm JG063005-1 Jl 72 Kgr ?
Jm Jm Jm Jm
Map Symbols Figure 7. Jm Jm Jm Town oph 64 30 Road JG061805-2 43 Geologic Map of the Amdo Basement, 55 Mapping limit Foliation & lineation Jm Jl ? 16 Bangong Suture, Central Tibet Kr Depositional or intrusive contact Orientation & plunge of fold-axis J. Guynn, A. Pullen and R. Waldrip Kr 64 Jm ? Fault contact Strike & dip Explanation of Units Jm Inferred fault contact 21 Overturned bed Jm Q Quaternary alluvium and basin fill Normal fault (ball on foot-wall) JG063005-1 Sample number Thrust fault (triangle of hanging-wall) Qt Quaternary tufa associated with hot springs Strike-slip fault
Ng Neogene limestone pebble conglomerate Cretaceous hypabyssal intrusions
Cretaceous conglomerate with clasts of multiple rock types, including gn, gr, Kc2 Kgr Cretaceous granitoids Pm, Ps, Pq and Kr Cretaceous conglomerate composed of largely limestone, Pm, Pq and some Kc1 Jgr Jurassic granitoids Kr clasts or exclusively limestone and Pm Cretaceous medium-coarse grained sandstone interbedded with siltstone; Kr Jgr2 Jurassic porphyritic granitoid with K-feldspar megacrysts and usually chloritized biotite cross-bedded, coarse-grained and pebbly red arenite
Kl Cretaceous (?) limestone that occurs as blocks in the valley, often including oph Jurassic ophiolite fragment, serpentinized breccia clasts Cretaceous tan, red and green shale with interbedded sandstone layers, Ks Pm Paleozoic (?) marble; includes some metapelites in the NE locally coarse and pebbly
Jm Jurassic turbidites, marine sandstones, dark shales; often transposed Ps Paleozoic (?) schists
Jl Jurassic (?) massive, dark gray limestone Pq Paleozoic (?) quartzite
Cambrian and Precambrian gneiss; mostly orthogneiss with some paragneiss & migmatites; gn includes small outcrops of metasediments and granitoid intrusions in places. 202
Figure 8. Guynn et al.
203
Figure 9. Guynn et al.
204
Figure 10. Guynn et al.
205
Figure 11. Guynn et al.
206
Figure 12. Guynn et al.
207
Figure 13. Guynn et al.
208
Figure 14. Guynn et al.
209
TABLES
Table 1. Sample location and summary of U-Pb age results
Zircon Sample Lat Lon Description Age (Ma) AP060504-B 31.946 92.138 Hypabyssal intrusion 116.2 ± 2.0 JG052704-1 31.766 92.073 Quartz-monzonite 115.4 ± 1.6 AP062004-B 31.934 92.224 Quartz-diorite 111.6 ± 1.5 JG062504-1 31.750 91.780 Hypabyssal intrusion 105.9 ± 2.5 JG061805-1 31.744 91.867 Red sandstone 176.2 ± 21.6 JG061805-2 31.616 91.738 Turbidite S.S. (J-K) 169.4 ± 9.6 JG062305-1 31.734 92.485 Turbidite S.S. (J-K) 145.9 ± 11.2 JG063005-1 31.671 91.857 Red sandstone 114.5 ± 9.0 AP052504-B 31.735 92.028 Red sandstone 108.4 ± 5.7 JG070605-1 31.680 91.777 Tan sandstone in white shale ~110 ? Shaded samples are detrital zircon maximum depositional ages
Table 2. U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector ICP Mass 210 Spectrometry Isotopic ratios Apparent ages (Ma) U 206Pb 207Pb* ±206Pb*± error 206Pb* ± 207Pb* ±206Pb*±Best age± Spot (ppm) 204Pb U/Th 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma) JG052704-1 granitoid: igneous zircon 35 ⎠ m spot size. 1 380 1458 1.8 0.13059 16.3 0.01817 1.8 0.11 116.1 2.1 124.6 19.2 291.4 372.9 116.1 2.1 2 252 524 1.3 0.15933 16.4 0.01968 5.4 0.33 125.6 6.7 150.1 22.8 556.9 338.9 125.6 6.7 3 282 2317 0.7 0.11943 15.2 0.01748 2.2 0.15 111.7 2.5 114.6 16.5 174.4 353.5 111.7 2.5 4c 427 3611 0.2 0.12586 14.8 0.01745 2.9 0.19 111.5 3.2 120.4 16.8 298.6 332.7 111.5 3.2 6 265 1097 0.6 0.12499 19.2 0.01839 2.0 0.10 117.5 2.3 119.6 21.6 161.5 449.6 117.5 2.3 7 372 349 0.7 0.15972 12.0 0.01830 1.9 0.16 116.9 2.2 150.5 16.8 718.1 252.9 116.9 2.2 9 202 1747 1.1 0.13778 30.7 0.01876 7.8 0.25 119.8 9.3 131.1 37.7 340.6 685.1 119.8 9.3 10 204 762 0.6 0.16241 23.1 0.02068 4.4 0.19 131.9 5.7 152.8 32.8 490.2 506.3 131.9 5.7 11c 518 1023 0.2 0.22380 17.1 0.02654 2.1 0.13 168.9 3.6 205.1 31.8 644.6 367.8 168.9 3.6 11t 263 10006 0.9 0.19443 9.7 0.02685 1.4 0.15 170.8 2.4 180.4 16.0 307.7 218.4 170.8 2.4 13 408 1772 1.5 0.14769 20.0 0.01859 2.2 0.11 118.7 2.6 139.9 26.2 515.5 441.1 118.7 2.6 13c 330 2323 0.8 0.11679 24.0 0.01827 2.3 0.09 116.7 2.6 112.2 25.5 17.0 580.4 116.7 2.6 14 131 3336 0.2 0.09590 21.6 0.01811 3.0 0.14 115.7 3.5 93.0 19.2 -456.5 570.8 115.7 3.5 15 297 613 0.6 0.13646 10.7 0.01763 1.1 0.10 112.7 1.2 129.9 13.0 457.9 236.8 112.7 1.2 16 361 4563 1.2 0.13459 16.5 0.01872 2.0 0.12 119.6 2.3 128.2 19.9 291.4 376.8 119.6 2.3 17 350 2368 1.1 0.12146 8.6 0.01824 1.2 0.14 116.5 1.4 116.4 9.4 113.7 200.9 116.5 1.4 18 433 6322 1.1 0.20917 5.4 0.02990 1.5 0.27 190.0 2.7 192.9 9.6 228.5 121.1 190.0 2.7 19 240 229 1.0 0.16523 35.7 0.02046 6.1 0.17 130.5 7.9 155.3 51.4 551.5 790.4 130.5 7.9 20 346 5154 0.8 0.13153 12.8 0.01801 1.6 0.13 115.1 1.9 125.5 15.1 327.2 288.5 115.1 1.9 21 277 6772 0.9 0.12516 12.4 0.01842 2.4 0.19 117.7 2.8 119.7 14.0 161.4 285.1 117.7 2.8 22 267 9019 0.4 0.14293 12.3 0.01827 4.1 0.33 116.7 4.7 135.6 15.6 481.6 256.4 116.7 4.7 23 454 12998 1.3 0.17763 6.1 0.02628 1.3 0.22 167.2 2.2 166.0 9.3 148.9 138.7 167.2 2.2 23c 258 9635 2.1 0.17766 12.3 0.02670 3.2 0.26 169.9 5.3 166.0 18.8 112.0 281.0 169.9 5.3 24 546 10208 0.8 0.12349 8.3 0.01784 1.9 0.23 114.0 2.2 118.2 9.2 204.9 187.1 114.0 2.2
AP060504-B hypabyssal intrusion: igneous zircon 50 ⎠ m spot size. 1 421 5380 0.5 0.11802 2.9 0.01825 1.5 0.53 116.6 1.8 113.3 3.1 43.8 58.0 116.6 1.8 2 611 1803 0.8 0.13210 7.0 0.01866 1.1 0.16 119.2 1.3 126.0 8.3 256.6 158.3 119.2 1.3 3 279 6199 1.4 0.11589 6.1 0.01855 1.8 0.29 118.5 2.1 111.3 6.4 -39.0 142.2 118.5 2.1 4 540 4291 0.7 0.12363 4.6 0.01865 2.4 0.51 119.1 2.8 118.4 5.2 102.6 94.4 119.1 2.8 5 474 809 0.3 0.13442 22.1 0.01783 1.8 0.08 113.9 2.0 128.1 26.6 399.0 498.9 113.9 2.0 6 652 6678 0.4 0.11415 2.6 0.01762 1.4 0.51 112.6 1.5 109.8 2.8 48.7 54.3 112.6 1.5 7 225 2725 0.5 0.12050 6.6 0.01849 4.0 0.60 118.1 4.7 115.5 7.2 62.8 126.1 118.1 4.7 8 300 3236 0.5 0.11355 3.9 0.01855 1.5 0.39 118.5 1.8 109.2 4.0 -88.1 88.2 118.5 1.8 9 329 3362 0.7 0.12106 4.9 0.01846 1.8 0.37 117.9 2.1 116.0 5.4 77.0 108.0 117.9 2.1 10 512 5650 0.6 0.11663 2.7 0.01811 1.0 0.38 115.7 1.1 112.0 2.8 34.1 59.0 115.7 1.1 11 575 7562 0.7 0.11564 2.3 0.01795 1.0 0.43 114.7 1.1 111.1 2.4 35.4 49.6 114.7 1.1 Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 211 ICP Mass Spectrometry
13 420 4268 0.6 0.11725 2.9 0.01786 1.2 0.41 114.1 1.4 112.6 3.1 80.5 63.6 114.1 1.4 14 562 8182 0.7 0.11843 2.9 0.01868 1.4 0.51 119.3 1.7 113.7 3.1 -3.4 59.4 119.3 1.7 15 433 6466 0.7 0.11917 3.0 0.01811 1.6 0.53 115.7 1.8 114.3 3.2 85.5 60.5 115.7 1.8 16 328 6902 0.7 0.15953 3.9 0.02463 2.1 0.53 156.9 3.2 150.3 5.5 47.9 79.2 156.9 3.2 17 528 4611 0.6 0.12089 3.1 0.01860 2.2 0.70 118.8 2.5 115.9 3.4 55.9 52.0 118.8 2.5 18 166 727 0.7 0.12434 20.0 0.01802 1.9 0.09 115.1 2.1 119.0 22.4 196.9 466.0 115.1 2.1
AP062004-B quartz-diorite: igneous zircon 50 ⎠ m spot size. 1C 511 3161 0.8 0.18952 19.4 0.02661 2.5 0.13 169.3 4.2 176.2 31.5 269.9 445.8 169.3 4.2 3C 292 903 0.8 0.11554 44.8 0.01737 3.2 0.07 111.0 3.5 111.0 47.2 111.7 1106.1 111.0 3.5 4C 503 5979 0.7 0.19337 7.8 0.02656 3.3 0.42 169.0 5.4 179.5 12.9 320.6 161.9 169.0 5.4 5C 194 1362 0.7 0.15101 29.0 0.01773 3.9 0.13 113.3 4.3 142.8 38.7 666.5 627.8 113.3 4.3 6C 111 445 0.7 0.12888 45.0 0.01871 7.5 0.17 119.5 8.9 123.1 52.2 192.7 1080.0 119.5 8.9 7C 487 3713 0.8 0.10944 23.7 0.01711 1.4 0.06 109.4 1.5 105.5 23.7 18.2 574.5 109.4 1.5 8C 488 2925 0.8 0.11848 26.4 0.01736 2.0 0.08 110.9 2.2 113.7 28.4 171.7 624.2 110.9 2.2 9C 539 2551 0.4 0.12369 14.6 0.01603 1.0 0.07 102.5 1.0 118.4 16.4 450.6 325.9 102.5 1.0 10C 155 1231 0.8 0.20503 26.4 0.01892 2.9 0.11 120.8 3.5 189.4 45.7 1161.6 529.3 120.8 3.5 11C 175 1858 0.5 0.11510 32.0 0.01778 2.8 0.09 113.6 3.1 110.6 33.6 47.0 780.1 113.6 3.1 12C 316 1445 1.0 0.23410 21.9 0.02859 5.7 0.26 181.7 10.1 213.6 42.1 581.5 463.5 181.7 10.1 13C 924 270 1.2 0.12626 23.4 0.01578 3.6 0.15 100.9 3.6 120.7 26.6 530.9 512.2 100.9 3.6 14C 210 1292 0.8 0.24588 27.4 0.01798 5.6 0.21 114.9 6.4 223.2 55.0 1608.7 509.8 114.9 6.4 15C 306 1845 0.7 0.14777 20.1 0.01737 3.5 0.18 111.0 3.9 139.9 26.2 663.6 427.1 111.0 3.9 16C 352 6876 0.9 0.62965 7.5 0.07240 5.3 0.71 450.6 22.9 495.9 29.2 710.8 112.1 450.6 22.9 17C 664 6390 0.7 0.10798 13.7 0.01796 1.3 0.10 114.7 1.5 104.1 13.5 -132.5 337.5 114.7 1.5 18C 222 1590 0.7 0.13742 52.9 0.01721 1.8 0.03 110.0 2.0 130.7 65.0 526.8 1245.3 110.0 2.0 19C 451 4809 0.5 0.12854 18.8 0.01677 1.3 0.07 107.2 1.3 122.8 21.8 435.8 421.4 107.2 1.3 20C 184 1287 1.4 0.35065 46.9 0.03051 31.2 0.67 193.8 59.6 305.2 124.3 1277.4 704.7 193.8 59.6 21C 313 3198 0.9 0.09786 28.9 0.01765 2.5 0.09 112.8 2.8 94.8 26.2 -338.0 755.2 112.8 2.8 22T 416 3429 1.1 0.12147 20.0 0.01703 1.4 0.07 108.8 1.5 116.4 22.0 274.2 460.4 108.8 1.5 24T 443 4117 1.0 0.13191 16.1 0.01785 1.8 0.11 114.0 2.0 125.8 19.0 354.6 362.9 114.0 2.0 25T 412 4872 0.7 0.12022 13.9 0.01756 1.5 0.11 112.2 1.7 115.3 15.2 178.9 324.2 112.2 1.7 26T 491 3784 1.0 0.14274 14.4 0.01794 1.5 0.11 114.6 1.8 135.5 18.3 518.9 315.8 114.6 1.8 27T 632 3331 1.1 0.11419 14.2 0.01806 2.7 0.19 115.4 3.1 109.8 14.8 -9.3 339.2 115.4 3.1 28T 526 3318 1.0 0.12177 9.0 0.01743 0.9 0.10 111.4 1.0 116.7 9.9 226.2 206.7 111.4 1.0 29T 536 2369 0.5 0.14911 20.3 0.01702 1.0 0.05 108.8 1.1 141.1 26.7 725.7 433.8 108.8 1.1 30T 429 4290 0.9 0.14681 9.5 0.01771 1.2 0.13 113.2 1.4 139.1 12.3 607.6 203.6 113.2 1.4 31T 347 1328 0.9 0.11196 30.9 0.01789 2.4 0.08 114.3 2.7 107.8 31.6 -34.5 764.2 114.3 2.7 32T 315 909 0.9 0.11555 19.9 0.01765 2.1 0.11 112.8 2.4 111.0 20.9 73.3 474.3 112.8 2.4 33T 334 1496 0.8 0.14097 19.8 0.01747 2.0 0.10 111.6 2.3 133.9 24.8 549.8 433.8 111.6 2.3 34T 589 4317 1.0 0.12834 16.2 0.01742 1.6 0.10 111.3 1.8 122.6 18.7 346.9 365.7 111.3 1.8 35T 421 3080 0.9 0.15122 12.4 0.01841 1.8 0.15 117.6 2.1 143.0 16.5 587.8 266.4 117.6 2.1 36T 464 2671 0.7 0.15870 10.3 0.01802 1.9 0.18 115.1 2.1 149.6 14.3 737.7 215.0 115.1 2.1
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 212 ICP Mass Spectrometry
JG062504-1 hypabyssal intrusion: igneous zircon 50 ⎠ m spot size. 1 417 68024 1.3 5.97954 2.1 0.26946 1.9 0.94 1538.1 26.4 1972.9 17.9 2465.6 11.8 2465.6 11.8 3 1629 12953 1.5 0.11097 4.2 0.01613 0.9 0.22 103.1 1.0 106.9 4.3 190.8 95.2 103.1 1.0 4c 1397 15504 3.0 0.28951 10.0 0.03847 9.6 0.96 243.3 22.8 258.2 22.8 395.1 65.1 243.3 22.8 6 589 4504 1.1 0.11506 12.0 0.01678 1.5 0.13 107.3 1.6 110.6 12.6 182.8 277.8 107.3 1.6 7 913 1363 1.3 0.11894 6.9 0.01671 0.8 0.12 106.8 0.9 114.1 7.4 269.1 157.1 106.8 0.9 9 772 3276 0.5 0.12367 18.4 0.01685 1.3 0.07 107.7 1.4 118.4 20.6 339.3 418.9 107.7 1.4 10 652 11487 5.2 0.15575 10.9 0.02345 0.7 0.06 149.4 1.0 147.0 14.9 108.0 257.5 149.4 1.0 11 1140 814 0.4 0.07095 37.6 0.01557 2.7 0.07 99.6 2.7 69.6 25.3 -870.3 1110.2 99.6 2.7 12 1291 2944 0.6 0.10112 7.5 0.01465 3.0 0.40 93.8 2.8 97.8 7.0 197.8 160.3 93.8 2.8 13 1032 1227 1.1 0.12328 8.9 0.01606 1.7 0.19 102.7 1.8 118.0 10.0 439.9 195.4 102.7 1.8 14 523 6312 2.0 0.11979 14.2 0.01642 2.0 0.14 105.0 2.1 114.9 15.4 325.0 319.6 105.0 2.1 15 1337 9093 1.2 0.11590 9.1 0.01686 1.5 0.16 107.8 1.6 111.3 9.6 188.4 209.6 107.8 1.6 17 461 2848 0.3 0.11657 14.3 0.01789 1.8 0.13 114.3 2.1 112.0 15.2 62.0 340.5 114.3 2.1 19 286 3186 0.5 0.09861 22.9 0.01789 6.0 0.26 114.3 6.8 95.5 20.8 -352.6 575.3 114.3 6.8 20 624 2200 0.5 0.10437 14.0 0.01628 2.0 0.14 104.1 2.0 100.8 13.4 22.8 334.3 104.1 2.0 21 740 757 0.5 0.11045 11.0 0.01360 6.1 0.56 87.1 5.3 106.4 11.1 563.9 198.5 87.1 5.3 22 408 3641 2.6 0.17933 18.2 0.02687 11.6 0.64 170.9 19.6 167.5 28.1 119.4 331.5 170.9 19.6 24 561 2200 0.9 0.13447 13.6 0.01707 2.7 0.20 109.1 2.9 128.1 16.4 497.0 296.1 109.1 2.9 25 222 4424 0.7 0.13680 31.3 0.02347 2.1 0.07 149.6 3.1 130.2 38.2 -210.3 799.5 149.6 3.1 26 455 787 0.8 0.13989 47.4 0.01634 3.9 0.08 104.5 4.0 132.9 59.2 677.4 1069.5 104.5 4.0
JG061805-1 red arenite: detrital zircon Bolded sample numbers used a 25 ⎠ m spot size; the rest were 35 ⎠ m. 102 595 1255 1.1 0.09261 15.8 0.01229 3.0 0.19 78.7 2.3 89.9 13.6 397.8 350.4 78.7 2.3 77 187 3730 1.4 0.14372 8.5 0.02422 1.9 0.22 154.3 2.9 136.4 10.9 -165.9 207.2 154.3 2.9 48 1136 11600 17.0 0.20981 15.6 0.02685 10.0 0.64 170.8 16.8 193.4 27.5 478.9 266.3 170.8 16.8 88 511 4598 0.6 0.19894 9.9 0.02691 3.6 0.36 171.2 6.0 184.2 16.6 355.2 208.3 171.2 6.0 92 503 6921 0.7 0.18170 2.8 0.02811 1.8 0.63 178.7 3.2 169.5 4.4 42.8 52.5 178.7 3.2 105 902 349 0.9 0.39493 20.0 0.02897 6.3 0.32 184.1 11.5 338.0 57.6 1602.8 357.2 184.1 11.5 22 503 18657 2.0 0.34537 3.0 0.04886 1.8 0.61 307.5 5.4 301.2 7.7 253.0 53.7 307.5 5.4 84 245 7320 0.8 0.49159 6.0 0.06396 5.7 0.96 399.7 22.1 406.0 20.0 442.1 38.8 399.7 22.1 96 568 19532 1.6 0.51767 1.4 0.06628 1.0 0.72 413.7 4.0 423.6 4.8 477.9 21.5 413.7 4.0 6 682 41642 1.4 0.54552 5.1 0.06768 4.5 0.89 422.2 18.5 442.1 18.3 547.0 51.9 422.2 18.5 56 210 8737 1.0 0.53161 5.4 0.06883 3.6 0.68 429.1 15.1 432.9 18.9 453.0 87.6 429.1 15.1 93 731 10470 1.6 0.54792 2.3 0.06892 1.8 0.79 429.6 7.6 443.6 8.3 516.8 31.0 429.6 7.6 60 571 12448 1.7 0.57309 4.2 0.07070 3.5 0.83 440.3 15.1 460.0 15.7 559.4 51.0 440.3 15.1 67 314 15323 0.8 0.60676 8.2 0.07144 4.3 0.52 444.8 18.3 481.5 31.4 660.2 150.1 444.8 18.3 65 545 2225 0.7 0.69275 11.1 0.07346 3.2 0.29 457.0 14.3 534.4 46.1 880.4 219.7 457.0 14.3 17 685 11819 1.3 0.62758 2.7 0.07416 1.8 0.66 461.2 8.0 494.6 10.6 652.5 43.6 461.2 8.0 87 169 8359 1.9 0.58755 6.7 0.07424 5.8 0.86 461.7 25.6 469.3 25.3 506.7 76.4 461.7 25.6 95 1224 4691 1.9 0.63108 7.5 0.07437 3.9 0.52 462.5 17.4 496.8 29.6 658.1 138.4 462.5 17.4 18 1054 39391 2.5 0.58647 5.6 0.07454 5.0 0.88 463.5 22.3 468.6 21.2 493.9 58.6 463.5 22.3 Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 213 ICP Mass Spectrometry
90 575 1888 2.0 0.61692 10.5 0.07455 9.2 0.88 463.5 41.4 487.9 40.5 604.1 105.3 463.5 41.4 64 506 35622 1.6 0.58575 2.9 0.07534 2.4 0.84 468.2 11.0 468.2 10.9 467.7 35.4 468.2 11.0 15 500 29313 0.9 0.60929 3.7 0.07584 2.7 0.72 471.2 12.1 483.1 14.3 539.9 56.7 471.2 12.1 57 783 36764 1.5 0.60551 2.6 0.07612 1.9 0.73 472.9 8.8 480.7 10.1 518.0 39.3 472.9 8.8 21 612 34303 4.0 0.62068 2.1 0.07658 1.1 0.50 475.7 4.9 490.3 8.3 558.9 40.3 475.7 4.9 72 512 13929 0.9 0.62958 5.5 0.07662 1.4 0.26 475.9 6.6 495.8 21.6 588.8 115.1 475.9 6.6 75 284 6718 1.5 0.62210 3.6 0.07677 1.8 0.50 476.8 8.3 491.2 14.2 558.7 68.9 476.8 8.3 52 619 29514 1.7 0.62138 6.4 0.07719 3.6 0.57 479.3 16.7 490.7 24.8 544.3 115.0 479.3 16.7 46 318 6753 1.3 0.59626 4.2 0.07720 2.2 0.53 479.4 10.4 474.9 16.0 453.0 79.5 479.4 10.4 26 746 16880 0.9 0.60292 6.1 0.07739 5.7 0.94 480.5 26.5 479.1 23.3 472.2 46.5 480.5 26.5 50 418 9780 1.2 0.60431 3.3 0.07742 2.6 0.78 480.7 12.0 480.0 12.7 476.5 46.0 480.7 12.0 8 839 76165 1.5 0.61888 2.2 0.07755 1.0 0.45 481.5 4.6 489.1 8.7 525.1 43.7 481.5 4.6 66 412 12669 0.6 0.62517 3.6 0.07769 2.3 0.65 482.3 10.8 493.1 13.9 543.4 58.9 482.3 10.8 58 592 17437 1.8 0.61102 2.7 0.07802 1.4 0.52 484.3 6.5 484.2 10.3 483.7 50.3 484.3 6.5 51 319 4209 1.9 0.62907 9.2 0.07821 3.9 0.42 485.4 18.3 495.5 36.2 542.4 183.4 485.4 18.3 7 1550 54709 1.4 0.62092 4.1 0.07821 3.3 0.82 485.4 15.5 490.4 15.8 513.8 51.7 485.4 15.5 70 249 16849 1.2 0.61613 2.8 0.07826 2.5 0.87 485.7 11.6 487.4 11.0 495.3 31.1 485.7 11.6 89 679 13164 1.7 0.62878 4.1 0.07832 2.9 0.71 486.1 13.8 495.3 16.2 538.4 63.6 486.1 13.8 91 637 30120 2.6 0.61977 2.0 0.07838 1.8 0.90 486.5 8.3 489.7 7.7 504.9 19.0 486.5 8.3 27 436 38438 1.3 0.63172 3.0 0.07848 1.7 0.57 487.0 7.9 497.2 11.8 544.1 54.0 487.0 7.9 104 338 17058 6.7 0.63088 5.8 0.07889 5.4 0.92 489.5 25.2 496.6 22.8 529.6 49.6 489.5 25.2 4 581 63978 1.3 0.64296 3.3 0.07896 2.5 0.76 489.9 11.8 504.1 13.1 569.1 46.4 489.9 11.8 13 635 49733 1.1 0.63543 3.5 0.07905 1.0 0.30 490.5 4.9 499.5 13.7 540.9 72.7 490.5 4.9 69 139 8331 1.1 0.64914 5.4 0.07907 4.8 0.89 490.6 22.5 508.0 21.4 586.9 53.5 490.6 22.5 20 397 24997 2.1 0.65367 3.8 0.07909 1.6 0.43 490.7 7.8 510.7 15.3 601.5 74.7 490.7 7.8 29 1744 63927 1.2 0.68750 4.3 0.07953 2.0 0.48 493.3 9.6 531.3 17.6 698.0 79.8 493.3 9.6 106 395 23055 1.0 0.62611 4.1 0.07972 3.8 0.93 494.5 18.2 493.7 16.0 490.0 32.0 494.5 18.2 23 406 20955 1.7 0.61718 3.8 0.07980 2.6 0.68 494.9 12.4 488.1 14.8 456.0 61.9 494.9 12.4 30 411 14790 1.8 0.63248 3.6 0.07984 2.3 0.65 495.2 11.1 497.6 14.2 509.0 60.3 495.2 11.1 109 178 12715 1.3 0.63395 2.7 0.08003 1.8 0.66 496.3 8.5 498.6 10.6 508.9 44.7 496.3 8.5 62 149 26642 1.3 0.61728 7.1 0.08018 6.3 0.88 497.2 30.0 488.1 27.7 445.8 76.0 497.2 30.0 99 328 15751 1.5 0.62294 2.6 0.08029 1.9 0.74 497.8 9.2 491.7 10.1 463.1 38.8 497.8 9.2 1 400 60468 2.2 0.63720 3.9 0.08030 1.6 0.41 497.9 7.6 500.6 15.4 512.8 78.3 497.9 7.6 40 524 16997 3.7 0.63752 2.2 0.08049 1.3 0.60 499.1 6.4 500.8 8.7 508.5 38.4 499.1 6.4 24 334 18865 1.6 0.64583 2.2 0.08055 1.6 0.73 499.4 7.7 505.9 8.7 535.4 32.5 499.4 7.7 78 803 41043 1.1 0.63515 6.5 0.08091 6.1 0.93 501.5 29.4 499.3 25.8 489.0 52.4 501.5 29.4 76 186 10125 1.4 0.63842 3.3 0.08096 1.3 0.40 501.8 6.3 501.3 13.1 499.0 67.0 501.8 6.3 32 442 6370 1.6 0.62822 4.5 0.08100 2.1 0.47 502.1 10.3 495.0 17.7 462.1 88.0 502.1 10.3 86 119 8427 2.3 0.62071 4.1 0.08107 2.8 0.68 502.5 13.3 490.3 15.8 433.6 66.2 502.5 13.3 107 175 11943 1.6 0.64171 2.7 0.08121 1.8 0.66 503.4 8.6 503.4 10.6 503.4 44.0 503.4 8.6 11 458 10422 1.4 0.65090 5.2 0.08135 3.4 0.65 504.2 16.6 509.0 21.0 530.9 86.9 504.2 16.6
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 214 ICP Mass Spectrometry
16 640 35164 1.3 0.64762 2.2 0.08144 1.5 0.69 504.7 7.2 507.0 8.6 517.4 34.1 504.7 7.2 97 303 15407 1.0 0.63308 2.8 0.08157 2.4 0.85 505.5 11.5 498.0 11.0 463.7 32.7 505.5 11.5 83 847 19406 1.5 0.65835 2.8 0.08158 1.5 0.56 505.6 7.5 513.6 11.1 549.5 50.1 505.6 7.5 85 339 27322 1.2 0.65454 2.5 0.08161 1.6 0.67 505.7 8.0 511.3 9.9 536.1 40.2 505.7 8.0 31 390 12218 1.4 0.65115 4.3 0.08196 3.4 0.79 507.8 16.7 509.2 17.3 515.3 57.5 507.8 16.7 101 203 9691 1.4 0.63630 3.3 0.08199 1.6 0.48 508.0 7.8 500.0 13.0 463.7 63.7 508.0 7.8 80 278 15675 1.4 0.64054 2.1 0.08220 1.2 0.58 509.3 5.8 502.6 8.2 472.6 37.3 509.3 5.8 36 387 4205 1.7 0.62429 6.7 0.08240 3.2 0.48 510.5 15.8 492.5 26.3 410.0 132.2 510.5 15.8 49 345 14478 1.1 0.65903 5.8 0.08288 3.7 0.63 513.3 18.0 514.0 23.5 517.3 99.4 513.3 18.0 39 211 12618 1.6 0.65469 3.9 0.08301 3.0 0.77 514.0 14.8 511.4 15.7 499.4 55.2 514.0 14.8 43 253 7661 1.2 0.67707 4.8 0.08364 3.8 0.80 517.8 19.1 525.0 19.7 556.3 63.3 517.8 19.1 79 455 23425 1.2 0.66268 3.7 0.08366 3.4 0.92 517.9 17.1 516.3 15.2 508.7 33.0 517.9 17.1 9 583 62426 1.1 0.64231 2.6 0.08389 1.9 0.73 519.3 9.4 503.7 10.3 433.8 39.8 519.3 9.4 94 206 2151 1.4 0.72558 4.7 0.08468 2.7 0.57 524.0 13.4 554.0 20.1 679.0 82.7 524.0 13.4 53 852 32747 1.4 0.67025 4.0 0.08471 3.5 0.89 524.2 17.7 520.9 16.1 506.4 39.9 524.2 17.7 61 260 24240 0.9 0.66403 3.0 0.08549 2.6 0.86 528.8 13.2 517.1 12.3 465.5 34.4 528.8 13.2 19 400 5273 2.0 0.99364 6.0 0.10359 2.0 0.34 635.4 12.2 700.6 30.4 915.5 116.3 635.4 12.2 63 215 21768 0.6 0.95494 4.3 0.10592 3.6 0.84 649.0 22.3 680.7 21.4 786.9 49.5 649.0 22.3 45 189 4906 1.4 0.86913 4.1 0.10778 3.3 0.80 659.8 20.8 635.1 19.6 548.0 54.0 659.8 20.8 38 1292 2481 0.9 1.14702 19.4 0.11185 5.2 0.27 683.5 33.6 775.8 105.4 1051.6 379.0 683.5 33.6 98 356 17870 0.9 1.21321 7.8 0.12345 6.9 0.88 750.4 48.5 806.7 43.5 965.3 76.3 750.4 48.5 14 95 13934 1.0 1.15343 3.5 0.12487 2.6 0.73 758.5 18.6 778.9 19.3 837.6 50.2 758.5 18.6 3 258 18430 1.8 1.19086 4.0 0.12492 2.5 0.63 758.8 17.9 796.4 21.9 902.9 63.7 758.8 17.9 28 342 5495 0.9 1.12583 7.0 0.12648 4.7 0.67 767.7 34.2 765.8 37.7 760.0 109.3 767.7 34.2 42 649 26870 1.1 1.21682 2.7 0.13038 2.5 0.91 790.0 18.4 808.3 15.1 859.0 22.7 790.0 18.4 100 454 42782 1.5 1.22408 1.7 0.13266 1.2 0.70 803.0 8.9 811.6 9.4 835.3 25.0 803.0 8.9 54 427 16866 1.1 1.20485 2.6 0.13317 1.9 0.73 805.9 14.5 802.8 14.4 794.2 37.1 805.9 14.5 59 1791 81378 1.2 1.25203 5.6 0.13916 4.3 0.77 839.9 33.7 824.3 31.4 782.5 74.4 839.9 33.7 34 90 19706 1.8 1.36163 6.5 0.14223 2.3 0.36 857.3 18.5 872.6 37.9 911.6 124.7 857.3 18.5 2 487 108314 4.1 1.46671 3.3 0.15345 2.9 0.89 920.3 24.9 916.8 19.7 908.4 30.9 920.3 24.9 55 336 25523 1.1 1.65707 2.9 0.16883 1.5 0.52 1005.7 14.3 992.3 18.7 962.7 51.5 1005.7 14.3 33 187 47614 1.5 3.26000 2.7 0.25581 1.8 0.68 1468.4 23.6 1471.6 20.7 1476.2 37.2 1476.2 37.2 41 897 116152 6.0 4.73500 4.9 0.30019 4.2 0.87 1692.2 62.5 1773.5 40.7 1870.5 43.8 1870.5 43.8 12 542 15089 3.4 5.23409 3.2 0.32728 1.1 0.35 1825.2 18.0 1858.2 27.7 1895.3 54.8 1895.3 54.8 71 102 32148 1.7 6.83665 2.7 0.36940 2.0 0.73 2026.6 34.3 2090.5 24.0 2154.0 32.5 2154.0 32.5 44 392 83096 1.7 10.09685 3.1 0.45765 2.2 0.72 2429.2 45.0 2443.7 28.5 2455.8 36.0 2455.8 36.0 25 321 69862 1.5 13.93991 6.2 0.53842 6.0 0.97 2776.8 135.7 2745.6 58.6 2722.8 23.1 2722.8 23.1 5 381 194227 2.1 19.43282 5.3 0.54586 3.1 0.59 2807.9 70.9 3063.6 51.2 3235.8 67.7 3235.8 67.7
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 215 ICP Mass Spectrometry
JG061805-2 turbidite sandstone: detrital zircon Bolded samples used a 15 ⎠ m spot size; the rest were 25 ⎠ m. 32 649 6464 2.9 0.17646 3.4 0.02600 2.5 0.73 165.5 4.1 165.0 5.2 158.0 55.1 165.5 4.1 33 499 1956 2.2 0.18954 6.7 0.02729 5.3 0.78 173.5 9.0 176.2 10.9 212.4 96.6 173.5 9.0 59 1092 241823 2.4 0.19584 4.7 0.02744 3.1 0.66 174.5 5.3 181.6 7.8 275.2 81.1 174.5 5.3 70 112 2440 0.7 0.15877 20.4 0.03147 7.8 0.38 199.8 15.2 149.6 28.4 -586.8 515.3 199.8 15.2 24 479 8591 1.9 0.23358 3.8 0.03274 1.8 0.48 207.7 3.8 213.1 7.3 273.9 76.7 207.7 3.8 48 1209 146018 0.7 0.23346 3.5 0.03291 2.9 0.84 208.7 6.0 213.0 6.7 260.8 43.9 208.7 6.0 80 745 13478 1.3 0.27894 7.1 0.03320 5.4 0.76 210.6 11.2 249.8 15.7 636.9 99.2 210.6 11.2 50 969 89277 2.7 0.27081 1.7 0.03783 1.3 0.75 239.4 3.1 243.3 3.8 282.0 26.4 239.4 3.1 57 1625 164400 24.7 0.28541 3.4 0.04003 2.5 0.73 253.0 6.2 254.9 7.7 272.4 54.1 253.0 6.2 66 817 19781 0.8 0.29060 4.7 0.04007 3.8 0.82 253.3 9.5 259.0 10.7 311.5 60.1 253.3 9.5 25 188 5962 1.1 0.27887 15.8 0.04066 10.1 0.64 256.9 25.5 249.8 35.1 182.9 284.4 256.9 25.5 54 297 41879 1.7 0.39823 12.1 0.04177 4.6 0.38 263.8 11.8 340.4 34.9 903.2 230.5 263.8 11.8 7 207 8579 1.7 0.29606 6.8 0.04290 2.4 0.35 270.8 6.4 263.3 15.8 197.5 147.9 270.8 6.4 76 427 8408 1.8 0.32375 4.0 0.04299 2.1 0.53 271.4 5.6 284.8 10.0 396.3 76.6 271.4 5.6 68 749 20483 1.3 0.34165 2.0 0.04718 1.0 0.49 297.2 2.9 298.4 5.2 308.2 40.2 297.2 2.9 45 341 37625 0.8 0.37445 5.0 0.05205 4.0 0.80 327.1 12.8 322.9 13.9 293.3 69.3 327.1 12.8 40 138 54244 0.8 0.41632 10.9 0.05317 4.9 0.45 333.9 16.0 353.4 32.6 483.4 215.8 333.9 16.0 6 228 5200 6.1 0.41721 8.7 0.05567 7.1 0.81 349.2 24.1 354.1 26.1 385.8 114.3 349.2 24.1 44 1613 82571 3.2 0.55885 15.0 0.05821 1.4 0.09 364.7 4.8 450.8 54.7 917.4 309.1 364.7 4.8 39 1847 36954 34.8 0.46555 5.5 0.06019 4.4 0.79 376.8 16.0 388.1 17.8 456.3 74.9 376.8 16.0 79 337 5317 1.3 0.50438 4.2 0.06313 3.9 0.93 394.6 14.9 414.7 14.3 527.6 34.8 394.6 14.9 75 969 18667 2.5 0.55697 4.7 0.07185 3.7 0.79 447.3 15.9 449.6 16.9 461.2 63.6 447.3 15.9 74 318 8633 0.6 0.57127 8.6 0.07198 8.2 0.95 448.1 35.4 458.8 31.6 513.0 56.1 448.1 35.4 58 436 367328 2.8 0.60425 4.2 0.07732 2.6 0.61 480.1 11.9 479.9 16.2 478.9 74.1 480.1 11.9 10 112 5246 0.7 0.68882 6.2 0.08958 3.6 0.58 553.1 19.0 532.1 25.7 443.1 112.3 553.1 19.0 37 608 51052 4.0 0.75138 3.1 0.09076 2.4 0.78 560.0 12.9 569.0 13.5 605.2 42.3 560.0 12.9 36 629 68523 4.0 0.74365 4.5 0.09139 1.9 0.42 563.8 10.2 564.5 19.5 567.6 88.9 563.8 10.2 52 992 459094 4.3 0.81806 2.6 0.09665 1.9 0.72 594.7 10.5 607.0 11.7 652.9 38.2 594.7 10.5 64 233 9352 1.6 0.87947 6.2 0.10327 6.0 0.97 633.5 36.4 640.7 29.5 666.0 32.2 633.5 36.4 5 545 34508 3.5 1.02113 4.5 0.10633 3.5 0.78 651.4 21.9 714.5 23.3 918.1 58.3 651.4 21.9 47 148 81750 1.1 0.97869 6.8 0.11748 6.2 0.92 716.0 42.1 692.9 33.9 618.6 57.4 716.0 42.1 2 1093 11939 1.2 1.15370 3.0 0.11916 2.5 0.84 725.7 17.4 779.0 16.5 934.7 34.0 725.7 17.4 31 269 12882 1.0 1.12870 4.5 0.12200 4.2 0.94 742.0 29.6 767.1 24.1 840.9 31.4 742.0 29.6 13 1045 62796 1.3 1.24654 3.3 0.12799 2.6 0.78 776.4 19.2 821.8 18.9 946.9 42.6 776.4 19.2 17 141 17458 0.8 1.13582 2.9 0.12995 2.1 0.73 787.6 15.5 770.5 15.4 721.3 41.3 787.6 15.5 27 1453 190375 1.9 1.26545 5.7 0.13684 5.1 0.91 826.8 39.9 830.4 32.1 840.0 48.7 826.8 39.9 35 407 2297 0.6 1.49402 4.3 0.13947 2.8 0.66 841.7 22.1 928.0 25.9 1139.0 63.9 841.7 22.1 19 500 32944 3.9 1.44346 4.5 0.15285 4.2 0.94 916.9 36.0 907.2 26.8 883.5 31.0 916.9 36.0 65 612 51360 2.2 1.61416 3.3 0.15912 1.8 0.57 951.9 16.4 975.7 20.4 1029.7 54.2 951.9 16.4 28 35 5239 1.2 1.56707 6.3 0.16257 3.0 0.48 971.0 27.1 957.3 39.1 925.7 113.7 971.0 27.1
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 216 ICP Mass Spectrometry
23 189 26862 1.0 2.05136 4.6 0.18840 4.1 0.88 1112.7 41.7 1132.7 31.7 1171.4 43.8 1171.4 43.8 1 655 35505 1.5 1.92293 2.3 0.17376 2.1 0.91 1032.8 20.2 1089.1 15.6 1203.5 19.3 1203.5 19.3 69 1011 10386 4.6 2.08046 3.7 0.17509 3.3 0.90 1040.1 31.9 1142.4 25.2 1342.1 30.5 1342.1 30.5 46 317 313500 0.9 2.55980 2.0 0.21328 1.4 0.67 1246.3 15.6 1289.2 14.9 1361.5 29.1 1361.5 29.1 14 171 27286 2.2 3.46528 8.5 0.26532 7.7 0.90 1517.0 103.9 1519.3 67.4 1522.6 70.4 1522.6 70.4 30 271 12724 0.8 3.38055 4.7 0.25634 4.3 0.91 1471.1 56.2 1499.9 36.6 1540.8 35.5 1540.8 35.5 73 347 9219 1.3 3.75291 3.8 0.26260 3.0 0.78 1503.1 40.0 1582.7 30.7 1690.5 44.1 1690.5 44.1 78 126 14498 0.6 4.88557 2.2 0.31338 1.6 0.73 1757.3 25.0 1799.8 18.7 1849.3 27.4 1849.3 27.4 63 296 98171 3.1 5.47249 3.5 0.34467 2.4 0.69 1909.1 39.4 1896.3 29.8 1882.3 45.4 1882.3 45.4 38 195 216687 4.9 5.82501 2.1 0.36226 1.4 0.67 1992.9 24.2 1950.1 18.4 1905.0 28.4 1905.0 28.4 72 171 9444 0.8 5.51649 2.5 0.34161 1.0 0.40 1894.4 16.4 1903.2 21.3 1912.7 40.7 1912.7 40.7 77 90 47051 0.8 5.41811 3.7 0.33362 2.6 0.70 1855.9 41.9 1887.7 32.0 1922.9 48.1 1922.9 48.1 15 147 5086 2.1 5.02732 6.5 0.30731 6.1 0.94 1727.5 92.2 1823.9 54.7 1935.9 38.8 1935.9 38.8 60 832 412781 1.9 5.34091 2.8 0.32519 1.0 0.36 1815.0 15.8 1875.4 23.8 1943.0 46.5 1943.0 46.5 42 180 867878 1.2 5.73135 3.5 0.32632 2.3 0.67 1820.5 37.2 1936.1 30.2 2062.1 45.5 2062.1 45.5 11 437 33388 2.9 4.82818 13.9 0.27218 8.3 0.60 1551.9 114.5 1789.8 117.8 2079.6 197.8 2079.6 197.8 49 961 121709 1.7 5.58090 3.8 0.29309 3.2 0.85 1657.0 46.9 1913.2 32.4 2203.5 33.9 2203.5 33.9 53 147 168088 2.4 6.35145 10.2 0.30646 9.5 0.93 1723.3 143.5 2025.6 89.9 2349.5 65.1 2349.5 65.1 26 631 104852 5.7 8.92177 2.0 0.40785 1.9 0.94 2205.1 35.1 2330.0 18.3 2441.3 11.7 2441.3 11.7 12 660 13554 1.4 9.19766 3.6 0.40544 3.4 0.94 2194.0 62.8 2357.9 32.9 2502.8 20.3 2502.8 20.3 61 959 585676 2.6 10.75454 1.8 0.46499 1.0 0.56 2461.6 20.5 2502.1 16.6 2535.2 24.8 2535.2 24.8 71 293 57142 2.6 10.56535 3.6 0.44725 2.6 0.72 2383.0 52.0 2485.7 33.7 2570.7 42.1 2570.7 42.1 8 269 16829 1.1 11.07553 2.4 0.44571 1.9 0.79 2376.1 37.8 2529.5 22.4 2654.9 24.3 2654.9 24.3 51 212 260562 1.1 13.74538 4.9 0.51595 1.5 0.30 2682.0 32.7 2732.3 46.4 2769.7 76.7 2769.7 76.7 34 147 3519 1.1 28.25904 10.5 0.64514 9.9 0.94 3209.2 249.9 3428.1 103.5 3558.7 55.5 3558.7 55.5 41 890 26962 0.9 2.05937 6.0 0.12778 4.5 0.75 775.2 32.8 1135.4 40.8 1909.2 70.2 1909.2 70.2 55 614 1568192 2.2 5.41154 3.4 0.35150 1.3 0.39 1941.7 22.3 1886.7 29.5 1826.6 57.5 1826.6 57.5 62 250 2738 1.0 0.20198 16.2 0.01722 2.8 0.18 110.0 3.1 186.8 27.6 1317.5 310.5 110.0 3.1 67 725 589 1.8 0.30399 24.5 0.01512 7.2 0.29 96.7 6.9 269.5 58.1 2297.2 409.3 2297.2 409.3 22 406 772 1.3 0.45032 12.6 0.03929 7.2 0.57 248.4 17.6 377.5 39.8 1272.2 202.2 248.4 17.6 4 76 1743 1.4 1.06892 7.0 0.09573 4.1 0.59 589.3 23.4 738.2 36.7 1221.0 110.6 589.3 23.4 18 154 2679 1.2 1.18885 8.7 0.11673 6.6 0.76 711.7 44.7 795.4 48.1 1037.7 113.9 711.7 44.7 20 135 323 1.1 0.74904 16.8 0.04646 7.8 0.46 292.8 22.2 567.7 73.2 1909.6 269.0 1909.6 269.0
JG062305-1 marine sandstone: detrital zircon 25 ⎠ m spot size. 67 430 1639 0.7 0.19042 20.5 0.02163 5.0 0.24 137.9 6.8 177.0 33.3 736.8 424.5 137.9 6.8 108 1006 14982 1.0 0.15451 3.4 0.02251 1.9 0.55 143.5 2.7 145.9 4.7 184.6 66.8 143.5 2.7 64 302 4297 1.9 0.14828 7.9 0.02334 4.0 0.51 148.7 5.9 140.4 10.4 1.5 164.4 148.7 5.9 34 936 5809 1.2 0.16107 2.7 0.02343 1.0 0.37 149.3 1.5 151.6 3.8 188.1 58.3 149.3 1.5 5 174 7105 0.9 0.15858 13.3 0.02350 5.3 0.40 149.8 7.8 149.5 18.5 144.7 288.0 149.8 7.8 27 2112 2602 2.5 0.19130 9.8 0.02381 1.2 0.12 151.7 1.7 177.7 16.0 539.5 214.2 151.7 1.7
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 217 ICP Mass Spectrometry
89 223 2335 1.0 0.15410 10.2 0.02398 2.7 0.27 152.8 4.1 145.5 13.8 29.1 236.4 152.8 4.1 26 727 1789 1.9 0.14763 15.2 0.02417 2.2 0.15 154.0 3.4 139.8 19.9 -94.3 370.9 154.0 3.4 60 889 14875 1.6 0.17048 3.4 0.02503 2.9 0.85 159.4 4.6 159.8 5.0 166.7 41.5 159.4 4.6 50 434 5461 2.5 0.17539 8.2 0.02635 4.2 0.50 167.7 6.9 164.1 12.5 112.5 168.1 167.7 6.9 97 465 7556 1.6 0.20283 9.3 0.02680 6.5 0.70 170.5 11.0 187.5 16.0 408.0 149.2 170.5 11.0 88 729 6106 0.6 0.20039 2.7 0.02901 1.7 0.63 184.3 3.1 185.5 4.6 199.7 49.5 184.3 3.1 51 889 17878 1.9 0.21746 3.2 0.03018 2.5 0.77 191.7 4.7 199.8 5.9 296.6 47.4 191.7 4.7 74 1684 34468 2.5 0.21043 2.6 0.03080 1.0 0.39 195.5 1.9 193.9 4.5 174.1 54.9 195.5 1.9 105 1140 9168 0.4 0.23376 7.6 0.03243 3.1 0.41 205.7 6.4 213.3 14.6 297.9 157.8 205.7 6.4 91 99 1912 1.3 0.23645 10.6 0.03411 4.6 0.43 216.2 9.7 215.5 20.6 207.8 221.8 216.2 9.7 99 308 9894 1.1 0.22396 3.9 0.03444 2.2 0.57 218.3 4.8 205.2 7.3 57.4 77.0 218.3 4.8 24 208 7519 1.5 0.24289 8.0 0.03544 2.0 0.25 224.5 4.4 220.8 15.9 181.1 181.0 224.5 4.4 56 850 21080 1.7 0.24806 4.7 0.03575 4.5 0.96 226.4 10.0 225.0 9.4 210.1 29.8 226.4 10.0 110 68 1032 1.5 0.33719 17.6 0.03833 7.6 0.43 242.5 18.2 295.0 45.1 735.1 337.8 242.5 18.2 102 546 11970 1.6 0.27786 6.2 0.03857 5.3 0.84 244.0 12.6 249.0 13.8 296.3 76.6 244.0 12.6 12 870 2709 2.1 0.32909 13.5 0.03901 4.0 0.30 246.7 9.7 288.9 33.9 645.7 277.3 246.7 9.7 28 429 9319 1.7 0.27118 4.1 0.03917 2.3 0.55 247.7 5.5 243.6 8.8 204.5 79.0 247.7 5.5 87 92 1936 1.9 0.24848 12.0 0.03928 5.2 0.43 248.4 12.7 225.3 24.3 -8.6 261.9 248.4 12.7 77 185 3525 1.7 0.32596 10.9 0.03935 5.6 0.51 248.8 13.6 286.5 27.1 606.3 201.6 248.8 13.6 43 322 6270 1.2 0.28451 5.6 0.03970 2.3 0.41 251.0 5.7 254.2 12.5 284.2 115.9 251.0 5.7 23 483 6678 1.7 0.30119 5.4 0.04195 3.7 0.69 264.9 9.5 267.3 12.6 288.8 88.8 264.9 9.5 62 546 9991 1.4 0.31335 4.8 0.04308 3.3 0.68 271.9 8.7 276.8 11.6 318.0 80.0 271.9 8.7 79 980 8905 1.9 0.31607 5.4 0.04349 4.6 0.85 274.4 12.4 278.9 13.1 316.1 63.7 274.4 12.4 104 153 3907 1.2 0.29877 6.9 0.04360 3.4 0.50 275.1 9.3 265.4 16.0 180.7 138.5 275.1 9.3 84 614 11321 1.6 0.30920 4.5 0.04381 2.0 0.45 276.4 5.5 273.6 10.9 249.4 93.6 276.4 5.5 29 736 18267 1.0 0.33605 2.2 0.04667 1.7 0.76 294.1 4.9 294.2 5.7 295.1 33.1 294.1 4.9 32 113 3374 1.3 0.33839 7.6 0.05089 3.9 0.51 320.0 12.1 295.9 19.6 110.4 155.6 320.0 12.1 90 367 9627 2.7 0.46039 6.4 0.06158 5.6 0.87 385.2 20.8 384.5 20.5 380.2 71.2 385.2 20.8 98 923 9165 1.6 0.49552 5.5 0.06384 5.1 0.92 398.9 19.6 408.7 18.5 463.9 46.8 398.9 19.6 55 759 18147 1.6 0.50225 2.9 0.06419 2.1 0.72 401.1 8.1 413.2 9.8 481.7 44.8 401.1 8.1 72 434 10740 1.7 0.51905 6.3 0.06542 4.6 0.73 408.5 18.2 424.5 21.8 512.5 93.9 408.5 18.2 35 231 4439 2.7 0.51992 4.9 0.06566 2.1 0.43 409.9 8.5 425.1 17.1 508.1 97.8 409.9 8.5 39 546 8594 1.3 0.56235 7.6 0.06796 4.0 0.52 423.8 16.4 453.1 27.8 604.1 140.4 423.8 16.4 85 160 2034 1.2 0.65171 12.1 0.07129 1.8 0.15 443.9 7.7 509.5 48.4 815.8 250.3 443.9 7.7 101 231 2663 0.6 0.63770 7.0 0.07200 5.9 0.84 448.2 25.4 500.9 27.6 749.5 80.0 448.2 25.4 49 198 5826 1.4 0.56024 5.0 0.07224 2.0 0.41 449.6 8.7 451.7 18.1 462.1 100.6 449.6 8.7 94 197 9490 1.1 0.56724 3.4 0.07498 2.0 0.59 466.1 9.0 456.2 12.6 406.9 62.0 466.1 9.0 41 168 6404 1.6 0.59394 6.9 0.07536 5.2 0.76 468.4 23.7 473.4 26.2 497.8 99.7 468.4 23.7 54 431 11648 3.3 0.66382 5.6 0.07953 1.6 0.29 493.3 7.7 516.9 22.8 622.7 116.4 493.3 7.7 93 129 4330 1.4 0.62230 8.1 0.08235 2.5 0.30 510.1 12.1 491.3 31.6 404.4 173.2 510.1 12.1 20 150 6846 0.5 0.66686 3.8 0.08364 2.8 0.73 517.8 13.7 518.8 15.4 523.1 57.1 517.8 13.7
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 218 ICP Mass Spectrometry
65 492 21359 4.6 0.76138 5.9 0.08420 5.7 0.97 521.1 28.7 574.8 26.0 793.1 31.2 521.1 28.7 57 434 16544 2.0 0.70644 6.4 0.08506 6.1 0.95 526.3 30.6 542.6 26.8 611.9 42.3 526.3 30.6 92 408 27054 10.3 0.78383 3.2 0.09534 2.8 0.88 587.1 15.7 587.7 14.2 590.0 32.4 587.1 15.7 76 467 50316 2.3 0.79953 3.6 0.09936 3.1 0.87 610.7 17.9 596.6 16.1 543.3 39.0 610.7 17.9 103 30 668 400.8 0.63560 15.1 0.11088 8.8 0.58 677.9 56.5 499.6 59.8 -252.3 312.7 677.9 56.5 11 95 10445 0.9 0.99409 4.2 0.11893 2.5 0.59 724.4 16.8 700.8 21.0 625.9 72.2 724.4 16.8 36 12 1047 2.5 0.65177 21.0 0.12202 7.5 0.36 742.2 52.4 509.6 84.4 -434.3 520.3 742.2 52.4 14 681 29017 1.1 1.17482 3.7 0.12867 3.2 0.87 780.3 23.4 788.9 20.1 813.3 38.4 780.3 23.4 100 156 2297 1.3 1.11337 3.4 0.13023 2.5 0.74 789.2 18.9 759.8 18.3 674.2 49.2 789.2 18.9 15 1706 45467 9.4 1.28124 9.0 0.13052 8.7 0.96 790.8 64.5 837.4 51.5 963.0 51.9 790.8 64.5 68 283 8309 0.8 1.36404 3.5 0.13812 2.6 0.73 834.0 20.1 873.6 20.6 975.3 48.8 834.0 20.1 38 879 59649 1.2 1.29522 5.1 0.13903 4.8 0.95 839.2 37.9 843.6 29.0 855.3 32.3 839.2 37.9 44 95 9858 1.0 1.26904 6.6 0.14506 5.5 0.85 873.2 45.3 832.0 37.3 723.3 74.4 873.2 45.3 69 430 35947 2.4 1.52387 6.2 0.15066 4.4 0.71 904.7 37.0 940.0 37.8 1023.8 87.7 904.7 37.0 73 198 14458 2.0 1.50262 2.8 0.15320 1.5 0.56 918.9 13.1 931.4 16.8 961.3 46.7 918.9 13.1 47 452 28835 1.2 1.51736 2.7 0.15402 2.3 0.83 923.5 19.6 937.4 16.8 970.3 31.3 923.5 19.6 40 230 18932 1.6 1.50966 2.9 0.15548 2.5 0.85 931.6 21.5 934.3 17.9 940.6 32.2 931.6 21.5 13 541 49226 2.6 1.55910 2.7 0.15789 1.9 0.72 945.0 16.9 954.1 16.5 975.1 37.8 945.0 16.9 96 318 47188 22.9 1.53553 2.6 0.15890 2.3 0.88 950.7 20.2 944.7 16.0 930.8 25.5 950.7 20.2 82 891 71671 4.2 1.58182 2.8 0.16273 2.1 0.74 971.9 18.7 963.1 17.4 943.0 38.5 971.9 18.7 71 555 48185 2.3 1.66623 1.7 0.16743 1.2 0.68 998.0 11.0 995.7 11.0 990.9 25.7 998.0 11.0 59 417 46486 1.8 1.70993 3.8 0.16939 2.4 0.62 1008.7 22.2 1012.3 24.6 1019.9 61.0 1019.9 61.0 52 345 30163 2.0 1.67184 2.3 0.16396 2.0 0.85 978.7 17.7 997.9 14.6 1040.2 24.3 1040.2 24.3 37 583 33359 4.4 1.74111 8.2 0.16887 8.1 0.99 1005.9 75.2 1023.9 52.8 1062.5 26.8 1062.5 26.8 78 189 23955 2.5 1.81871 3.9 0.17424 3.1 0.81 1035.4 29.9 1052.2 25.3 1087.2 45.7 1087.2 45.7 109 139 15910 0.6 2.06356 2.0 0.19232 1.4 0.73 1133.9 15.0 1136.8 13.6 1142.3 27.1 1142.3 27.1 31 246 22400 1.0 2.15708 8.0 0.19999 7.7 0.96 1175.3 82.2 1167.3 55.4 1152.6 45.2 1152.6 45.2 106 842 41890 1.4 1.96926 2.6 0.18213 2.0 0.77 1078.6 20.0 1105.1 17.6 1157.5 33.2 1157.5 33.2 81 136 20488 1.4 2.39482 4.7 0.20849 4.2 0.88 1220.8 46.2 1241.1 33.7 1276.4 43.0 1276.4 43.0 7 216 131594 1.7 2.63841 4.5 0.22582 3.8 0.86 1312.6 45.7 1311.4 33.1 1309.5 44.8 1309.5 44.8 10 248 69525 2.9 3.35271 7.3 0.26147 6.7 0.92 1497.4 89.3 1493.4 57.2 1487.8 55.5 1487.8 55.5 58 316 76374 2.9 3.13417 7.9 0.24429 7.4 0.93 1408.9 93.8 1441.1 61.2 1488.9 54.0 1488.9 54.0 80 208 22052 1.0 2.90580 7.7 0.22640 7.3 0.96 1315.6 87.3 1383.4 57.9 1489.6 41.5 1489.6 41.5 18 86 10848 1.2 3.74805 6.4 0.27440 6.2 0.97 1563.1 86.4 1581.7 51.7 1606.6 31.0 1606.6 31.0 86 363 49399 1.4 3.49561 3.4 0.24809 2.5 0.73 1428.6 32.3 1526.2 27.2 1664.2 43.3 1664.2 43.3 66 206 14882 1.1 4.21388 10.4 0.26493 10.1 0.97 1515.0 136.0 1676.7 85.7 1885.5 47.8 1885.5 47.8 45 902 105572 1.7 5.18629 2.6 0.32038 1.0 0.39 1791.6 15.6 1850.4 21.7 1917.1 42.2 1917.1 42.2 19 333 56308 2.3 7.63960 2.3 0.39873 1.4 0.60 2163.2 25.2 2189.5 20.5 2214.3 31.7 2214.3 31.7 9 166 72860 0.6 8.44318 12.2 0.38585 11.6 0.95 2103.6 208.2 2279.8 110.9 2441.9 62.5 2441.9 62.5 16 537 118565 4.2 10.07169 3.2 0.43686 3.1 0.96 2336.6 61.1 2441.4 29.9 2529.9 14.3 2529.9 14.3 61 162 25569 2.1 12.69661 4.2 0.51325 3.4 0.81 2670.5 74.3 2657.4 39.3 2647.5 40.2 2647.5 40.2
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 219 ICP Mass Spectrometry
70 171 47448 1.5 12.87657 8.4 0.49603 8.3 0.99 2596.7 177.8 2670.7 79.4 2727.1 21.1 2727.1 21.1 46 551 34250 2.6 14.13823 12.5 0.53764 12.3 0.98 2773.6 277.3 2759.0 119.2 2748.4 38.5 2748.4 38.5 83 59 23522 1.2 14.86146 3.2 0.54085 2.3 0.73 2787.0 52.6 2806.4 30.1 2820.4 35.1 2820.4 35.1 4 719 4409 0.8 0.22551 24.4 0.02130 8.9 0.36 135.8 11.9 206.5 45.5 1115.9 458.7 135.8 11.9 8 63 32107 3.8 1.73945 10.4 0.16709 8.8 0.85 996.1 81.5 1023.3 67.1 1081.9 110.3 1081.9 110.3 22 116 17097 0.8 3.61048 5.1 0.27905 4.5 0.88 1586.6 63.7 1551.8 40.8 1504.8 45.3 1504.8 45.3 25 234 33761 2.4 5.43768 6.1 0.35576 5.6 0.91 1962.0 94.5 1890.8 52.7 1813.5 46.2 1813.5 46.2 17 1257 2750 5.6 2.33124 9.5 0.12901 9.2 0.96 782.2 67.5 1221.9 67.7 2112.1 45.3 2112.1 45.3
JG063005-1 red arenite: detrital zircon 25 ⎠ m spot size. 72 3172 5753 1.4 0.01050 9.3 0.00169 2.9 0.31 10.9 0.3 10.6 1.0 -57.8 216.6 10.9 0.3 4 313 1797 0.9 0.11860 10.8 0.01728 2.8 0.26 110.4 3.0 113.8 11.6 184.8 243.3 110.4 3.0 63 794 10310 1.3 0.11624 8.9 0.01782 7.3 0.82 113.9 8.2 111.7 9.4 65.0 122.9 113.9 8.2 60 825 4707 1.6 0.11555 3.7 0.01785 2.1 0.58 114.1 2.4 111.0 3.9 46.6 72.7 114.1 2.4 27 550 5741 2.4 0.12060 5.3 0.01817 1.7 0.32 116.1 2.0 115.6 5.8 105.7 118.1 116.1 2.0 77 494 3520 1.8 0.12675 11.6 0.01834 3.5 0.30 117.1 4.0 121.2 13.3 201.3 258.3 117.1 4.0 32 347 3663 1.6 0.12668 5.5 0.01857 2.2 0.39 118.6 2.6 121.1 6.3 170.7 118.6 118.6 2.6 51 540 5887 1.3 0.12707 7.2 0.01876 3.7 0.51 119.8 4.3 121.5 8.2 154.2 144.1 119.8 4.3 83 304 1828 2.2 0.18052 34.0 0.02101 6.0 0.18 134.1 7.9 168.5 52.9 684.7 734.4 134.1 7.9 25 838 8165 2.0 0.20138 8.3 0.02702 2.7 0.33 171.9 4.6 186.3 14.1 373.1 176.9 171.9 4.6 84 862 11165 1.0 0.18115 2.9 0.02705 1.5 0.52 172.1 2.5 169.1 4.4 127.1 57.4 172.1 2.5 66 584 13479 0.7 0.18506 5.5 0.02745 2.4 0.44 174.6 4.2 172.4 8.7 142.8 115.9 174.6 4.2 12 449 7699 0.7 0.18961 10.9 0.02748 3.4 0.31 174.7 5.8 176.3 17.7 197.1 242.5 174.7 5.8 80 829 6813 2.2 0.19720 8.7 0.02813 2.5 0.29 178.8 4.4 182.8 14.5 233.6 191.8 178.8 4.4 46 1072 3666 1.1 0.22382 14.6 0.02938 2.8 0.19 186.6 5.1 205.1 27.0 422.8 320.5 186.6 5.1 1 1468 1419 5.4 0.32580 27.8 0.03503 2.2 0.08 221.9 4.8 286.4 69.6 851.9 587.8 221.9 4.8 53 588 5933 4.2 0.26954 7.2 0.03684 2.1 0.29 233.2 4.8 242.3 15.6 331.7 156.9 233.2 4.8 105 712 2578 0.9 0.30794 11.8 0.03836 7.3 0.62 242.7 17.5 272.6 28.1 538.2 201.6 242.7 17.5 79 322 6227 0.9 0.26356 6.3 0.03932 2.5 0.40 248.6 6.2 237.5 13.3 129.1 135.0 248.6 6.2 7 584 1550 1.0 0.40635 25.5 0.04392 8.2 0.32 277.1 22.3 346.2 74.9 840.9 508.8 277.1 22.3 20 128 1391 1.1 0.44708 22.9 0.05100 6.6 0.29 320.7 20.6 375.2 71.8 727.4 469.4 320.7 20.6 106 673 2817 1.6 0.59882 8.3 0.06655 2.6 0.31 415.4 10.3 476.5 31.6 782.6 166.3 415.4 10.3 30 1791 43363 97.1 0.51118 4.7 0.06708 4.5 0.95 418.5 18.3 419.2 16.3 423.2 32.5 418.5 18.3 89 1751 2273 5.1 0.63572 8.4 0.06876 4.4 0.53 428.7 18.4 499.7 33.2 839.5 148.7 428.7 18.4 98 354 34326 1.4 0.51790 2.1 0.06902 1.3 0.60 430.2 5.2 423.8 7.3 388.7 37.6 430.2 5.2 49 819 36129 7.4 0.53919 3.2 0.07015 1.8 0.57 437.1 7.7 437.9 11.4 442.1 58.2 437.1 7.7 74 533 46761 2.3 0.55637 3.5 0.07115 2.2 0.64 443.1 9.6 449.2 12.8 480.4 59.9 443.1 9.6 16 1000 1930 5.7 0.68328 17.1 0.07189 2.6 0.15 447.5 11.4 528.8 70.6 896.7 351.3 447.5 11.4 47 184 6340 0.9 0.49975 3.9 0.07190 1.7 0.44 447.6 7.4 411.5 13.2 213.9 81.1 447.6 7.4 15 230 14285 1.1 0.58849 4.4 0.07526 3.2 0.73 467.8 14.5 469.9 16.5 480.3 66.4 467.8 14.5 99 678 2725 2.3 0.65988 6.7 0.07582 1.6 0.24 471.1 7.4 514.5 27.0 712.2 138.2 471.1 7.4
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 220 ICP Mass Spectrometry
2 592 20339 1.9 0.63271 1.6 0.07910 1.2 0.73 490.7 5.7 497.8 6.4 530.2 24.5 490.7 5.7 55 395 17917 0.9 0.61566 2.6 0.07930 1.3 0.50 491.9 6.3 487.1 10.2 464.5 50.6 491.9 6.3 40 648 1332 0.7 0.77670 18.1 0.08298 7.0 0.39 513.9 34.6 583.6 80.5 865.0 348.5 513.9 34.6 57 729 12291 1.2 0.67932 3.8 0.08340 1.8 0.47 516.4 8.8 526.4 15.5 569.9 72.7 516.4 8.8 45 621 7886 1.3 0.69684 7.3 0.08362 2.1 0.28 517.7 10.2 536.9 30.5 619.4 151.6 517.7 10.2 93 881 49040 3.1 0.79773 1.5 0.09668 1.0 0.68 594.9 5.7 595.5 6.7 597.8 23.6 594.9 5.7 38 1591 4007 5.2 0.88665 6.5 0.09687 5.0 0.78 596.1 28.6 644.6 30.8 818.4 84.8 596.1 28.6 8 794 13004 1.3 1.01786 3.6 0.10969 2.2 0.60 670.9 13.8 712.8 18.6 847.1 60.6 670.9 13.8 91 60 2892 0.8 0.97158 7.1 0.11357 2.4 0.34 693.5 15.8 689.3 35.7 675.6 144.0 693.5 15.8 48 458 1837 0.5 1.14381 4.9 0.11747 3.3 0.68 716.0 22.6 774.3 26.5 946.5 73.3 716.0 22.6 102 102 2081 0.7 1.16317 21.5 0.11864 3.8 0.18 722.7 26.2 783.4 118.0 960.5 437.2 722.7 26.2 92 220 13154 1.2 1.07710 2.4 0.11981 1.9 0.79 729.5 12.9 742.2 12.4 780.8 30.2 729.5 12.9 95 272 6979 0.9 1.18583 7.0 0.12739 6.7 0.96 773.0 48.8 794.0 38.5 853.6 41.2 773.0 48.8 26 291 3931 2.8 1.36417 19.4 0.13013 3.7 0.19 788.6 27.7 873.7 114.3 1095.9 384.9 788.6 27.7 33 317 16168 1.2 1.17479 3.6 0.13274 1.7 0.48 803.5 13.0 788.9 19.7 747.8 66.6 803.5 13.0 58 336 17656 2.9 1.41213 2.8 0.14637 2.3 0.83 880.6 19.3 894.1 16.9 927.6 32.8 880.6 19.3 90 467 50566 3.7 1.49248 5.5 0.15025 4.3 0.79 902.4 36.5 927.3 33.4 987.1 68.8 902.4 36.5 81 261 27048 9.1 1.54822 2.6 0.15951 2.4 0.93 954.0 21.3 949.8 15.9 939.9 18.8 954.0 21.3 13 327 37911 1.9 1.70209 4.9 0.16893 3.5 0.71 1006.2 32.4 1009.3 31.5 1016.1 70.4 1016.1 70.4 9 490 50926 2.0 1.74827 2.1 0.17233 1.9 0.87 1024.9 17.6 1026.5 13.9 1029.9 21.7 1029.9 21.7 65 178 11752 0.9 1.71465 2.7 0.16819 2.4 0.89 1002.1 22.2 1014.0 17.2 1039.8 24.1 1039.8 24.1 42 306 3581 0.6 1.65363 4.8 0.16211 1.8 0.37 968.5 16.0 990.9 30.4 1041.0 89.9 1041.0 89.9 39 489 19342 1.0 1.97190 3.5 0.18570 3.4 0.96 1098.0 34.1 1106.0 23.8 1121.5 20.5 1121.5 20.5 19 417 56569 2.9 2.09089 3.5 0.19528 2.8 0.80 1149.9 29.4 1145.8 23.9 1138.1 41.2 1138.1 41.2 85 258 35750 1.6 2.24460 2.5 0.20520 2.1 0.83 1203.2 22.6 1195.1 17.4 1180.5 27.2 1180.5 27.2 104 282 30681 1.5 2.61196 2.0 0.22382 1.0 0.49 1302.0 11.8 1304.0 14.8 1307.3 34.1 1307.3 34.1 29 1906 2377 0.9 2.13119 8.6 0.17396 7.4 0.86 1033.9 71.1 1159.0 59.6 1401.0 82.9 1401.0 82.9 36 291 20112 3.9 3.58019 4.8 0.25937 3.5 0.72 1486.6 46.2 1545.1 38.2 1626.1 61.7 1626.1 61.7 37 219 18247 0.7 4.70952 2.6 0.31470 1.8 0.69 1763.8 27.9 1768.9 21.8 1775.0 34.3 1775.0 34.3 24 581 2837 1.3 3.87132 9.0 0.25572 3.2 0.36 1467.9 42.6 1607.7 73.1 1796.1 154.0 1796.1 154.0 11 288 68290 2.0 5.05362 2.1 0.32528 1.7 0.81 1815.5 26.9 1828.4 17.8 1843.0 22.5 1843.0 22.5 54 723 130233 2.3 5.17397 3.1 0.32707 2.7 0.85 1824.2 42.5 1848.3 26.8 1875.6 29.9 1875.6 29.9 5 2224 8362 2.3 4.64995 3.2 0.29273 2.1 0.66 1655.1 30.9 1758.3 26.8 1883.1 43.2 1883.1 43.2 73 182 31018 2.0 5.83792 3.0 0.35282 2.1 0.70 1948.0 35.1 1952.1 25.9 1956.3 38.3 1956.3 38.3 100 459 19559 7.0 5.45946 4.0 0.32612 1.8 0.46 1819.5 28.9 1894.2 34.1 1977.1 62.9 1977.1 62.9 10 350 78589 1.9 6.21672 1.3 0.36772 1.0 0.79 2018.7 17.3 2006.8 11.1 1994.6 13.9 1994.6 13.9 14 309 48583 2.0 6.03167 3.6 0.35218 3.0 0.84 1945.0 51.0 1980.4 31.5 2017.6 34.6 2017.6 34.6 62 358 56427 2.0 6.35430 6.6 0.37085 6.1 0.92 2033.4 106.8 2026.0 58.4 2018.4 45.8 2018.4 45.8 68 381 17793 3.2 5.84563 11.7 0.33286 6.0 0.51 1852.2 96.7 1953.2 101.8 2062.0 177.7 2062.0 177.7 69 930 9799 0.7 6.15590 6.2 0.33944 5.7 0.92 1884.0 92.8 1998.2 54.0 2118.5 42.5 2118.5 42.5 70 380 94760 1.6 7.72111 2.7 0.38448 1.8 0.68 2097.2 32.4 2199.1 24.0 2295.4 33.7 2295.4 33.7
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 221 ICP Mass Spectrometry
17 1575 32309 4.5 6.73396 4.5 0.31560 3.8 0.84 1768.2 58.1 2077.1 39.5 2399.1 41.2 2399.1 41.2 61 462 22774 2.4 9.47385 5.2 0.42563 4.7 0.91 2286.0 91.0 2385.0 47.9 2470.7 36.8 2470.7 36.8 28 501 52832 2.2 10.49799 6.7 0.46254 6.7 0.99 2450.8 136.4 2479.7 62.6 2503.6 14.8 2503.6 14.8 64 419 44600 1.2 11.32814 3.7 0.49901 3.3 0.90 2609.5 71.3 2550.5 34.4 2503.9 26.9 2503.9 26.9 35 52 27316 3.1 10.83446 3.3 0.47414 2.4 0.73 2501.7 49.2 2509.0 30.3 2515.0 37.5 2515.0 37.5 87 173 45387 1.0 10.76060 1.6 0.46765 1.0 0.62 2473.2 20.5 2502.7 15.0 2526.6 21.3 2526.6 21.3 59 118 3617 0.8 10.41879 5.2 0.44206 4.4 0.86 2359.9 87.9 2472.7 47.9 2566.8 43.9 2566.8 43.9 50 148 1946 1.0 10.71715 8.4 0.43841 6.9 0.82 2343.5 135.2 2498.9 77.7 2627.7 78.7 2627.7 78.7 34 228 3222 1.3 10.16718 6.1 0.40352 4.0 0.65 2185.2 73.6 2450.1 56.7 2677.9 77.3 2677.9 77.3 31 176 44890 1.1 13.33572 2.4 0.49367 2.0 0.87 2586.5 43.5 2703.7 22.2 2792.5 19.2 2792.5 19.2 22 161 81502 3.2 20.43944 2.0 0.60900 1.3 0.65 3066.0 31.2 3112.4 18.9 3142.5 23.5 3142.5 23.5 56 347 366 0.9 0.19317 26.2 0.01658 5.1 0.20 106.0 5.4 179.3 43.1 1304.2 506.8 106.0 5.4 23 1151 1538 1.3 0.25424 33.9 0.02524 6.5 0.19 160.7 10.3 230.0 69.8 1015.4 691.9 160.7 10.3 82 2208 601 1.3 0.28703 11.1 0.02531 4.0 0.36 161.1 6.4 256.2 25.1 1251.6 202.9 161.1 6.4 21 965 4076 2.8 0.29203 45.7 0.02783 9.8 0.21 177.0 17.1 260.2 105.3 1097.7 941.6 177.0 17.1 75 926 1352 1.5 0.65151 9.0 0.06221 5.1 0.56 389.1 19.2 509.4 36.1 1093.8 149.1 389.1 19.2 6 1087 662 0.8 0.84819 11.7 0.06899 3.0 0.26 430.1 12.5 623.7 54.6 1407.7 217.2 430.1 12.5 88 446 763 5.2 1.13602 29.9 0.09464 1.9 0.06 582.9 10.7 770.6 162.8 1361.9 588.1 582.9 10.7 67 146 13268 1.1 2.21479 3.9 0.20865 2.7 0.69 1221.6 30.1 1185.7 27.4 1120.8 56.4 1120.8 56.4 86 209 4882 1.1 1.67085 7.9 0.15712 3.9 0.49 940.8 34.2 997.5 50.4 1124.4 137.8 1124.4 137.8 18 300 3718 1.9 1.52014 15.4 0.13207 3.8 0.25 799.7 28.9 938.5 94.5 1280.4 291.9 1280.4 291.9 97 151 757 1.4 1.60500 14.1 0.12832 4.2 0.29 778.3 30.5 972.2 88.6 1440.6 258.4 1440.6 258.4 76 349 739 0.7 1.70810 26.3 0.12188 10.2 0.39 741.4 71.6 1011.6 170.2 1654.3 456.5 1654.3 456.5 44 429 31960 2.0 5.79914 3.4 0.37113 3.1 0.90 2034.7 53.8 1946.3 29.8 1853.4 27.5 1853.4 27.5 71 232 86198 2.4 11.53360 3.8 0.52060 3.7 0.95 2701.7 81.0 2567.3 36.0 2462.8 19.4 2462.8 19.4
AP052504-B red arenite: detrital zircon 35 ⎠ m spot size. 5 767 7614 1.2 0.11448 5.0 0.01667 3.1 0.63 106.6 3.3 110.1 5.2 185.7 89.9 106.6 3.3 86 483 4595 2.1 0.10535 2.9 0.01680 1.5 0.52 107.4 1.6 101.7 2.8 -30.7 60.8 107.4 1.6 23 751 5387 1.1 0.11608 5.7 0.01738 2.9 0.52 111.1 3.2 111.5 6.0 120.3 113.9 111.1 3.2 43 796 7848 1.6 0.13007 3.4 0.01972 1.7 0.51 125.9 2.2 124.2 4.0 91.4 68.9 125.9 2.2 95 1265 1180 1.5 0.20888 12.5 0.02571 2.2 0.17 163.7 3.5 192.6 21.9 564.0 268.8 163.7 3.5 83 725 6409 0.3 0.17342 4.1 0.02577 2.9 0.69 164.0 4.6 162.4 6.2 138.5 69.9 164.0 4.6 89 886 10013 1.3 0.17756 3.0 0.02622 1.4 0.47 166.8 2.3 166.0 4.6 153.6 62.7 166.8 2.3 8 580 5870 0.7 0.18453 4.2 0.02655 1.6 0.39 168.9 2.7 171.9 6.6 213.4 89.3 168.9 2.7 91 412 1393 0.6 0.19481 3.6 0.02685 2.5 0.69 170.8 4.1 180.7 5.9 312.5 59.3 170.8 4.1 18 322 1314 0.4 0.24169 17.3 0.02754 5.3 0.30 175.2 9.1 219.8 34.1 729.8 350.7 175.2 9.1 60 903 8083 0.7 0.19711 3.4 0.02808 2.2 0.64 178.5 3.9 182.7 5.8 237.0 60.9 178.5 3.9 36 1050 11014 0.8 0.19360 2.0 0.02824 1.4 0.71 179.5 2.5 179.7 3.3 182.3 32.6 179.5 2.5 7 333 8448 4.4 0.23563 3.6 0.03431 1.6 0.43 217.5 3.3 214.8 7.0 186.0 76.4 217.5 3.3 3 114 3473 1.3 0.25334 6.1 0.03617 2.4 0.39 229.0 5.3 229.3 12.5 232.0 129.9 229.0 5.3
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 222 ICP Mass Spectrometry
45 757 17984 0.8 0.28395 2.5 0.04091 2.1 0.85 258.5 5.3 253.8 5.5 210.6 30.3 258.5 5.3 48 369 10509 2.1 0.30736 7.5 0.04262 2.3 0.30 269.0 5.9 272.1 17.9 298.8 163.1 269.0 5.9 22 1888 6338 6.1 0.49580 5.8 0.05980 5.5 0.95 374.4 19.9 408.9 19.5 608.4 41.0 374.4 19.9 2 899 3756 0.7 0.56572 7.0 0.06519 6.4 0.92 407.1 25.4 455.2 25.7 706.2 57.9 407.1 25.4 59 1504 1574 1.9 0.61144 5.2 0.06695 2.9 0.55 417.8 11.5 484.5 20.0 813.7 90.9 417.8 11.5 26 721 2600 1.4 0.60935 8.0 0.06910 6.8 0.85 430.7 28.2 483.2 30.8 740.1 90.2 430.7 28.2 76 44 1886 1.5 0.52689 6.8 0.07115 2.5 0.36 443.1 10.5 429.7 23.9 359.0 143.7 443.1 10.5 50 416 9753 1.5 0.55558 2.5 0.07140 1.9 0.75 444.6 8.0 448.6 9.1 469.6 36.9 444.6 8.0 77 68 2850 1.1 0.49269 9.3 0.07206 1.9 0.20 448.5 8.2 406.7 31.1 175.8 212.0 448.5 8.2 20 974 12126 1.0 0.59156 5.3 0.07234 2.4 0.44 450.2 10.3 471.9 20.0 578.4 103.3 450.2 10.3 94 737 15855 1.6 0.57728 1.8 0.07286 1.0 0.57 453.3 4.4 462.7 6.6 509.5 32.1 453.3 4.4 54 569 7585 1.8 0.61109 3.4 0.07476 1.7 0.49 464.8 7.5 484.2 13.2 577.4 64.8 464.8 7.5 25 169 6179 1.4 0.54981 5.0 0.07515 4.5 0.90 467.1 20.4 444.9 18.1 331.5 49.6 467.1 20.4 85 968 33026 1.8 0.58450 6.4 0.07524 5.6 0.88 467.7 25.4 467.4 23.9 465.7 66.2 467.7 25.4 30 224 16229 1.1 0.60615 2.0 0.07551 1.3 0.65 469.2 5.8 481.1 7.5 538.1 32.6 469.2 5.8 32 698 29604 1.0 0.59013 2.0 0.07565 1.6 0.83 470.1 7.4 471.0 7.5 475.0 24.5 470.1 7.4 33 696 23124 1.5 0.60960 2.9 0.07604 2.7 0.95 472.5 12.4 483.3 11.1 535.1 20.7 472.5 12.4 70 504 8892 1.8 0.62378 4.1 0.07624 2.3 0.57 473.6 10.6 492.2 15.9 579.6 72.6 473.6 10.6 39 560 16673 1.2 0.61292 2.8 0.07629 1.8 0.63 473.9 8.1 485.4 10.8 539.8 47.2 473.9 8.1 19 319 9220 1.4 0.57388 2.7 0.07633 2.0 0.74 474.2 9.1 460.5 10.0 392.9 40.4 474.2 9.1 21 542 33329 2.0 0.59852 1.8 0.07639 1.4 0.80 474.5 6.5 476.3 6.7 484.8 23.6 474.5 6.5 14 861 19681 1.2 0.60032 7.8 0.07660 7.5 0.96 475.8 34.5 477.4 29.8 485.3 47.9 475.8 34.5 88 371 22464 7.1 0.66640 9.0 0.07675 8.7 0.97 476.7 40.1 518.5 36.6 707.4 47.1 476.7 40.1 15 451 26864 2.4 0.61098 2.5 0.07684 1.8 0.74 477.2 8.4 484.2 9.5 517.2 36.0 477.2 8.4 62 653 15598 2.2 0.63546 4.3 0.07726 3.0 0.70 479.8 13.8 499.5 16.9 590.9 66.3 479.8 13.8 74 423 7662 1.2 0.62624 2.8 0.07726 2.0 0.70 479.8 9.2 493.8 11.1 559.1 43.8 479.8 9.2 57 453 13461 1.4 0.62170 10.5 0.07729 10.1 0.96 479.9 46.7 490.9 41.0 542.5 66.3 479.9 46.7 53 412 20201 1.0 0.61329 2.8 0.07758 2.3 0.84 481.7 10.7 485.6 10.6 504.4 33.4 481.7 10.7 84 465 22822 1.7 0.61906 4.1 0.07777 2.4 0.58 482.8 11.1 489.3 15.9 519.5 73.2 482.8 11.1 35 249 9953 1.8 0.63357 7.0 0.07820 3.3 0.47 485.4 15.3 498.3 27.5 558.3 134.5 485.4 15.3 65 522 19604 1.2 0.61988 1.4 0.07845 1.0 0.73 486.9 4.7 489.8 5.3 503.2 20.6 486.9 4.7 80 1030 35103 2.7 0.62971 1.4 0.07934 1.0 0.70 492.2 4.7 495.9 5.6 513.3 22.1 492.2 4.7 55 385 17999 1.3 0.62432 3.3 0.07959 3.1 0.94 493.7 14.6 492.5 12.8 487.3 25.6 493.7 14.6 98 797 31278 1.5 0.63561 2.8 0.07982 1.8 0.62 495.1 8.4 499.6 11.2 520.3 49.2 495.1 8.4 16 1238 64487 2.3 0.63353 2.2 0.07994 1.0 0.46 495.8 4.8 498.3 8.6 509.8 42.7 495.8 4.8 81 657 20349 1.5 0.65303 9.6 0.08025 9.0 0.93 497.6 43.0 510.3 38.7 567.7 77.1 497.6 43.0 34 323 5552 1.6 0.69359 7.4 0.08047 1.9 0.26 498.9 9.3 535.0 30.8 691.7 152.4 498.9 9.3 58 835 23398 1.7 0.64995 1.5 0.08090 1.1 0.69 501.5 5.1 508.4 6.1 539.9 23.9 501.5 5.1 69 278 13541 1.5 0.64319 4.2 0.08090 4.0 0.95 501.5 19.1 504.3 16.6 516.9 29.9 501.5 19.1 90 609 21010 1.6 0.65090 3.5 0.08104 3.0 0.86 502.3 14.7 509.0 14.2 539.3 39.2 502.3 14.7 13 857 54593 3.1 0.64126 2.9 0.08126 2.4 0.82 503.6 11.7 503.1 11.6 500.5 36.7 503.6 11.7
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 223 ICP Mass Spectrometry
1 105 3526 1.4 0.67638 11.2 0.08171 4.0 0.36 506.3 19.4 524.6 46.0 605.0 227.6 506.3 19.4 46 226 11475 2.1 0.62950 1.9 0.08185 1.3 0.67 507.2 6.1 495.8 7.3 443.5 30.7 507.2 6.1 64 236 12890 1.4 0.65297 2.2 0.08186 1.1 0.50 507.2 5.3 510.3 8.9 524.1 42.1 507.2 5.3 61 473 18206 1.3 0.65377 2.1 0.08187 1.4 0.67 507.3 6.9 510.8 8.5 526.6 34.6 507.3 6.9 72 950 38687 1.4 0.64411 2.1 0.08200 1.6 0.79 508.0 8.0 504.8 8.2 490.4 28.3 508.0 8.0 42 149 11113 1.7 0.64679 2.0 0.08204 1.3 0.64 508.3 6.2 506.5 8.0 498.4 33.9 508.3 6.2 28 523 33638 1.6 0.65773 2.3 0.08209 2.0 0.86 508.6 9.6 513.2 9.1 534.0 25.1 508.6 9.6 75 282 14632 1.6 0.64890 4.4 0.08261 4.1 0.93 511.7 20.3 507.8 17.7 490.4 34.7 511.7 20.3 87 338 12820 1.0 0.64547 1.9 0.08401 1.5 0.80 520.0 7.6 505.7 7.6 441.4 25.5 520.0 7.6 96 641 32450 2.0 0.69482 3.5 0.08486 3.1 0.87 525.1 15.4 535.7 14.6 581.1 37.6 525.1 15.4 66 276 17532 2.1 0.73401 2.5 0.09226 2.4 0.95 568.9 13.1 558.9 10.9 518.4 17.6 568.9 13.1 93 231 10469 1.2 0.99206 3.3 0.11367 2.7 0.79 694.0 17.5 699.8 16.9 718.2 43.0 694.0 17.5 11 296 23311 1.1 1.04474 3.2 0.11616 2.8 0.85 708.4 18.5 726.3 16.8 781.8 35.3 708.4 18.5 78 331 23007 2.0 1.14495 3.8 0.12517 3.3 0.88 760.3 24.0 774.9 20.7 817.1 38.2 760.3 24.0 73 90 3213 1.1 1.16256 8.5 0.12646 2.2 0.26 767.7 16.1 783.2 46.5 827.5 171.8 767.7 16.1 100 559 31270 1.1 1.18359 3.3 0.12650 3.1 0.92 767.8 22.3 793.0 18.3 864.3 26.5 767.8 22.3 37 329 21590 1.6 1.17925 1.9 0.12699 1.7 0.87 770.7 12.1 791.0 10.5 848.6 19.3 770.7 12.1 6 288 11988 1.0 1.19961 2.7 0.12882 2.1 0.76 781.1 15.1 800.4 15.1 854.4 37.0 781.1 15.1 27 287 23929 1.8 1.17399 1.8 0.12896 1.5 0.87 781.9 11.4 788.5 9.7 807.1 18.0 781.9 11.4 49 666 63544 1.9 1.20714 2.6 0.13032 2.4 0.92 789.7 17.5 803.9 14.3 843.3 21.5 789.7 17.5 92 741 67957 1.3 1.22043 2.2 0.13142 1.0 0.45 796.0 7.5 810.0 12.5 848.6 41.6 796.0 7.5 97 948 57936 1.7 1.22920 1.4 0.13305 1.0 0.71 805.2 7.6 814.0 7.8 837.9 20.5 805.2 7.6 56 365 9933 1.6 1.27854 3.6 0.13428 2.9 0.80 812.3 21.9 836.2 20.6 900.4 45.1 812.3 21.9 41 730 72866 1.2 1.24449 1.1 0.13572 1.0 0.89 820.4 7.7 820.9 6.3 822.2 10.6 820.4 7.7 82 186 3695 2.4 1.30831 9.1 0.13770 1.6 0.18 831.6 12.6 849.4 52.2 896.0 184.2 831.6 12.6 99 351 14954 1.3 1.28011 4.1 0.13986 3.3 0.79 843.9 25.7 836.9 23.5 818.5 52.6 843.9 25.7 17 643 18269 2.0 1.46368 2.1 0.15049 1.2 0.56 903.7 10.0 915.5 12.8 944.2 36.0 903.7 10.0 40 169 14317 0.5 1.67729 8.4 0.16931 8.1 0.97 1008.3 76.0 1000.0 53.3 981.6 38.6 1008.3 76.0 4 258 26779 1.0 1.78208 2.1 0.17378 1.7 0.78 1032.9 15.8 1038.9 13.8 1051.6 26.9 1051.6 26.9 71 539 31472 3.8 2.24368 8.5 0.18632 8.3 0.97 1101.4 83.7 1194.8 59.7 1367.9 37.6 1367.9 37.6 29 359 58900 2.5 2.95173 6.2 0.21296 5.4 0.87 1244.6 60.8 1395.3 47.1 1633.8 57.6 1633.8 57.6 44 260 61612 4.2 5.27478 2.1 0.33816 1.9 0.91 1877.8 31.6 1864.8 18.2 1850.3 16.3 1850.3 16.3 24 722 126249 3.3 5.28108 2.3 0.33574 1.9 0.83 1866.2 30.5 1865.8 19.3 1865.4 22.6 1865.4 22.6 9 429 87858 4.6 5.54841 4.1 0.35216 3.5 0.87 1944.9 59.3 1908.1 34.9 1868.4 36.1 1868.4 36.1 47 43 10069 0.7 5.28102 2.2 0.33327 1.2 0.57 1854.2 20.1 1865.8 18.7 1878.7 32.4 1878.7 32.4 52 335 47035 6.5 5.61310 3.5 0.35058 3.1 0.91 1937.4 52.6 1918.1 29.9 1897.3 26.1 1897.3 26.1 68 150 38654 1.7 6.80141 2.5 0.38168 1.1 0.45 2084.1 19.9 2085.9 22.2 2087.6 39.4 2087.6 39.4 10 582 64113 2.4 6.40116 6.1 0.30175 5.7 0.94 1700.0 85.4 2032.4 53.4 2389.2 35.4 2389.2 35.4 51 216 79095 1.9 11.64913 2.0 0.48036 1.8 0.89 2528.8 37.0 2576.6 18.6 2614.4 15.2 2614.4 15.2 79 120 22208 2.4 13.07006 2.7 0.52112 2.5 0.94 2703.9 56.1 2684.7 25.4 2670.3 15.0 2670.3 15.0 67 746 151982 9.2 12.62547 2.2 0.48794 1.7 0.79 2561.8 36.3 2652.1 20.4 2721.8 21.8 2721.8 21.8
Table 2. (Cont’d). U-Pb geochronologic analyses for igneous and detrital zircon determined by Laser-Ablation Multicollector 224 ICP Mass Spectrometry
12 192 20139 1.5 18.10500 2.4 0.50663 2.2 0.92 2642.2 47.6 2995.3 23.0 3241.8 15.0 3241.8 15.0 63 308 584 1.7 0.77483 28.2 0.07012 5.7 0.20 436.9 24.2 582.5 125.7 1200.6 555.1 436.9 24.2
JG070605-1 sandstone (white shale): detrital zircon 25 ⎠ m spot size. 7 1199 2268 0.6 0.02070 11.2 0.00328 4.2 0.38 21.1 0.9 20.8 2.3 -11.9 252.1 21.1 0.9 22 1682 2143 0.7 0.02638 18.0 0.00368 6.4 0.36 23.7 1.5 26.4 4.7 286.3 387.9 23.7 1.5 13 454 4259 1.1 0.10362 6.2 0.01652 3.3 0.54 105.6 3.5 100.1 5.9 -29.6 126.3 105.6 3.5 28 644 7098 1.0 0.14323 18.4 0.01821 3.6 0.20 116.3 4.2 135.9 23.5 493.7 401.5 116.3 4.2 31 645 3631 2.2 0.18240 9.0 0.02307 2.4 0.26 147.0 3.5 170.1 14.2 504.8 192.2 147.0 3.5 32 550 3984 0.8 0.17399 7.6 0.02337 3.6 0.47 148.9 5.3 162.9 11.5 370.7 151.6 148.9 5.3 2 1993 14859 126.6 0.19519 5.2 0.02669 2.9 0.56 169.8 4.9 181.0 8.6 330.1 98.0 169.8 4.9 26 1126 12460 1.7 0.19168 4.2 0.02809 3.9 0.92 178.6 6.8 178.1 6.9 170.8 39.8 178.6 6.8 8 334 5833 1.2 0.22977 8.2 0.03368 6.5 0.79 213.5 13.6 210.0 15.5 170.8 116.3 213.5 13.6 23 647 9901 2.3 0.27042 6.4 0.03753 3.2 0.50 237.5 7.4 243.0 13.7 296.7 125.7 237.5 7.4 21 582 11314 1.2 0.29839 4.6 0.04218 2.3 0.50 266.3 5.9 265.1 10.6 254.5 90.9 266.3 5.9 20 475 14789 6.3 0.56439 4.4 0.07101 4.1 0.91 442.3 17.4 454.4 16.3 516.1 39.5 442.3 17.4 10 511 42432 1.7 0.60958 1.7 0.07801 1.3 0.78 484.2 6.2 483.3 6.6 478.8 23.8 484.2 6.2 24 114 3418 1.2 0.74208 11.8 0.07882 3.2 0.27 489.1 15.2 563.6 51.0 877.2 235.3 489.1 15.2 16 377 8258 1.9 0.68806 9.6 0.07901 8.4 0.88 490.2 39.9 531.6 39.8 713.7 97.4 490.2 39.9 9 290 9029 1.1 0.62273 2.9 0.07952 2.5 0.86 493.3 12.0 491.6 11.4 483.6 32.7 493.3 12.0 25 288 3138 2.8 0.87120 4.8 0.09364 2.1 0.43 577.0 11.4 636.2 22.6 852.6 89.9 577.0 11.4 17 186 4828 2.4 0.92917 5.6 0.10381 3.0 0.54 636.7 18.5 667.2 27.4 771.6 99.0 636.7 18.5 11 382 10849 1.3 1.03751 5.7 0.11542 5.5 0.96 704.2 36.6 722.7 29.7 780.6 35.1 704.2 36.6 15 310 18247 2.9 1.09622 2.6 0.12316 2.2 0.82 748.8 15.3 751.5 14.0 759.8 31.6 748.8 15.3 30 219 13927 1.1 1.16294 6.4 0.12323 5.1 0.79 749.1 35.8 783.3 34.9 882.1 80.9 749.1 35.8 19 47 4472 2.3 1.12248 4.7 0.12452 2.6 0.55 756.5 18.4 764.2 25.0 786.5 81.6 756.5 18.4 4 743 37611 2.3 1.51102 3.5 0.15382 3.2 0.90 922.3 27.1 934.9 21.3 964.5 30.7 922.3 27.1 1 39 4975 0.7 2.00972 4.7 0.19189 2.2 0.48 1131.6 23.3 1118.8 31.7 1094.0 82.1 1094.0 82.1 29 54 4395 0.8 1.88075 6.3 0.17680 3.8 0.60 1049.4 36.6 1074.3 41.5 1125.2 99.7 1125.2 99.7 33 360 15969 1.4 3.64259 3.8 0.23655 2.2 0.58 1368.7 26.9 1558.9 30.1 1827.0 56.0 1827.0 56.0
Igneous analyses in italics were not used in the average age calculation Detrital analyses in italics were not plotted in the probability density plots because they 1) are more than 30% discordant, 2) are more than 5% reverse discordant, or 3) have greater than 10% uncertainty Shaded values were used in the maximum depositional age calculations 225
APPENDIX D: U-Pb geochronology of basement rocks in central Tibet and paleogeographic implications
Manuscript for submittal to Journal of Asian Earth Sciences
Jerome Guynn University of Arizona
Paul Kapp University of Arizona
George Gehrels University of Arizona
Lin Ding Institute of Tibetan Plateau Research and Institute of Geology and Geophysics 226
U-Pb Geochronology of Basement Rocks in Central Tibet and Paleogeographic Implications
Jerome Guynn
Paul Kapp,
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 100029 China
227
Abstract
U-Pb geochronologic data from basement exposures in central Tibet provide new
constraints on the Gondwanan paleogeography of the Lhasa and Qiangtang terranes. The
Amdo basement is composed largely of granitic orthogneisses with subordinate
paragneisses and metasedimentary rocks. The intermediate-felsic compositions of the
orthogneisses suggest a continental arc or orogenic origin for the granitoid protoliths.
The orthogneisses have a bimodal distribution of Middle-Late Proterozoic (ca. 850-910
Ma) and Ordovician-Cambrian (ca. 480-530 Ma) crystallization ages. The former
correspond in time with Rodinia post-assembly magmatism while the latter granitoid ages
are common throughout Gondwana, including the Himalaya, and indicate post-
Gondwana assembly tectonics. In addition, detrital zircon analysis of three
metasedimentary rocks from the Amdo basement reveals an abundance of Middle-Late
Proterozoic and Cambrian ages with strong peaks at ~550 Ma and ~950 Ma, scattered
early Middle-Early Proterozoic ages, a distinctive 2500 Ma peak and very few Archean
ages. The detrital zircon age spectra are distinct from those determined for Tethyan and
Greater Himalaya rocks from the northern margin of India, which have a much stronger
“Grenville” signature (1000-1300 Ma ages) indicating the two were not as proximal as indicated in most reconstructions. Considering the geochronologic data, the necessity of
Greater India in addition to the current Indian continent, the constraints of southeast
Asian terranes that were rifted from northwest Australia, and the age and distribution of cratons and orogenic belts in Gondwana, we propose a Gondwana location for the combined Lhasa and Qiangtang terranes that is farther west than in most reconstructions, 228
closer to Iran, Afghanistan, Arabia and northwest India than to northeast India and
western Australia. We also suggest that the source for the Nepalese Himalayan rocks is
to the east near western Australia, eastern India and Antarctica.
1. Introduction
The Phanerozoic evolution of Asia is characterized by rifting of continental terranes off of Gondwana, their travel across the Tethys and their accretion to a Eurasian core
(e.g. Metcalfe, 1996; Şengör, 1979; 1984; see Fig. 1). The delineation and timing of rifting and accretion of these terranes has important consequences for the formation and subsequent breakup of supercontinents and the process of accretion. The location of these terranes in time also has implications for the timing and causes of orogenesis along the margins of Gondwana in the Paleozoic and in Asia during the Mesozoic and
Cenozoic. However, the reconstruction of these terranes along Gondwana’s margin is hampered by a scarcity of good paleomagnetic data due to the lack of appropriate
lithologies, difficult access and remagnetization as a result of Mesozoic and Cenozoic
tectonics. The latter has also resulted in the modification and overprinting of rifting and
collisional events by younger deformation. As a result, there is still a great deal of debate
concerning the position of peri-Gondwanan terranes in the Paleozoic and even less
constraint on pre-Gondwana locations. Here we present new geochronologic data from
Tibet that provides information on the Neoproterozoic-Cambrian history and Paleozoic
location of two peri-Gondwana terranes that comprise the bulk of the Tibetan Plateau. 229
The Tibetan Plateau is the largest orogenic feature on Earth, encompassing an area circa 5,000,000 km3 with an average elevation around 5 km. The primary terranes that
compose the Tibetan Plateau (Fig. 1) accreted to the southern margin of Asia throughout
the late Paleozoic and Mesozoic (Allégre et al., 1984; Chang and Zheng, 1973; Dewey et
al., 1988; Şengör and Natal'in, 1996; Yin and Harrison, 2000; Yin and Nie, 1996) and, from north to south, are the Kunlun-Qaidam, Songpan-Ganzi, Qiangtang and Lhasa; this paper focuses on the last two. The Tibetan Plateau has been the focus of many recent geologic and geophysical investigations but due to its large size, high elevation and
remoteness, there is still a great deal unknown about the geology of the plateau,
particularly the interior. Furthermore, the older geologic history of the terranes that make
up Tibet is obscured by the paucity of basement exposures and the predominance of
supercrustal assemblages that are late Paleozoic or younger. We present new
geochronologic data from the Amdo gneisses, the only confirmed crystalline basement in
central Tibet, that gives information on the Precambrian and early Cambrian history of
the Lhasa and Qiangtang terranes and, combined with detrital zircon ages of Paleozoic
sedimentary rocks, provides new constraints on their position along the margin of
Gondwana. We propose that the primary source for Tibetan sediments in the Paleozoic
was northwest India and east Africa and consequently we suggest a location for the
terranes further west, towards northwest Indian and northeast African, than most
reconstructions indicate. Furthermore we show that the Tibetan Paleozoic detrital zircon
signature is distinct from that of the Himalaya and suggest that the sedimentary source for 230
the early Paleozoic Himalayan sediments is western Australia, eastern Indian and
Antarctica and not northwest Africa as previously proposed (DeCelles et al., 2000).
2. Regional Geology and Paleozoic Paleogeography
In this paper, in the context of the Lhasa and Qiangtang terranes, the Tethys Ocean is subdivided into three oceanic regions after Şengör (1984) and Metcalfe (1996): the Paleo-
Tethys, the ocean between Qiangtang and Songpan-Ganzi (Eurasia); the Meso-Tethys, the ocean between Lhasa and Qiangtang (Eurasia); and the Neo-Tethys, the ocean between India and Lhasa (Eurasia). Note that these three terms do not necessarily indicate separate, unconnected oceans, but rather designate ocean crust that was created and consumed between distinct continental terranes. Also, we use the terms terrane and block interchangeably to indicate a fragment of continental or ocean island crust that is significantly smaller than a continent (e.g. India). While authors have referred to the
North and South China “blocks”, we will use the alternative designation of “craton”, since both contain Archean crust and are not much smaller than India.
The northernmost Tibetan terrane is the Kunlun-Qaidam terrane, a composite of Late
Paleozoic and Triassic arcs, Proterozoic gneisses and Paleozoic to Triassic sedimentary rocks (Gehrels et al., 2003c; Yin and Harrison, 2000), followed by the Songpan-Ganzi flysch complex, a thick accumulation of Triassic and early Jurassic deep marine sediments thought to have come from the Triassic Qinling-Dabie orogeny which resulted from the collision between the North China and South China cratons (Hacker et al., 1996;
Weislogel et al., 2006; Yin and Nie, 1996; Zhou and Graham, 1996). The Qiangtang 231
terrane is separated from the Songpan-Ganzi by the Jinsha suture and is dominated by
upper Paleozoic (Carboniferous and Permian), Jurassic and Triassic shallow marine
strata, fluvial deposits and volcanic sequences (Yin and Harrison, 2000). The southern- most Tibetan terrane is Lhasa, bounded by the Bangong suture zone to the north and the
Indus-Yarlung suture to the south. The Bangong suture zone is defined by a broad and discontinuous series of ophiolites and is a result of Meso-Tethys Ocean closure and
subsequent collision between the Lhasa and Qiangtang terranes in the Early Cretaceous
(Dewey et al., 1988; Girardeau et al., 1984; Guynn et al., 2006; Kapp et al., 2003a).
Rocks exposed in the Lhasa terrane consist of minor exposures of Paleozoic shelf
sequences, Mesozoic marine and fluvial rocks, and, especially to the south, Cretaceous-
Tertiary volcanic sequences and plutons (Yin and Harrison, 2000). The Indus suture is
the sight of the collision of India with Asia in the early Tertiary and rocks to the south of
the suture have an Indian affinity (Hodges, 2000).
The Lhasa and Qiangtang terranes are typically placed at the northern edge of India in
most Gondwana reconstructions (e.g. Metcalfe, 1996, 1999; Scotese, 2001; Torsvik and
Smethurst, 1999 - see Fig. 2) in essentially the same configuration they have today. The
two terranes are considered to have been one prior to Permian-Triassic rifting on the basis
of similar lithologies, including the presence of Carboniferous-Permian diamictites that
are also found in northern Arabia, northern India and Australia (Leeder et al., 1988), and
shared Gondwanan flora and fauna (Dewey et al., 1988; Metcalfe, 1996). The timing of
rifting from Gondwana and between each other is largely based on Permo-Triassic
basalts, syn-rift sedimentation and normal faulting recorded in the Lhasa terrane, 232
Qiangtang terrane and northern India (Gaetani and Garzanti, 1991; Leeder et al., 1988;
Pearce and Mei, 1988; Yin and Harrison, 2000). Unfortunately, the roughly east-west orientation of the northern edge of Gondwana in the late Paleozoic (see Fig. 2; Scotese,
2001) precludes the use of paleomagnetic data to constrain the lateral position of the
Lhasa-Qiangtang terrane and there is little data in the early Paleozoic. A summary of the limited available paleomagnetic data by Li et al. (2004) suggests that the two terranes were at approximately the same latitude until the Qiangtang terrane started drifting northward in the late Permian and their ensuing latitudinal paleopositions are broadly consistent with the timing of collisions along their bounding sutures.
The Qiangtang terrane is thought to have been part of a possibly continuous strip of continent, the Cimmerian continent, that rifted off of northern Gondwana in the Permian and drifted northward, closing the Paleo-Tethys and opening the Meso-Tethys (Metcalfe,
1996; Şengör, 1979, 1984). The Cimmerian continent is composed, from west to east, of
Turkey, Iran, Qiangtang and Sibumasu (Sino-Burma-Malaya-Sumatra) and its existence is based on similarities in paleolatitudes, flora, fauna and age of accretionary deformation between the different terranes. The longitudinal location of the terranes is largely based on their current position and their exact arrangement in the Paleozoic is debatable.
Within this framework, in the Paleozoic the Iranian blocks (Yazd, Tabas, Lut) are typically placed next to the northern margin of the Arabian plate, Afghan blocks (Farah,
Helmand) are tucked in a triangular region between Arabia and India, the Lhasa and
Qiangtang blocks are located along northern India and Sibumasu and West Burma sit against northwestern Australia (Fig. 2) (e.g. Metcalfe, 1996). The South China craton 233
and Indochina block were likely located outboard of Arabia and India in the early
Paleozoic and than drifted to a position closer to Australia in the late Paleozoic (Fig. 2).
An important consideration in positioning terranes on the northern edge of India that is often ignored is the space occupied by Greater India, that portion of the Indian continent whose lower crust has been underthrust beneath Asia (Veevers et al., 1975) and whose upper crust was composed of the three tectono-stratigraphic units that make up the
Himalaya, the Lesser Himalayan Sequence, the Greater Himalayan Sequence and the
Tethyan Himalaya (e.g. DeCelles et al., 2002). Shortening estimates suggest that at least
600-700 km of Indian lower crust has been thrust beneath Tibet (DeCelles et al., 2002 and references therein). The extra ~600 km on the northern edge of India, combined with the known location of Indo-China terranes to the northwest of Australia, leaves much less room for the placement of the Lhasa-Qiangtang terrane directly north of India. The room problem is exacerbated by shortening in the Lhasa and Qiangtang terranes which plate reconstruction models do not typically take into account. Geologic and geodetic data suggest at least 50% shortening of the Lhasa and Qiangtang terranes due to Mesozoic and
Cenozoic deformation as a result of accretionary tectonics and the Indo-Asian collision
(Murphy et al., 1997; Yin and Harrison, 2000).
3. Amdo Basement Geology
In general, exposures of rocks within the Tibetan terranes consist of supracrustal assemblages: low-grade sedimentary rocks, volcanic sequences and igneous plutons of
Phanerozoic age. Within the Qiangtang and Lhasa terranes, the only known Precambrian 234
basement exposure occurs near the town of Amdo along the Bangong suture zone
(Dewey et al., 1988; Guynn et al., 2006; Harris et al., 1988b; Xu et al., 1985). The Amdo
basement is composed of high-grade orthogneisses with subordinate metasedimentary
rocks, mafic amphibolites and migmatites that have been intruded by extensive Jurassic
granitoids (see Fig. 3; Coward et al., 1988; Guynn et al., 2006; Harris et al., 1988a;
Schärer et al., 1986). The Amdo basement has generally been considered part of the
Lhasa terrane because of ophiolite exposures at the town of Amdo north of the basement
(Coward et al., 1988), but Jurassic metamorphism of the basement indicates a Qiangtang affinity since Lhasa did not arrive at the Eurasian margin until the Cretaceous and Meso-
Tethys subduction is considered to have been northward (Dewey et al., 1988; Girardeau et al., 1984; Guynn et al., 2006). However, since the evidence indicates the two terranes were one until Permo-Triassic rifting, the differentiation between them is not critical for
Paleozoic paleogeography and the Amdo basement can be considered to have a combined
Lhasa-Qiangtang affinity and we will refer to the two terranes together for the rest of the paper.
4. U-Pb Geochronology
We analyzed nine samples of orthogneisses from the Amdo basement using U-Pb zircon geochronology to determine crystallization ages. The igneous composition of the gneisses consists largely of granite and granodiorite, with smaller outcrops of monzonite,
tonalite, and quartz-diorite, and most contain biotite and/or hornblende. The majority of
the gneisses are strongly banded, though some show only a slight foliation fabric. A 235
more complete description of each dated sample is given in Appendix A. Migmatites,
mafic garnet amphibolites and the presence of sillimanite indicate upper amphibolite
facies conditions. U-Pb ages were also obtained for detrital zircons from two quartzites and a paragneiss that occur in the basement. Locations of all samples are shown in
Figure 3 and sample coordinates and ages are listed in Table 1. The zircons were analyzed using the Laser Ablation-MultiCollector-Inductively Coupled Plasma Mass
Spectrometer (LA-MC-ICPMS) facility at the University of Arizona.
4.1. Methods
Zircons were separated from samples of fresh rock using standard crushing and
separation techniques. For magmatic analyses (including orthogneiss samples),
individual zircon crystals were hand-picked using a binocular microscope. All sizes and
morphologies were selected for analysis, though an attempt was made to choose crystals
that were free of inclusions and cracks. Approximately 50 grains were than mounted in
epoxy. For detrital analyses, zircons were poured in a layer onto the mount, generally
resulting in several hundred grains per sample. While the addition of other heavy
minerals on the mount is generally not a problem, any pyrite was removed from the
mount as the mineral generally results in lead contamination. The mounts are than
polished to approximately 2/3 of the crystal thickness. If possible, magmatic zircons
were imaged using cathode-luminescence on the SEM prior to analysis. As a rule, the
number of grains analyzed is: 25 for a simple magmatic sample, 50 for a more complex 236
magmatic (such as lead loss, young zircon growth or abundant inheritance) and 100 for a
detrital sample.
The grains were ablated with a New Wave DUV193 Excimer laser, operating at a
wavelength of 193 nm and using a spot diameter of 25, 35, or 50 microns. Larger laser
spot sizes are more accurate, due to the larger volume of material, but small grain sizes or
metamorphic overgrowth may necessitate a smaller spot size. The ablated material is
carried via argon gas into the plasma source of a Micromass Isoprobe multicollector ICP-
MS and ionized. The Micromass Isoprobe is equipped with a flight tube of sufficient
width such that U, Th, and Pb isotopes are measured simultaneously using nine Faraday
collectors, an axial Daly detector and four ion-counting channels. All measurements
were made in static mode, using Faraday detectors for 238U, 232Th, and 208-206Pb and an ion-counting channel for 204Pb. 235U is determined from the 238U measurement assuming
238U/235U = 137.88. Ion yields are ~1 mV per ppm. Each analysis consists of one background run with 20-second integration on peaks and the laser off, 20 1-second integrations with the laser firing, and a 30-second delay to purge for the next analysis.
The laser ablates at ~1 micron/second, resulting in an ablation pit ~20 microns in depth.
A common lead correction was performed by using the measured 204Pb and assuming an initial Pb composition from Stacey and Kramers (1975) (with uncertainties of 1.0, 0.3, and 2.0 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, respectively). Measurement of 204Pb is unaffected by the presence of 204Hg because backgrounds are measured on peaks
(thereby subtracting any background 204Hg and 204Pb) and because very little Hg is present in the argon gas. 237
Isotopic fractionation of Pb is generally <5%, while inter-element fractionation of
Pb/U is typically <20%. These fractionations were corrected by analysis of a standard with a known, concordant ID-TIMS (Isotope Dilution – Thermal Ion Mass Spectromety) age; standard analyses were conducted after every three unknowns for magmatic samples and after every five unknowns for detrital samples. The zircon standards are fragments of a large zircon crystal from a Sri Lankan pegmatite (e.g. Dickinson and Gehrels, 2003) with an age of 564.0 ± 4.0 Ma (2-σ). The uncertainty resulting from the calibration correction is generally ~3% (2-σ) for both 207Pb/206Pb and 206Pb/238U ages. U and Th concentrations were determined by analyzing a piece of NIST 610 glass with known U and Th concentrations of approximately 500 ppm and using the resulting measured intensities to calibrate the zircon U and Th intensities. The errors in determining
206Pb/238U and 206Pb/204Pb typically leads to several percent measurement uncertainty (2-
σ) in the 206Pb*/238U age. Analyses which have very low 206P/204Pb ratios, high 204Pb intensities or very low U concentrations are discarded due to the impractically high uncertainty.
The low concentration of 235U relative to 238U produces low concentrations of 207Pb in younger (approximately < 1000 Ma) samples which leads to substantially larger measurement uncertainty for 206Pb/207Pb. Consequently, the 207Pb*/235U and
206Pb*/207Pb* ages for younger grains have larger uncertainties and are less precise than the 206Pb*/238U ages. Therefore, if a magmatic sample contains a cluster of concordant ages and the age is less than ~1000 Ma, the reported crystallization age is based on a weighted average of the individual, concordant spot analyses using the 206Pb*/238U ages. 238
For most of the orthogneisses, however, the high-grade metamorphism resulted in lead loss and metamorphic overgrowth so that many of the samples do not have clear clusters of crystallization ages. In these cases, the reported crystallization age is based on a discordia regression through the zircon analyses or on averages of 206Pb*/207Pb* ages.
These crystallization ages are accordingly less precise and for a couple of samples an age with reasonable uncertainty simply cannot be defined. If Jurassic ages were obtained for some zircons or tips of the crystals, a weighted average of those ages was used to fix the lower intercept of the discordia since the high-grade metamorphism is known to be Early-
Middle Jurassic in age (Guynn et al., 2006), while for samples lacking at least two young, concordant ages, the lower intercept was fixed at 180 ± 10 Ma. The exact choice of a lower intercept (within ± 50 Ma) is actually not that important as it has a negligible effect on the upper intercept. In order to best define the discordia, those analyses with large error or discordance were not used in the regression; these analyses are shown with gray error ellipses in the concordia plots. For the quartzite detrital samples, the zircon age spectra are based on 206Pb*/238U ages if less than 1000 Ma and 206Pb*/207Pb* ages otherwise. However, in the paragneiss, lead loss and magmatic zircon growth is significant enough that we feel more confident using only the 206Pb*/207Pb* ages despite the large uncertainty in those ages. In addition, detrital zircon analyses are rejected if they have more than 10% uncertainty, 30% discordance or 5% reverse discordance.
All analyses are plotted with 2σ or 95% confidence limits, while apparent ages of individual zircon analyses in Table 2 are reported at the 1σ level. The stated uncertainties (2σ) on the assigned crystallization ages are absolute values and include 239
contributions from all known random and systematic errors. Random errors are included
in the data tables. Systematic errors (2σ) are as follows: JG053104-1, 1.18%; JG053104-
1, 0.94%; JG060504-2, 1.31%; JG061504-1, 0.81%; JG061504-2, 1.24%; JG061604-1,
1.15%; JG063004-1, 1.34%; PK97-6-4-1A, 1.30%; PK97-6-4-1B, 1.30%.
All U-Pb plots, weighted average calculations and discordia regressions were made using Isoplot 3.00 (Ludwig, 2003).
4.2. Igneous Zircon Analysis
Despite zircon lead loss and metamorphic growth, dating of individual crystals has allowed us to broadly define a bimodal distribution of orthogneiss ages for the Amdo basement (Table 1). The younger age group is Cambrian-Ordovician (~450-550 Ma) and the older is Middle-Late Proterozoic (~800-950 Ma). The latter are the first documented
Precambrian basement ages in the Lhasa and Qiangtang terranes. Most of the orthogneisses do not have inherited zircons, with the possible exception of a few discordant analyses and sample JG053104-2.
4.2.1. JG053104-1
The zircons in this sample exhibit significant lead loss and most analyses are discordant (Fig. 4). A discordia anchored at 180 ± 10 Ma gives an upper intercept age of
910 ± 16 Ma and an average of 206Pb*/207Pb* ages for the least discordant analyses results in an overlapping age of 915 ± 14 Ma; the latter is reported as the crystallization age. 240
4.2.2. JG061504-2
Some of these zircons appear to have suffered lead loss (Fig. 5). One zircon tip
yielded an age of 185.6 ± 1.5 Ma and though the U/Th ratio is only 5.6, it is significantly
higher than the rest of the analyses (U/Th ~ 0.8). A discordia line anchored at 180 ± 10
Ma gives an upper intercept of 878 ± 15 Ma.
4.2.3. PK970604-3A
The age of this sample was previously reported in Guynn et al. (2006) as 852 ± 18
Ma. The crystallization age is based on a distinct cluster of older, concordant zircon
analyses.
4.2.4. JG060504-2
The majority of zircon analyses in this sample are discordant, though there are some concordant analyses both in the Precambrian and the Jurassic, and several analyses have large uncertainties (Fig. 6). Two concordant zircon analyses from the tips of zircons with older cores have Jurassic ages of 175.2 ± 2.1 Ma and 178.8 ± 5.6 Ma and moderate U/Th ratios (U/Th ~ 8). Other tip analyses yielded older ages. Using an average age of 177 ± 6
Ma for the lower intercept of an anchored discordia line and only using analysis with low uncertainty, the upper intercept of the regression is 838 ± 23 Ma. Given the lack of concordant analyses of that age and the large uncertainties relative to the regression line, 241
we regard this sample as having an earliest Late Proterozoic crystallization age of ~840
Ma.
4.2.5. JG061504-1
Many of the zircons in this sample had a significant common lead component (i.e. very high 204Pb concentrations) and therefore did not yield reliable ages, resulting in
fewer analyses shown than most samples (Fig. 7). The two Proterozoic ages probably
represent inherited zircon grains. One zircon gave a 206Pb*/238U age of 187.4 ± 2.1 Ma which we interpret as young, metamorphic zircon growth (U/Th = 114.2). A small cluster of concordant ages around ~530 Ma was used to define a 260Pb*/238U crystallization age of 532 ± 7 Ma.
4.2.6. PK970604-1B
This sample yielded many discordant analyses with large uncertainties (Fig. 8).
There are too few precise ages and the ages are too young to define a meaningful discordia. A cluster of concordant 206Pb*/238U ages provides a weighted average age of
501 ± 12 Ma.
4.2.7. JG061604-1
The older analyses are mostly discordant without a clear cluster of ages (Fig. 9).
There is a distinct cluster of Jurassic ages with a mean 206Pb*/238U age of 181 ± 3 Ma, the majority of which are from zircon cores. Although there is no distinct cluster of older 242
ages and the Jurassic ages are clustered, we interpret the crystallization age to be
Precambrian and the Jurassic age to be metamorphic growth based on the high U/Th
ratios (15-60) of the Jurassic zircon analyses and the discordant nature and wide range of
the older ages. A discordia regression yields a poorly defined upper intercept of ~470
Ma, consistent with the other early Paleozoic crystallization ages, and we interpret it
simply to have a Cambro-Ordovician protolith.
4.2.8. PK970604-1A
Three of these zircons yield young ages, one discordant, but the majority cluster
around 500 Ma (Fig. 10). An average of clustered 206Pb*/238U ages yields an interpreted crystallization age of 483 ± 15 Ma, statistically equivalent to the age of PK970604-1B which was collected from approximately the same location.
4.2.9. JG053104-2
The analyses for this sample yielded two clusters of ages, a large group around 900
Ma and a smaller group around 500 Ma, some of which have large uncertainties (Fig. 11).
While the population of ~500 Ma zircons is much smaller, their concordance and the lack of intermediate ages (500-900 Ma) makes lead loss an unlikely reason for these younger ages. As a result, we interpret the younger ages as representing crystallization age and the older ages as inherited, though the large group suggests a significant component of crustal melting of the Precambrian gneiss in the generation of this sample. A weighted average of 206Pb*/238U ages was calculated for four concordant, low uncertainty analyses 243
around 500 Ma and yielded an age of 482 ± 18 Ma. A weighted 206Pb*/238U average was also calculated for the older population. Similarly, only four analyses were used for this population as well since there appears to be minor lead loss that pulled many of the ages down. The resulting age is 839 ± 21. An average of all 206Pb*/207Pb* ages between 800 and 900 Ma yielded 866 ± 22 Ma. Considering the uncertainty and discordance, we only confidently report a Cambro-Ordovician crystallization age with significant earliest
Neoproterozoic inheritance.
4.3. Detrital Zircon Analysis
U-Pb detrital zircon analysis was performed on three samples of metasedimentary rocks from the Amdo basement. The concordia diagrams of all three samples are shown in Figure 12 and the probability density functions are shown in Figure 13.
4.3.1. JG061504-4
This sample is a paragneiss from the central portion of the Amdo basement (Fig. 3).
The zircons in this sample have a wide range of morphologies and while some of the grains are well-rounded, others have very well-defined crystal shapes with sharp points.
However, CL-SEM images reveal that the latter generally have rounded or angular cores separated from the tips by bright zones (Fig. 14). Many of the tips were large enough to analyze with a 25 or 35 µm laser spot size and most of these analyses yielded mid-
Jurassic ages, revealing that the majority of the euhedral zircon shapes are due to metamorphic growth. The high-grade metamorphism also appears to have resulted in 244
lead loss for some of the grains so we prefer to rely on the 206Pb*/207Pb* ages for all of the grains in the probability density function, despite the larger uncertainty. There were a total of 161 analyses, 16 of which were Jurassic tips and 135 were used in the age spectra. The resulting age spectra has large overlapping peaks at ~850 and ~950 Ma, small peaks at ~1150 and ~1430 Ma, and large peaks at ~1600, ~1700 and ~2500 Ma.
Given the lead loss and metamorphic overgrowth, we assign a conservative estimate of the maximum depositional age as ~750 Ma.
4.3.2. JG062504-3
All 99 zircon grains analyzed from this quartzite sample are of sufficient quality to include in the age spectrum. The maximum depositional age is 493 ± 64 Ma, a weighted average of the youngest 11 zircons, all of which are concordant and overlap at the 2σ confidence level.
4.3.3. AP061304-A
A total of 92 zircons from this quartzite were analyzed and 85 of these passed the minimum constraints to be used in the age spectrum. The two youngest concordant ages combined give a maximum depositional age of 447 ± 30 Ma; the next youngest group of ages (n=4) gives a maximum age of ~510 Ma.
5. Discussion
245
Unfortunately there are no other reported exposures of crystalline basement rock in the Lhasa or Qiangtang terranes to compare with the Amdo basement ages. Instead we will emphasize a comparison of basement and detrital zircon ages to regional Gondwanan continents and terranes. First we will generate a “Tibetan” Paleozoic detrital zircon signature from several samples and compare these to regional detrital zircon records.
Second, we will discuss both the Amdo basement and “Tibetan” detrital zircon ages together in the framework of Gondwanan tectonics and magmatic provinces.
5.1. Tibetan Paleozoic Detrital Zircon Signature
Based on the youngest zircons, the Amdo quartzites had to be deposited no earlier than the Cambro-Ordovician, but it must be emphasized that this is only a maximum depositional age. If northern Gondwana developed a passive margin (Stern, 1994), the early Paleozoic magmatism would have been the last magmatic event until Gondwana started to break up in the late Paleozoic. Thus Cambro-Ordovician zircons could easily be the youngest in a Carboniferous sandstone or quartzite and in fact there is only one zircon age younger than Ordovician in two Tibetan samples that are known to be
Carboniferous in age (see below). Carboniferous sandstones and quartzites are very common in the Lhasa and Qiangtang terranes and there is a high probability that the
Amdo quartzites are in fact Carboniferous. The abundance of Cambro-Ordovician magmatism in Gondwana, including Amdo and the Himalaya, does make it highly 246
unlikely, however, that the Amdo paragneiss is Paleozoic in age and therefore we regard
this sample as Neoproterozic.
The three Amdo detrital zircon samples share some significant characteristics, notably a group of 800-1000 Ma Proterozoic ages and a 2500 Ma peak at the Archean-
Proterozoic boundary, as well as various peaks in the late Paleoproterozic and early
Mesoproterozoic (1100-2000 Ma) and few early Paleoproterozoic (2000-2400 Ma) or
Archean ages. The quartzites also share a Cambro-Ordovician group. In order to provide a broader context for comparing a Tibetan Paleozoic detrital zircon signature to magmatism and basement ages in Gondwanan continents, we combine the Amdo quartzite age spectra with three additional detrital zircon age spectra from Tibet in order to create a “Tibetan” Paleozoic detrital zircon signature. PK052402-2 is a Carboniferous sandstone from northern Qiangtang and PK061302-1 is a Carboniferous sandstone from the Nyainqentanglha mountains of the Lhasa terrane, about 150 km south of Amdo; both are unpublished data. NAMCO and DMXNG are Paleozoic sandstones, probably
Carboniferous, also from near the Nyainqentanglha mountains (Leier, accepted).
PK970619-1 is a quartzite from the Qiangtang blueschist belt in central Qiangtang (Kapp et al., 2003b). Mfab 4LL7 is an Eocene redbed also located in the Nyainqentanglha
Mountains, also unpublished. While the sample is Eocene in age, over 80% of the grains
yield ages older than 500 Ma and given its immature nature and the lack of pre-
Cretaceous igneous rocks in the area, it likely represents a proxy of the large exposures of
Carboniferous sedimentary rocks in the region. The post-Cambro-Ordovician Paleozoic
represents a stable time period for the Lhasa-Qiangtang terrane, between Cambro- 247
Ordovician Gondwana assembly and Permo-Triassic rifting, such that even if the Amdo quartzites are Ordovician they should record similar ages as the Carboniferous samples.
The Amdo paragneiss sample is kept as an individual spectrum.
The probability density curves for all nine samples are shown in Fig. 13 and they share many commonalities, including Cambro-Ordovician peaks, abundant 550-1000 Ma ages, scattered Middle and Early Proterozic ages and peaks around 2500 Ma. Despite the geographical and possible age differences, the Tibetan samples show remarkable similarities, even for samples separated by several 100s kms, and the shared characteristics increase our confidence in combining the six samples into a common
Paleozoic Tibetan signature.
5.2. Regional Detrital Zircon Record
We compare the Paleozoic Tibetan age spectra to detrital zircon (DZ) populations from other Gondwanan terranes, in particular those from the Nepalese Himalaya. The
Tibetan terranes are generally placed along the northern edge of India in Paleozoic reconstructions, so it might be expected that Tibetan and northern Indian sediments should have some similarities in their detrital zircon signatures. In addition to the
Nepalese sedimentary rocks, we also compare our ages to Cambrian sandstones of the
Spiti region in the northwestern Indian Himalaya, approximately 700 km west-northwest of Nepal (Myrow et al., 2003), Permian sandstones of the Paleozoic Perth Basin of southwestern Australia (Cawood and Nemchin, 2000) and Cambrian-Ordovician sandstones from Israel and Jordan (Kolodner et al., 2006). The probability density 248 function of each group is shown in Figure 15, together with the Amdo orthogneiss signature based on the ages defined by our U-Pb zircon geochronology, and ages of magmatism and tectonics from Gondwana which are discussed in detail in the next section.
The early Paleozoic Tethyan sedimentary rocks have a signature that is very similar to the GHS and the only significant difference between them is the Cambro-Ordovician peak in the Tethyan rocks, a result of Cambro-Ordovician granites in Greater India and the Neoproterozic-Cambrian depositional age of the GHS (DeCelles et al., 2000). The two are dominated by broad peaks of ~900-1300 Ma ages, centered ~1050 Ma, and also contain peaks ~2500 Ma. Both are very different from the LHS sequence which is characterized by a dominant group between 1800-2100 Ma with a peak ~1900 Ma. The
Lesser Himalayan Sequence (LHS) of metasedimentary rocks is presumed to have been sourced from the Indian continent in the Paleoproterozoic, while the metamorphosed
Greater Himalayan Sequence (GHS) and the Tethyan sedimentary rocks appear to have been derived from a source external to India (DeCelles et al., 2004; DeCelles et al., 2000;
Gehrels et al., 2003a).
The Tibetan rocks, including the Amdo paragneiss, have a DZ signature that is quite different from the Himalayan rocks. The Paleozoic rocks have a very wide spread in zircon ages, resulting in small peaks or regions that have very low wavelength peaks.
The dominant age groups are ~500-550, ~900-1000 and ~2450-2550 Ma, with smaller and broader peaks ~800-900, ~1050-1250 and ~1700-1900 Ma. The paragneiss has three main groups, ~750-1000, ~1550-1800 and ~2450-2550 Ma, that overlap with the 249
Paleozoic age groups. The Tibetan rocks do not contain the late Mesoproterozic ages of
the Tethyan and GHS and the latter are missing the 800-900 Ma ages of the Tibetan
rocks. Interestingly, the DZ signature of the Indian Himalayan rocks from farther west in
the Himalaya have an age spectrum that contains elements of both Tibetan and Nepalese
Himalayan rocks, including abundant 800-1000 Ma ages similar to the former and some
1000-1100 Ma ages and a lack of Paleoproterozoic ages like the latter. These rocks also contain peaks at ~575 and ~750 Ma, as well as a Cambrian peak. The Indian Himalaya and Tibetan samples contain ages similar to those in the Amdo basement, while the only commonality between the Amdo basement and the Himalayan rocks is the Cambro-
Ordovician peak in the Tethyan sedimentary units.
The DZ age spectrum that most closely matches the Nepalese Tethyan and GHS distributions is that from the Perth Basin in western Australia. It contains a dominant peak ~1100 Ma, within a large group from 850-1250 Ma, and a significant Cambro-
Ordovician peak. The primary difference is a ~2600 Ma Archean peak instead of the
~2500 Ma peak observed in the Himalayan and Tibetan spectra. Cawood and Nemchin
(2000) also analyzed a single Ordovician sandstone sample from the Perth Basin and it is broadly similar to the Permian sandstones, though it has less 1100-1250 Ma ages.
The Cambo-Ordovician sandstones from Jordan and Israel were deposited on the edge of the Arabian-Nubian Shield (ANS) and the dominance of ~550-750 Ma ages is a result of being largely derived from the ANS to the south (Avigad et al., 2003; Kolodner et al., 2006). The units also contain a population of 750-1100 Ma ages and a few ages in the mid-Paleozoic and late Archean. The age spectrum is very distinct from the others 250
and like the LHS is dominated by one large group of locally derived zircons. This
signature is similar to that from the immature, clastic-rich late-Neoproterozoic
Hammamat Group from eastern Egypt, which shows a large group of ages from 600-800
Ma, with peaks ~650 and ~750 Ma, a few 1800-2000 Ma grains a few 2400-2600 Ma
grains (Wilde and Youssef, 2002).
All the rocks contain limited Archean grains, with the exception of ubiquitous peaks
in the 2450-2650 Ma age range. Even in the LHS, which was sourced from cratonic
India, only about 20% of the ages are Archean and half of those are between 2500 and
2600 Ma.
In summary, the Tibetan rocks have a detrital zircon age signature that is distinct from that of the Nepalese Himalaya, indicating a difference in source region and suggesting a more distal location than generally assumed. Furthermore, the variation in the Indian and Nepalese detrital zircon signature may indicate a lateral change in
Himalayan sources, though the limited number of ages in the Indian distribution makes this a tentative conclusion. Finally, DeCelles et al. (2000) suggest that the source of the
800-1000 Ma ages in the Tethyan and GHS rocks was the Pan-African orogen, specifically the ANS, but the Jordan and Israel units have a week 800-1100 Ma signature and the Himalayan samples do not show the strong 600-800 Ma age group as seen in the
Middle Eastern sandstones. In addition, the 900-1100 Ma ages in the Jordanian and
Israeli samples are not known in the ANS and are proposed to have come from a southern, distal source unless there are unexposed rocks of that age in the ANS (Avigad et al., 2003). In light of the striking similarity between the Nepalese Himalayan ages and 251
those of western Australia, we propose an alternate source in eastern India, western
Australia and Antarctica.
5.3. Regional Basement Ages
By the end of the Cambrian, Gondwana had been assembled and the continent of
India was surrounded by (clockwise order) Australia, Antarctica, Africa and the newly created Arabian-Nubian Shield (Fig. 2), a configuration that remained until the initiation of its break-up in the Jurassic. The South China Craton also appears to have been located near Arabia and India in the early Paleozoic, but than drifted further north and east to a position north of Australia by the middle Paleozoic (Fig. 2). These continents and possibly some of the peri-Gondwanan terranes are the possible sources for Tibetan sediments. Our discussion of source areas is concentrated on regions proximal to northern India, such as northeastern Africa, western Australia and “east” Antarctica (~30
E-120 E). While it has been shown that sediments can be dispersed thousands of kms from there source regions (e.g. Dickinson and Gehrels, 2003), the Neoproterozoic and
Cambrian tectonics surrounding India resulted in mountain ranges that decrease the likelihood of sediments being derived from the interiors of the large continents (Fig. 16).
Archean rocks occur in cratons throughout all the major continents of Gondwana, particularly in India and western Australia, but Archean ages are not well represented in any of the detrital zircon signatures, with the exception of ages in the 2500-2700 Ma range. The lack of Archean ages may be due to a lack of exposed Archean material, to the interior location of most Archean cratons, or to Archean zircons not surviving transport as well, possibly due to long-term radiation damage. The ~2500 Ma peak is 252
common to most of the detrital zircon records and may indicate a common crust-forming event at that time (Condie et al., 2005). The Aravalli Craton in western India contains
abundant granitoids with ~2500 Ma ages, as well as 2600-2700 Ma and 2900-3300 Ma
(Choudhary et al., 1984; Pandit et al., 2003; Roy and Kroner, 1996; Wiedenbeck et al.,
1996) and modern sands of northwest India show a peak of 2500 Ma ages as well
(Condie et al., 2005). The Bundelkhand Massif in central India also contains basement
with ages of ~2500 Ma, ~2700 Ma, and ~3300 Ma and may be part of the Aravalli Craton
(Mondal et al., 2002). Metavolcanics of the northern Indian Himalaya have Nd depleted
mantle model ages of 2400-2600 Ma (Miller, 2000), suggestive of underlying ~2500 Ma
basement. The Dharwar craton of southern India also has significant 2500-2550 Ma
magmatism (Chadwick et al., 2000; Collins et al., 2003; Fitzsimons and Hulscher, 2005;
Jayananda et al., 2000) as does the Rayner complex in Antarctica (Cox et al., 2004)
which would have been next to the Dharwar craton in Gondwana. In addition, Dharwar
contains appreciable basement ages of ~2650 and 3000-3400 Ma (Collins et al., 2003;
Fitzsimons and Hulscher, 2005; Jayananda et al., 2000) and detrital zircon data from
southeast India include peaks at ~2500 Ma, ~2650 Ma and ~2950 Ma (Cox et al., 2004).
Ages of ~2500 Ma are also common in Madagascar, in both bedrock and detrital
sediments, as well as ages of ~2700 Ma and ~3200 Ma (Collins et al., 2003; Cox et al.,
2004; Fitzsimons and Hulscher, 2005; Tucker et al., 1999); these ages are also common
in Tanzania and Zimbabwe, as well as 3000-3100 Ma (Fitzsimons and Hulscher, 2005;
Johnson et al., 2003). Inherited zircons and Neodymium model ages suggest there may
be 2400-2600 Ma basement within the much younger Neoproterozoic crust of the 253
Arabian-Nubian Shield, including Somalia and Ethiopia (Johnson and Woldehaimanot,
2003; Whitehouse et al., 1998). The cratons of central and western Australia contain
abundant Archean ages (2600-3200 Ma), including 2600-2850 Ma from the Yilgarn
Craton and 2100-2700 Ma and ~3200 Ma in the Gawler, but generally lack significant
~2500 Ma crust (Veevers, 2004; Veevers et al., 2005). Some Archean ages have been
documented in the South China Craton in the Kongling region of the north-central
Yangtze craton (Qiu et al., 2000), but much of the craton is covered in Meso- and
Neoproterozoic sedimentary rocks (Goodwin, 1996) that indicate it was not well exposed
in the Paleozoic either.
Paleoproterozoic and early Mesoproterozoic ages are also sparse in the Gondwanan
DZ spectra, with the exception of a 1550-1700 Ma group in the Amdo paragneiss. The
Indian and Nepalase Himalaya contain Middle Proterozoic gneisses with ages ~1800-
1900 Ma, generally from within the LHS (DiPietro and Isachsen, 2001; Le Fort and Rai,
1999; Miller et al., 2000; Zeitler et al., 1989). The Singhbhum craton contains 2100-
2400 Ma granitoids and orthogneisses while the nearby Bhandara craton contains ~2200
Ma magmatism (Goodwin, 1996) and the Aravalli Craton contains sources with 1500-
1700 Ma and 1900-2000 Ma ages (Choudhary et al., 1984; Deb et al., 2001). The lack of
1700-2100 Ma ages in India is surprising given the very strong peak in the LHS, but this could be due to a lack of exposure, including the large extent of Paleoproterozoic sedimentary rocks which may themselves be sources, and to a paucity of geochronologic data. The ANS also has indications of 1600-1800 Ma inherited crust (Johnson and
Woldehaimanot, 2003) and ~1400 Ma and 1700-2500 Ma crust are indicated in Somalia 254
(Kroner and Sassi, 1996; Stern, 1994). Detrital zircon data from the Rayner complex in
Antarctica has significant ages in the 1250-1400 Ma range (Cox et al., 2004). In Western
Australia there are various Early and Middle Proterozoic basement rocks, particularly
1500-1800 Ma and ~2000 Ma (Veevers, 2004; Veevers et al., 2005) and the 1600-1900
Ma Capricorn Orogeny at the northern edge of the Yilgarn Craton (Bruguier et al., 1999).
These ages are also scarce in the South China block, though Chen, 2003 suggest 1800-
1900 Ma ages.
The late Mesoproterozoic is dominated by extensive orogenies that are thought to mark the amalgamation of most of the continents into a supercontinent termed “Rodinia”
(see Weil, 1998 for some references). The time period of most intense activity is ~1000-
1300 Ma and herein termed the “Grenville” based on the North American orogenic province of the time period. A Grenville signature is most apparent in the Nepalese
Himalaya and the Perth Basin samples, though these ages are also present in the Tibetan,
Indian Himalaya and Jordan/Israel sandstones. Magmatism and crustal genesis associated with Grenville orogenies is limited in eastern Africa, Madagascar and western
India (Kroner & Cordani, 2003). The nearest Rodinia-related orogenic belts are the
Kibaran, Irumide and Zambezi belts located south and west of the Tanzania Craton in
Tanzania, Malawi, Zambia and Zaire. The Neoproterozoic Molo quartzite in Madagascar contains abundant zircons of that age (Cox et al., 2004) which probably came from these belts and magmatism of that age is also reported in the Mozambique belt of Tanzania and
Malawi (Kroner, 2001) to the west of these belts. The Eastern Ghats of India contain
Grenville age metamorphism and magmatism and may related to 1000-1100 Ma 255
magmatism in the Darling Mobile belt of northwestern Australia and 900-1100 Ma
magmatism in the Rayner and Napier complexes and northern Prince Charles Mountains of Antarctica (Boger et al., 2000; Bruguier et al., 1999; Collins and Pisarevsky, 2005;
Dobmeier and Raith, 2003; Meert and Torsvik, 2003; Mezger and Cosca, 1999). The
Albany-Fraser belt of southwestern Australia and the Windmill and Bunger Hills regions of Antarctica are probably related to a Grenvillian collision between a proto-India and proto-Antarctica and contain ages between 1150-1350 Ma (Clark et al., 2000; Condie,
2003; Meert, 2003; Veevers et al., 2005). Magmatism ~1100 Ma is also seen in the
Leeuwin block along the southwest coast of Australia (Cawood and Nemchin, 2000) and there is some 1000-1100 Ma magmatism in the South China craton (Chen et al., 2003).
The most significant ages in the Tibetan and Indian Himalaya spectra are the 800-
1000 Ma group because this age range, between the amalgamation and break-up of
Rodinia, is not especially common. These ages appear in the Israeli/Jordanian and Perth
Basin sandstones, yet there is no currently local source for either (Cawood and Nemchin,
2000; Kolodner et al., 2006). It is also the presence of these ages in the Nepalese
Himalayan units that prompted DeCelles et al. (2000) to suggest a northwest African
source for these sediments. Magmatism of this age occurs in the South China Craton,
though it is unlikely to have contributed to the Perth Basin sediments. Along the northern
edge of the Yangtze Craton (part of the South China Craton) in the Qinling Mountains,
825-880 and 900-960 Ma granites may be related to subduction and Rodinia assembly
(Chen et al., 2006; Ling et al., 2003; Xue et al., 2006) while in the central and southern
Yangtze Craton, 800-900 Ma magmatism is proposed to be related to subduction and the 256
subsequent collision with the Huanan block during the Jinning orogeny (Li, 1999; Wang
et al., 2006; Wu et al., 2006) or to an arriving plume prior to rifting (Li et al., 2003a; Li,
1999; Li et al., 1999; Li et al., 2003b). Magmatism in the Irumide belt of Mozambique and Malawi is 950-1050 Ma (De Waele et al., 2003). As discussed earlier, magmatism in the Eastern Ghats continued past the “Grenville” time period to as young as ~900 Ma and there is evidence of an 800-1000 tectonothermal event in the Sausar Mobile belt of the
Central Indian Tectonic Zone, though the extent of magmatism is unclear (Roy et al.,
2006). The most extensive area of magmatism this age is in the Aravalli Craton, which
includes 800-900 Ma granitoids in the Aravalli-Delhi orogenic belt (Choudhary et al.,
1984; Rathore et al., 1999) and volcanics, granitoids and orthogneisses of the Amaji-
Sendra belt, a possible arc terrane (Deb et al., 2001; Pandit et al., 2003). Orthogneisses in
the Indian Himalaya northeast of the Aravalli Craton have also been dated at ~825 Ma
(DiPietro and Isachsen, 2001; Singh et al., 2002). Another possible source is Antarctica,
where 900-1020 Ma granitoids have been reported in the Prince Charles Mountains,
which would have been opposite the Eastern Ghats in the Paleozoic (Boger et al., 2000;
Fitzsimons, 2003). There are also ~825 Ma and 920-930 Ma plutons in the Qaidam-
Qiling terrane of northern Tibet, though this terrane may have been much farther east and north in the Paleozoic (Gehrels et al., 2003b; Gehrels et al., 2003c). Finally, it is important to recognize that the Amdo basement itself is a possible source of 800-925 Ma ages and if the Amdo paragneiss was sourced locally, the Tibetan basement could contain plutons from 750-1000 Ma. 257
The earliest magmatism in the Arabian-Nubian shield, including parts of Ethiopia and
Somalia, is ~870 Ma and related to initial rifting (Grenne et al., 2003; Pallister et al.,
1988; Stern, 1994), though inherited zircons and Neodymium model ages indicate the presence of older (800-1200 Ma and older) crust (Grenne et al., 2003; Teklay et al.,
1998). Rifting was closely followed by seafloor spreading, calc-alkaline arc-related magmatism and terrane accretion between 600-870 Ma (Ayalew et al., 1990; Grenne et al., 2003; Johnson et al., 2004; Kroner and Sassi, 1996; Pallister et al., 1988; Stern, 1994;
Teklay et al., 1998), with higher volumes of pluton production around 620-660 Ma and
720-780 Ma (Johnson and Woldehaimanot, 2003). The ANS had amalgamated by ~600
Ma, possibly by collision of older crustal material to the east (Windley et al., 1996), and younger activity is post-orogenic (Johnson and Woldehaimanot, 2003; Stern, 1994). This magmatic history matches fairly well with the distribution of ages in the Jordanian and
Israeli Cambrian sandstones, which show a peak ~630 Ma, though the low number of
700-850 Ma ages is surprising. The 850-1250 Ma ages in this sample could represent older basement buried beneath Phanerozoic sedimentary rocks or may simply represent more distal sources (Kolodner et al., 2006).
Magmatism of 600-800 Ma age is fairly common in other locations around India, though these ages are scarce in the Perth Basin and Nepalese Himalayan units and are only a minor component of Tibetan distributions. Igneous activity between 700 and 850
Ma in the Yangtze block is thought to represent the break-up of Rodinia (Chen et al.,
2006; Li et al., 2003b), the Jinning orogeny which resulted from the collision of the
Huanan block with the Yangtze block (Chen et al., 2003; Wang et al., 2006) or 258
subduction and back-arc spreading along the Yangtze continental margin (Druschke et
al., 2006). The primary age of the Milani complex in the Aravalli Craton is ~750 Ma
(Choudhary et al., 1984; Rathore et al., 1999; Torsvik et al., 2001b) and is proposed to be related to 700-800 Ma magmatism in the Seychelles (Tucker et al., 2001) and
Madagascar (Collins et al., 2003; Handke et al., 1999; Tucker et al., 1999), with the three areas forming a Neoproterozoic subduction zone (Handke et al., 1999; Torsvik et al.,
2001a). Magmatism in the 600-700 Ma range is also found in Madagascar (Kroner,
2001; Tucker et al., 1999). The Leeuwin block in western Australia contains abundant plutons between 680 and 800 Ma (Cawood and Nemchin, 2000; Collins, 2003) and 700-
800 Ma magmatism occurs in the Yilgarn and Pilbara Cratons (Wingate and Giddings,
2000).
Latest Neoproterozoic and early Paleozoic (450-600 Ma) magmatism is almost exclusively Gondwanan in origin (Veevers, 2004) and granitoids of this age are common
along the margins of Gondwanan continents. Cambrian-Ordovician magmatism in
Gondwana (e.g. Himalayan events in northern India (Gehrels et al., 2003a) and the Ross-
Delamerian Orogeny in eastern Australia-eastern Antarctica (Boger and Miller, 2004))
may be related to a reorganization of plate boundaries following late Neoproterozoic
Gondwana amalgamation (Boger and Miller, 2004). Around India, late Neoproterozoic
and early Paleozoic orogenic events include the Mozambique-Malagasy, Kuunga and
Pinjarra orogenies (Collins and Pisarevsky, 2005; Veevers, 2004). All the Paleozoic
detrital zircon distributions show peaks or age groups of ~550 Ma, which could be
represented by plutonism in the ANS of Arabia and Ethiopia (Ayalew et al., 1990; 259
Johnson and Woldehaimanot, 2003), the Malagasy-Mozambique orogeny between West
and East Gondwana (Boger and Miller, 2004; Collins and Pisarevsky, 2005; Kroner,
2001; Tucker et al., 1999), the Kuunga-Pinjarra orogenies in western Australia, eastern
India and Antarctica (Boger and Miller, 2004; Cawood and Nemchin, 2000; Collins,
2003; Fitzsimons, 2003; Mezger and Cosca, 1999), and magmatism in Iran (Ramezani
and Tucker, 2003).
Cambro-Ordovician plutonism is also represented in Himalaya and Tibet. Half the
Amdo orthogneiss ages presented here are of this age and Kapp et al. (2000) reported a
Cambrian-Ordovician garnet-amphibolite gneiss within a blueschist-bearing mélange in
the central Qiangtang terrane which they suggest was derived from the northern
Qiangtang basement, an interpretation consistent with the presence of the Amdo Cambro-
Ordivician orthogneisses. Inherited ~550 Ma (and ~2500 Ma) zircons have been reported
in the Cretaceous-Tertiary granitoids and orthogneisses of the Nyainqentanglha mountains of the Lhasa terrane (Kapp et al., 2005). Cambro-Ordovician orthogneisses are also widespread in the GHS of the Himalaya (DeCelles et al., 1998; Gehrels et al.,
2003a; Godin et al., 2001; Le Fort, 1986; Schärer and Allègre, 1983) and may be related to early Paleozoic collisional tectonics (Gehrels et al., 2003a and references therein). In the Tethyan Himalaya, north of the GHS and the high Himalayan peaks, orthogneisses are exposed in the cores of several gneiss domes. An initial U-Pb zircon date of the
Kangmar dome orthogneiss yielded an age of ~565 Ma (Schärer et al., 1986), but more recent U-Pb zircon work yielded two well-constrained ages of ~510 Ma (Lee et al.,
2000). In the Malashan dome further west, Aoya et al. (2005) concluded that the 260
crystallization age of the orthogneiss is Miocene and older ages are due to inherited
xenocrysts, though it may be as likely that some of the older cores are magmatic and the young rims are a result of Miocene metamorphism. Early Paleozoic orthogneiss exposed in the Kangmar and possibly other gneiss domes may be related to the orthogneisses of the GHS (Gehrels et al., 2003a). The Tibetan and Himalayan magmatism is slightly younger (~480-510 Ma) than the main peaks seen in most of the DZ signatures (~550
Ma), suggesting that the local magmatism may not have contributed a large signal to the age spectra. The ~550 Ma ages are representative of final Gondwana assembly and post-
collison magmatism represented by the Mozambique-Malagasy-Kuunga orogenies
around the margins of India and could have come from East Africa, Madagascar, western
Australia, Antarctica or peri-Gondwanan terranes which are not well dated.
Alternatively, this slightly older magmatism may exist in Tibet and the Himalaya but has
not been exposed or dated.
The Cambrian tectonics may have resulted in an early episode of metamorphism and
deformation of the Amdo orthogneisses (Coward et al., 1988; Xu et al., 1985), which
could explain why the Jurassic granitoids that intrude the gneisses are undeformed
despite being coeval with Jurassic metamorphism. The region may have than developed
into a passive continental-shelf margin, generating sandstones, pelites and carbonates that
eventually became the quartzites, schists and marbles seen in the Amdo basement as well
as the Carboniferous-Permian shales, sandstones, carbonates and diamictites that are
common throughout the Lhasa and Qiangtang
261
5.4. Implications for Paleogeography of the Lhasa-Qiangtang terrane
The Tibetan Paleozoic sedimentary rocks have a DZ age spectrum that is significantly different from those of the northern margin of India, indicating that they did not have the same spatial relationship in the Paleozoic that they do today. The Nepalese Himalayan samples are surprisingly similar to age spectra from Permian sandstones of the Perth basin, suggesting common source terranes. The most significant group of ages in the
Nepalese Himalayan samples is Grenville in age. Grenville age mountain belts are lacking in eastern Africa and western India but common in western Australia, eastern
India and Antarctica (Fig. 16), making this region a likely source for the Perth basin
(Cawood and Nemchin, 2000) and Nepalese Himalayan sedimentary rocks. The Tibetan samples have much fewer Grenville ages with a more significant peak in the 900-1000
Ma range.
Of all the Gondwanan source regions, northwestern India is the most likely for the
Tibetan signatures, as well as Madagascar and northwestern Africa, while southern and central Africa, western Australia and much of Antarctica seem the least likely. The former areas have significant crust of 2500 Ma age, orogenic events from 750-1000 Ma and little 1000-1300 Ma orogenesis. In particular, the Aravalli Craton of India and the island of Madagascar contain significant 2500 Ma rocks, the Aravalli Craton and northwestern India contain the uncommon 900-1000 Ma ages as well as other
Neoproterozoic ages and Madagascar and east Africa are sources for 500-750 Ma ages.
This would place Lhasa-Qiangtang farther east (Fig. 16), closer to the Helmand block
(Afghanistan) and Karakorum (Pakistan) which have many similarities to the Tibetan 262 terranes (Le Fort et al., 1994) and also to Iran, whose blocks include Cambrian arc magmatism (Ramezani and Tucker, 2003). The South China craton also contains many of the appropriate ages but could not have contributed to late Paleozoic sediments as it was distally located from the Africa and Indian margins at that time (see Fig. 2; Cocks and Torsvik, 2002; Li et al., 2004; Torsvik and Cocks, 2004).
The peaks in the paragneiss are surprisingly similar to magmatic and metamorphic ages in the Aravalli-Delhi fold belt, the Aravalli Craton and Madagascar. The Aravalli-
Delhi orogeny produced 1500-1700 Ma magmatism and this area also contains 800-900
Ma granitoids (Choudhary et al., 1984); the Amaji-Sendra belt, a possible arc terrane, contains ~986 Ma rhyolites, ~970 Ma granitoids and ~820-840 Ma orthogneisses and granitoids (Deb et al., 2001; Pandit et al., 2003); and the Malani complex contains ~730-
850 granitoids and volcanics (Deb et al., 2001; Pandit et al., 2003; Rathore et al., 1999;
Torsvik et al., 2001b). The early Neoproterozoic magmatism appears to extend to both the northeast, where ~825 Ma granitoids occur in the Himalaya (DiPietro and Isachsen,
2001; Singh et al., 2002), and to the southwest, with 750-800 Ma magmatism in
Madagascar (Collins et al., 2003; Handke et al., 1999; Tucker et al., 1999) and 750-810
Ma magmatism in the Seychelles (Torsvik et al., 2001a; Tucker et al., 2001), which are proposed to have formed a continuous northern Madagascar-Seychelles-Malani arc in the
Neoproterozoic (Handke et al., 1999; Torsvik et al., 2001a; Tucker et al., 1999).
Druschke et al. (2006) even proposed that ~700-850 South China magmatism is related to this postulated continental arc in and is a result of rifting of continental fragments from 263
Rodinia, creation of back-arc basins and the re-establishment of continental subduction zones.
One speculative possibility is that the ~840-920 Ma Amdo orthogneisses and the
Neoproterozoic paragneiss are related to the ~850-1000 Ma magmatism in the Aravalli
Craton, the Indian Himalaya and the northern Yangtze craton of South China and part of this continental arc system. The South China Craton is sometimes placed to the east of
Australia in ~750 Ma Rodinia reconstructions (e.g. Li et al., 1995; Meert and Torsvik,
2003), but the paleomagnetic poles also allow it to be placed further east near northwest
Australia (e.g. Evans et al., 2000; Macouin et al., 2004). India may also have been located at this approximate latitude and position (Meert and Torsvik, 2003; Torsvik et al.,
2001b), but others place it further north (Evans et al., 2000; Yang et al., 2004). If India and the South China Craton were adjacent to one another, they could have shared a subduction zone along their margins; alternatively, an ocean could have been consumed between them, resulting in coeval but spatially distinct arcs. Regardless, a continental arc margin may have stretched from Madagascar and the Seychelles, across Malani and the
Aravalli Craton, through northwestern India and into the Lhasa-Qiangtang terranes between at least 750-950 Ma. Paleomagnetic data for these terranes and this time period is relatively sparse and more work, including more extensive geochronology, is necessary to test the validity of this hypothesis.
The proposed paleogeography and source areas implies that Himalayan rocks further to the west should have a signature more similar to Tibet, with a northwestern Indian influence and less western Australia-Antarctica input, and that is indeed the case for 264
Cambrian sandstones from the northwestern Indian Himalaya. They contain smaller
Grenville populations, a large group of ages between 900-1000 Ma and peaks between
600-900 Ma that could be a result of the East African Orogen. The occurrence of
Grenville ages in the Indian Himalaya and the Tibetan samples and the presence of 900-
1000 Ma zircons in the Nepalese Himalaya suggest that there was some mixing along the
northern Gondwana margin between the different source areas.
One possible problem with northeastern Africa and western India as a source region
are the limited number of 650-800 Ma ages in the Tibetan detrital zircon record despite the protracted history of the East African Orogeny. One possibility is that if the ocean containing the arc terranes of the ANS closed as a result of continental fragments, such as
Turkey, Iran and Lhasa-Qiangtang, colliding with Africa (Fig. 2; 16), the collision produced a mountain belt along the northeast edge of the ANS that hindered sediment transport in that direction. In modern times, Himalayan foreland basin rocks and modern
Neogene sands record mostly Proterozoic ages with few zircons of Himalaya orogenesis or Gangdese arc age (DeCelles et al., 2000; DeCelles et al., 1998). The Tibetan signature does show a small ~650 Ma peak similar to an age peak in the East African orogen that is seen in the Jordan/Israel DZ ages (Kolodner et al., 2006; Meert, 2003). Another issue is the very few Archean ages ≥ 3000 Ma in the detrital zircon signature, despite the association of crust that age with the ~2500 Ma crust. It is difficult to assess the meaning of the lack of these ages, particularly given that the LHS, which is understood to be derived from cratonic India, also has very few older (>2600 Ma) Archean ages. Finally, while the age associations discussed suggest a general source region, the exact area may 265 be unknown due to a lack of data. Many of the possible source regions, such as northwest Arabia, the Horn of Africa and northern India are extensively covered by younger deposits of sedimentary and volcanic rocks (e.g. the Deccan traps and the
Himalayan foreland basin in India) or have not been adequately dated. Basement ages of
800-1000 Ma and ~2500 Ma may also be more common in Ethiopia, Somalia, Arabia and northwest India. Indeed, Condie et al. (2005) propose a widespread episode of juvenile crust creation at ~2500 Ma based on U-Pb ages and Hf isotopic compositions of detrital zircons from Brazil, Australia, India and the Ukraine, which suggests that crust of this age is presently disproportionately exposed.
6. Conclusions
Our basement and detrital zircon geochronologic data, combined with detrital zircon data from other Tibetan Paleozoic sandstones, suggest a location of the combined Lhasa-
Qiangtang terrane that is farther west along the Gondwanan margin than most current models indicate. The presence of post-Grenville and Pan-African ages, a strong and consistent 2500 Ma peak and a lack of Grenville ages is more consistent with a Paleozoic location near northeastern African and northwestern India than a location towards eastern
India and Australia. Likewise the available data indicate an eastern source for sediments of the Late Proterozoic and early Paleozoic rocks of the Nepalese Himalaya which have less early Pan-African ages and more Grenville ages, indicating a smaller input from
Africa and western India and a larger input from western and southern Australia, eastern
India and Antarctica. 266
While a limited exposure, the Amdo basement suggests extensive Neoproterozoic and early Paleozoic magmatism and reworking of a Proterozoic Lhasa-Qiangtang crust, consistent with a location along the margin of Rodinia and than Gondwana. The
Neoproterozoic magmatism may be related to a continental arc in eastern India and even possibly the South China craton. The Cambro-Ordovician orthogneisses are part of an extensive region of magmatism and metamorphism along the margins of Gondwana following its final assembly, which occurred as the Indian continent collided with East
Africa and Australia/Antarctica in the latest Neoproterozoic (Boger and Miller, 2004;
Collins and Pisarevsky, 2005; Meert, 2003). The continued tectonism along the margin of Gondwana into the Paleozoic is likely a result of the initiation of subduction along its edges (Boger and Miller, 2004) and the accretion of additional terranes, such as Iran or the South China craton.
Our model predicts similarities in Cambrian detrital zircon records between the
Nepalese Himlaya, western Australia and western Sibumasu, similarities between the
Iranian, Afghan and Lhasa-Qiangtang terranes and specific changes in Paleozoic detrital zircon age populations along the northern margin of Gondwana, from the west
(Afghanistan, Lhasa-Qiangtang, northwest Himalaya), to central (Nepalese and
Bhutanese Himalaya, South China) to the east (east Indian Himalaya, Indochina,
Sibumasu). The Cambrian sandstones from the Indian Himalaya hint at this change in provenance along strike but further analyses are needed. The model preferred here is geologically permissible, but not unique, and requires more data to resolve. 267
Recent work has resulted in many proposed redefinitions of Gondwana assembly, the
discovery of previously unknown magmatic provinces and new suggestions for
reconstructions between the breakup of Rodinia and the assembly of Gondwana. Most of
these new reconstructions or definitions include, and are the result of, additional
recognized tectonic complexity. Detrital zircon geochronology is one method that is
being used to resolve the location of continents and terranes in the absence of
paleomagnetic data and has the potential to further elucidate some of the questions
concerning paleogeographic reconstructions.
Acknowledgments
We thank Jen Fox, Alex Pullen, Jenn Pullen and Kelley Stair for sample preparation, zircon analysis and SEM imaging. We also thank Paul Myrow and especially Peter
Cawood for providing copies of their data. Alex Pullen also provided invaluable field assistance. This manuscript benefited from discussions with Pete DeCelles.
Appendix A.1 Description of Rock Samples
A.1.1 JG053104-1 orthogneiss
This sample is a felsic gneiss composed of quartz, plagioclase and microline with minor biotite. It is located in the south-central part of the gneiss about 10 km from the southern boundary.
A.1.2. JG061504-2 orthogneiss
268
This granodiorite orthogneiss has a well-developed foliation defined by biotite, with only minor chlorite alteration of the biotite, and contains stretched quartz grains. It was collected ~10 km west of the Lhasa-Golmud highway. This gneiss and JG061504-1 were interlayered within the same outcrop, along with garnet amphibolite.
A.1.3. PK970604-3A orthogneiss
This is a medium-grained, hornblende-biotite granite orthogneiss located about halfway across the Amdo basement along the Lhasa-Golmud highway. Both the hornblende and biotite display chlorite alteration. Many of the zircons in this sample display thin (< 10 µm) bright rims in CL-SEM images that were interpreted as Jurassic zircon resorption by Guynn et al. (2006).
A.1.4. JG060504-2 orthogneiss
This is a mylonitic orthogneiss of granite composition from the central part of the
Amdo basement with stretched quartz grains, quartz and feldspar porphroblasts, and fine- grained biotite defining the foliation.
A.1.5. JG061504-1 orthogneiss
This sample is a weakly foliated, felsic orthogneiss that contains abundant accessory magnetite with some crystals up to several mm across. It was collected from the same outcrop as JG061504-1 and is interlayered with that orthogneiss.
269
A.1.6. PK970604-1B orthogneiss
This is a felsic granite orthogneiss from the southern boundary of the Amdo basement along the Lhasa-Golmud highway. It has a weak foliation defined by flattened quartz grains and highly chloritized biotite.
A.1.7. JG061604-1 orthogneiss
Located along the Lhasa-Golmud highway just south of the northern basement
contact, this fine-grained granitic orthogneiss has a very weak biotite foliation.
A.1.8. PK970604-1A orthogneiss
Though collected from the same area as PK970604-1B, this sample is a hornblende-
biotite granodiorite with a weak foliation defined by aligned hornblende.
A.1.9 JG053104-2 orthogneiss
This is a biotite-granite orthogneiss located close to JG053104-1. It is compose largely of plagioclase with quartz and minor biotite and K-feldspar. The biotite displays some chlorite alteration. The orthogneiss has a well-developed foliation defined by layers of felsic minerals and aligned biotite.
A.1.10 JG061504-4 paragneiss
This dark, banded gneiss is located along the Lhasa-Golmud highway about halfway across the Amdo basement. It is largely composed of quartz, plagioclase and biotite that 270
has some chlorite alteration. It has a well-developed banding defined by layers of
coarser, more felsic minerals and aligned biotite rich layers. It also contains minor
epidote, muscovite and garnet and the biotite has some chlorite alteration.
A.1.11. JG062504-3 quartzite
Quartzite sample JG062504-3 comes from a small outcrop located within the gneisses
near the northern edge of the Amdo basement. This quartzite is very clean with only minor mica.
A.1.12. AP061304-A quartzite
This is a quartzite from a narrow outcrop located at the southwest edge of the orthogneiss. The quartzite is associated with a garnet-kyanite schist that is a lower grade
(lower amphibolite facies) than the orthogneisses. It is also a very clean quartzite with only minor muscovite and biotite.
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Figure Captions
Figure 1. Location of terranes and continental blocks within southeast Asia. All the terranes shown were originally part of East Gondwana with the exception of Northeast
China and the Kazakh Block. The questions marks highlight an area that is poorly defined due to Cenozoic deformation and poor access; the Qiangtang terrane and
Sibumasu are generally considered to have been joined, but the relationship is unclear.
Likewise, the Qiangtang and Lhasa terranes may continue into the Pamirs. The
Himalayan mountains are made up of Indian upper-crustal rocks whose lower crust is 291
presumed to have been underthrust northward beneath Tibet (“Greater India”). AB is the location of the Amdo Basement.
Figure 2. One view of Gondwana and other continents in the Ordovician and
Carboniferous from Scotese (2001; http;//www.scotese.com/newpage1.htm). The main differences in the Carboniferous are 1) the assembly of Laurasia and Gondwana to form
Pangea, 2) the counter-clockwise rotation of Gondwana and 3) the rifting and relocation of the South China Craton and Indochina. With the exception of South China and
Indochina, the configuration of Gondwana is essentially the same from the late Cambrian through the early Permian. Note that India does not include Greater India, the part of the continent underthrust beneath Tibet in the Cenozoic Indo-Asian collision. The Sibumasu and West Burma terranes are part of Malaya, near northwest Australia. Af = Africa; Ar =
Arabia; Au = Australia; Av = Avalonia; Ba = Baltica; Gr = Greenland; IC = Indochina; In
= India; Kz = Khazakstania; La = Laurentia; LQ = Lhasa-Qiangtang; Ma = Malaya; Md =
Madagascar; NC = North China; SA = South America; Si = Siberia; SC = South China.
Figure 3 Simplified geologic map of the Amdo basement and surrounding sedimentary rocks.
Figure 4. Pb/U concordia plot of zircon analyses of orthogneiss JG053104-1. The regression line is anchored at 180 ± 10 Ma and is based on all analyses. Inset shows the weighted average of clustered 207Pb*/206Pb* ages, which are the black error ellipses in the 292
concordia plot. Error ellipses in gray were not used in the discordia regression. In this
and all following concordia and weighted average figures, error ellipses and error bars are
at the 2-σ (95% confidence) level. Plots, regressions and averages were made using the programs of Ludwig (2003).
Figure 5. Pb/U concordia plot of zircon analyses of orthogneiss JG061504-2. The regression line is anchored at 180 ± 10 Ma and is based on all analyses.
Figure 6. Pb/U concordia plot of zircon analyses for orthogneiss JG060504-2. Ellipses in gray were not used in the regression. Regression is anchored at 177 ± 6 Ma, the mean of the two concordant, overlapping Jurassic 206Pb*/238U ages.
Figure 7. Pb/U concordia plot of zircon analyses of orthogneiss JG061504-1. Inset shows the weighted average of clustered 206Pb*/238U ages, which are the black error ellipses in the concordia plot.
Figure 8. Pb/U concordia plot of zircon analyses of orthogneiss PK970604-1B. Inset shows the weighted average of clustered 206Pb*/238U ages, which are the black error ellipses in the concordia plot.
293
Figure 9. Pb/U concordia plot of zircon analyses for orthogneiss JG061604-1. Ellipses in gray were not used in the regression. Regression is anchored at 181 ± 3 Ma, the mean of the concordant, overlapping Jurassic 206Pb*/238U ages.
Figure 10. Pb/U concordia plot of zircon analyses of orthogneiss PK970604-1A. Inset shows the weighted average of clustered 206Pb*/238U ages, which are the black error ellipses in the concordia plot.
Figure 11. Pb/U concordia plot of zircon analyses from orthogneiss JG053104-2. Insets show the most concordant analyses with the least uncertainty for the old and young cluster of ages and the weighted 206Pb*/238U average of each.
Figure 12. Pb/U concordia plots for the Amdo metasedimentary detrital zircon samples.
Analyses with gray error ellipses are not included in the probability density functions due to discordance, large uncertainty or metamorphic overgrowth. Ages less than 1000 Ma are based on 206Pb*/238U apparent ages, while ages older than 1000 Ma are based on the
207Pb*/206Pb* age. The dashed lines in sample JG061504-4 show interpreted lead loss paths as a result of the Jurassic metamorphic event.
Figure 13. Probability density functions of detrital zircon ages for the three Amdo metasedimentary samples and other Paleozoic Tibetan sedimentary samples. The number of individual zircon analyses in each curve is given beside the group name. The curves 294 have been normalized so that each has the same area. Spikes in the curves are caused by relatively high precision analyses, generally by an ion-microprobe, while lower precision analyses form broader peaks.
Figure 14. Catholde-luminescence SEM images of JG061504-4 zircons showing rounded or irregular cores with sharp tips. The cores are older, detrital zircon grains and the tips represent Jurassic metamorphic growth, as revealed by the U-Pb dating, shown for two of the zircons. 295
Figure 15. Probability density plots for Gondwanan detrital age data and Amdo orthogneisses (upper panel) compared to periods of magmatism and orogenesis in
Gondwanan terranes (lower panel). In the upper panel, the number in parentheses is the number of individual zircon analyses in each curve. The curves have been normalized so that each has the same area. As in Figure 13, the spikes are due to high precision TIMS
(Thermal-Ion Mass Spectrometry) or ion-microprobe analyses. Perth Basin from
Cawood and Nemchin (2000), Jordan/Israel from Kolodner et al. (2006), Nepalese
Himalaya from Gehrels et al. (2003a) and DeCelles et al. (2004), Indian Himalaya from
Myrow et al. (2003) and Tibetan data from this study and unpublished data. In the lower panel, dark bars represent significant, concentrated magmatism; medium bars are broader times of orogenesis based on magmatic and metamorphic thermochronology; light bars are based on sparse geochronology, inherited zircon and isotopic model ages. Sources for Gondwana orogenesis discussed in text. LHS = Lesser Himalaya Sequence; GHS =
Greater Himalaya Sequence; ANS = Arabian-Nubian shield and includes parts of
Ethiopia and Somalia.
296
Figure 16. Our interpretation of the position of the Lhasa-Qiangtang terrane in the
Cambrian, ~500 Ma. The map is based on Boger et al., 2004 and shows major continents, cratons terranes and orogenic belts of Gondwana. The location of detrital zircon samples are: 1) Amdo basement, 2) Nepalese Himalaya, 3) NW Indian Himlaya,
4) Perth Basin and 5) Jordan/Israel. The Cambro-Ordovician orogenic belt across northern India is only generally defined due to the overprint by rifting and the Himalayan thrust belt. The Lhasa-Qiangtang terrane is based on the current boundaries with 50% extension and rotation at the ends to account for Mesozoic and Cenozoic deformation; the internal line represents the Bangong suture between the Lhasa and Qiangtang terranes.
Other peri-Gondwana terranes are only generally defined. Extent of Greater India is based on Ali and Aitchison (2005). AD = Aravalli-Delhi craton; AF = Africa; AG =
Afghanistan blocks; ANS = Arabian-Nubian shield; AN = Antarctica; AU = Australia;
BH-LC = Bunger Hills-Leeuwin Complex; BK = Bundelkhand craton; DW = Dharwar
craton; EG = Eastern Ghats; GI = Greater India; GL = Gawler craton; HB = Huanan
block; IC = Indo-China; IR = Iranian blocks; L-Q = Lhasa-Qiangtang terranes; MD =
Madagascar; NC = Northampton Complex; NPCM; Northern Prince Charles Mountains;
PC = Pilbara craton; SA = South America; SB = Singhbhum craton; SC = South China craton; SEY = Seychelles; SPCM = Southern Prince Charles Mountains; SWB =
Sibumasu-West Burma; TB = Tarim Block; TK = Turkey; YB = Yangtze block; YC =
Yilgarn craton.
297
Figures
Figure 1. Guynn et al.
298
Figure 2. Guynn et al.
299
Figure 3. Guynn et al.
300
Figure 4. Guynn et al.
Figure 5. Guynn et al.
301
Figure 6. Guynn et al.
Figure 7. Guynn et al.
302
Figure 8. Guynn et al.
Figure 9. Guynn et al.
303
Figure 10. Guynn et al.
Figure 11. Guynn et al.
304
Figure 12. Guynn et al.
305
Figure 13. Guynn et al.
306
Figure 14. Guynn et al.
307
Figure 15. Guynn et al.
308
Figure 16. Guynn et al.
309
Tables
Table 1. Location and ages of Amdo basement samples
Zircon Other Zircon Ages Sample Lat Lon Description Age (Ma) Type Age (Ma) JG053104-1 31.884 92.048 Granitie gneiss 915 ± 14 JG061504-2 31.932 91.838 Granodiorite gneiss 878 ± 15 PK970604-3A 32.124 91.711 Granite gneiss 852 ± 18 JG060504-2 31.946 92.136 Granodiorite gneiss ~ 840 meta. 177 ± 6 JG061504-1 31.932 91.838 Granite gneiss 532 ± 7 PK970604-1B 31.881 91.699 Granite gneiss 501 ± 12 JG061604-1 32.130 91.716 Granite gneiss ~ 470 meta. 181 ± 3 PK970604-1A 31.881 91.699 Granodiorite gneiss 483 ± 15 JG053104-2 31.885 92.060 Granite gneiss 482 ± 18 inherited ~ 840 JG063004-1 31.817 91.752 Quartz-diorite gneiss ? meta. 180 ± 3 JG061504-4 32.123 91.711 Paragneiss max. depo. ~750 " " " " " " " " meta. 177 ± 3 JG062505-3 32.048 92.209 Quartzite max. depo. 493 ± 64 AP061304-A 31.794 91.773 Quartzite max. depo. 447 ± 30 meta. = metamorphic age; max. depo. = maximum age of deposition; Maximum depositional ages based on clusters of the youngest, overlapping 206Pb*/238U ages. Uncertainty is at the 2σ level Table 2. Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass Spectrometry 310 Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ±207Pb*±206Pb*±Best age± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
Orthogneiss JG053104-1 04 2018 6305 1.7 1.2509 4.2 0.1292 3.5 0.84 783.3 25.9 823.8 23.6 934.8 46.5 783.3 25.9 07 1402 15363 1.2 1.3258 1.0 0.1382 0.7 0.71 834.8 5.8 857.1 6.1 915.2 15.2 834.8 5.8 11 1045 134293 0.9 1.3549 1.4 0.1404 1.2 0.86 847.0 9.4 869.7 8.1 927.9 14.5 847.0 9.4 12 1430 84931 1.4 1.3251 1.8 0.1376 1.7 0.92 831.0 12.9 856.8 10.4 923.9 14.4 831.0 12.9 13 1300 105755 1.2 1.3898 1.6 0.1443 1.5 0.90 868.8 12.0 884.6 9.7 924.2 14.4 868.8 12.0 14 679 53443 0.9 1.3759 1.0 0.1438 0.7 0.70 865.8 5.7 878.7 5.9 911.3 14.7 865.8 5.7 17 1473 112261 1.1 1.2362 1.9 0.1298 1.8 0.93 787.0 13.0 817.2 10.6 900.2 14.5 787.0 13.0 18 2054 2530 1.6 1.1601 4.4 0.1198 3.2 0.73 729.4 22.2 782.0 24.1 935.1 62.0 729.4 22.2 21 1272 82698 1.1 1.2940 4.0 0.1355 4.0 0.98 819.4 30.4 843.1 23.0 905.9 14.5 819.4 30.4 22 1951 91430 1.5 1.3031 1.0 0.1365 0.7 0.71 825.1 5.4 847.1 5.7 905.2 14.4 825.1 5.4 24 760 97904 1.1 1.3787 2.2 0.1432 2.1 0.95 862.5 16.9 879.9 13.0 923.9 14.4 862.5 16.9 25 2579 1628 1.3 1.1216 2.4 0.1143 2.1 0.89 697.7 13.9 763.8 12.7 962.3 22.4 697.7 13.9 28 1939 47177 1.2 1.0971 5.1 0.1177 5.0 0.99 717.5 34.1 751.9 26.9 855.8 14.9 717.5 34.1 32 1755 72528 1.8 1.2526 1.5 0.1341 1.3 0.88 811.1 10.1 824.6 8.5 861.0 14.7 811.1 10.1 34 1374 12596 1.0 1.3773 1.0 0.1442 0.7 0.68 868.6 5.7 879.3 6.0 906.3 15.4 868.6 5.7 36 1385 60243 1.1 1.3190 1.1 0.1409 0.7 0.65 850.0 5.6 854.1 6.2 864.5 17.1 850.0 5.6 38 1218 31672 1.7 1.3249 1.0 0.1411 0.7 0.71 851.0 5.6 856.6 5.7 871.2 14.6 851.0 5.6 40 1289 93209 1.5 1.1657 1.7 0.1253 1.5 0.88 760.9 11.1 784.6 9.5 852.9 16.9 760.9 11.1 44 1929 1808 0.9 0.6358 2.4 0.0703 1.9 0.77 438.2 7.8 499.7 9.5 792.0 32.5 438.2 7.8 48 1531 14646 1.3 1.2683 1.1 0.1344 0.8 0.72 813.0 5.8 831.6 6.0 881.6 15.1 813.0 5.8 52 1314 44169 1.2 1.2819 1.4 0.1361 1.1 0.82 822.4 8.8 837.7 7.9 878.3 16.3 822.4 8.8 53 2023 66468 1.1 1.3138 1.2 0.1393 1.0 0.82 840.8 7.8 851.8 7.0 880.6 14.5 840.8 7.8 54 1396 48616 1.6 1.2576 1.1 0.1340 0.9 0.78 810.4 6.7 826.8 6.4 871.1 14.6 810.4 6.7 57 1904 18262 1.2 1.3170 1.0 0.1390 0.7 0.70 838.9 5.6 853.2 5.8 890.6 14.8 838.9 5.6 58 2406 9441 1.2 1.3008 1.0 0.1375 0.7 0.69 830.7 5.5 846.1 5.9 886.6 15.3 830.7 5.5 59 1034 24812 3.1 0.9468 6.5 0.1057 6.4 0.98 647.8 39.6 676.4 32.3 773.1 26.9 647.8 39.6 60 1054 4206 1.6 0.9490 4.7 0.1022 4.5 0.96 627.5 27.0 677.6 23.1 847.9 25.9 627.5 27.0 63 1230 91586 1.0 1.3456 1.0 0.1417 0.8 0.73 854.4 6.1 865.6 6.0 894.5 14.5 854.4 6.1
Orthogneiss JG061504-2 01c 272 18248 0.7 1.3950 1.3 0.1494 0.7 0.56 897.8 5.9 886.8 7.4 859.4 21.7 897.8 5.9 01t 271 27949 0.8 1.3464 1.5 0.1422 0.8 0.56 857.2 6.6 866.0 8.6 888.4 25.3 857.2 6.6 02 197 4769 0.7 1.2174 3.7 0.1276 0.7 0.19 774.4 5.1 808.6 20.8 904.0 75.4 774.4 5.1 03 341 14268 0.7 1.3409 1.7 0.1428 0.7 0.41 860.3 5.6 863.6 9.8 872.1 31.9 860.3 5.6 04 252 15451 0.5 1.3498 1.7 0.1447 0.7 0.40 871.2 5.7 867.5 10.2 858.0 33.3 871.2 5.7
Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 311 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ±207Pb*±206Pb*±Best age± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
06 298 26227 0.7 1.3763 1.9 0.1464 1.4 0.71 880.9 11.3 878.9 11.3 873.6 28.0 880.9 11.3 07 388 11405 0.8 1.3567 1.5 0.1437 0.7 0.48 865.6 5.7 870.4 8.6 882.8 26.7 865.6 5.7 08 400 16652 0.7 1.2614 1.4 0.1338 1.0 0.72 809.6 7.7 828.6 7.9 879.8 19.9 809.6 7.7 09 679 27528 0.6 1.3642 1.0 0.1445 0.7 0.69 870.3 5.7 873.7 6.0 882.3 15.3 870.3 5.7 10 325 19822 0.8 1.2008 1.8 0.1292 1.1 0.61 783.3 8.0 800.9 9.9 850.2 29.6 783.3 8.0 11 490 7216 0.6 1.1667 1.7 0.1236 0.7 0.42 751.2 5.1 785.1 9.2 882.6 31.6 751.2 5.1 13 425 46859 0.9 1.2535 1.6 0.1343 0.7 0.45 812.3 5.3 825.0 8.9 859.3 29.1 812.3 5.3 14 596 23906 1.6 1.3064 1.8 0.1389 1.5 0.84 838.5 12.1 848.5 10.5 874.8 20.3 838.5 12.1 15 610 57193 0.7 1.2821 1.0 0.1361 0.7 0.71 822.3 5.4 837.8 5.7 878.9 14.5 822.3 5.4 16 297 34382 0.8 1.3451 3.3 0.1448 3.1 0.93 871.8 24.9 865.5 19.1 849.3 25.1 871.8 24.9 17 522 30514 0.7 1.2862 1.5 0.1368 1.2 0.81 826.3 9.5 839.6 8.6 874.9 18.2 826.3 9.5 18 204 7534 1.5 0.9324 5.8 0.1025 4.1 0.71 629.2 24.8 668.9 28.6 805.1 86.3 629.2 24.8 19 388 20690 0.8 1.3380 2.4 0.1426 2.2 0.91 859.6 17.5 862.4 13.9 869.4 20.5 859.6 17.5 20 266 29916 0.9 1.5867 9.1 0.1602 6.9 0.75 957.9 61.2 965.0 56.8 981.3 121.9 957.9 61.2 21 440 6778 0.5 1.3323 2.0 0.1404 1.6 0.77 847.0 12.4 859.9 11.8 893.3 27.0 847.0 12.4 22 288 34587 0.7 1.2937 1.4 0.1400 0.7 0.51 844.6 5.5 842.9 7.9 838.6 24.7 844.6 5.5 23 2313 13363 1.0 1.3327 1.1 0.1413 0.9 0.76 852.1 6.9 860.1 6.5 880.7 15.0 852.1 6.9 24 303 19830 0.7 1.2449 2.4 0.1330 2.0 0.83 804.8 15.1 821.1 13.6 865.5 28.1 804.8 15.1 25 353 31912 1.0 1.3014 2.6 0.1397 2.4 0.92 843.1 18.9 846.3 14.9 854.8 21.3 843.1 18.9 25c 454 43190 0.8 1.4154 3.9 0.1492 3.8 0.98 896.6 32.0 895.5 23.2 892.7 15.8 896.6 32.0 26 408 29624 0.6 1.3304 1.2 0.1412 0.8 0.63 851.2 6.2 859.1 7.2 879.4 20.0 851.2 6.2 27 395 33655 0.7 1.2958 1.0 0.1388 0.7 0.70 838.0 5.5 843.9 5.7 859.4 14.8 838.0 5.5 29t 1266 5249 5.6 0.2130 3.3 0.0292 0.8 0.26 185.6 1.5 196.1 5.8 324.2 71.3 185.6 1.5 30 455 43389 0.9 1.4374 1.9 0.1530 1.7 0.91 917.8 14.6 904.6 11.3 872.8 16.3 917.8 14.6
Orthogneiss JG060504-2 02C 774 35290 0.8 1.0754 5.6 0.1207 5.0 0.88 734.4 34.4 741.4 29.7 762.4 56.3 734.4 34.4 04C 531 14026 1.3 1.0489 5.2 0.1110 3.3 0.64 678.4 21.4 728.4 26.8 885.2 81.7 678.4 21.4 05C 2673 11665 0.4 0.4471 8.8 0.0533 8.4 0.95 335.0 27.4 375.2 27.7 632.0 59.5 335.0 27.4 05T 1537 30736 8.7 0.1888 2.1 0.0275 1.2 0.57 175.2 2.1 175.6 3.4 181.6 40.9 175.2 2.1 06C 1121 3587 0.4 1.0742 7.7 0.1075 6.1 0.79 658.5 38.1 740.8 40.5 998.4 95.7 658.5 38.1 06T 1315 34851 1.1 1.1881 4.0 0.1273 3.8 0.95 772.6 27.4 795.1 22.0 858.5 26.8 772.6 27.4 07C 618 4268 0.9 0.7104 7.9 0.0751 6.6 0.83 466.8 29.5 545.0 33.4 887.0 91.8 466.8 29.5 07T 1705 7691 2.6 0.4954 4.5 0.0614 4.2 0.94 383.9 15.7 408.6 15.1 550.3 34.4 383.9 15.7 08C 390 36365 2.8 1.0137 2.6 0.1127 1.8 0.70 688.5 12.0 710.7 13.3 781.4 38.9 688.5 12.0 09 1424 48798 2.2 0.7874 4.1 0.0913 3.7 0.90 563.3 19.9 589.7 18.4 692.8 38.2 563.3 19.9 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 312 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
10 727 5710 0.6 0.7037 11.3 0.0724 7.8 0.69 450.8 34.0 541.0 47.4 941.5 167.4 450.8 34.0 11 1331 31741 1.5 0.9258 6.7 0.1042 6.0 0.89 638.8 36.2 665.4 32.5 756.7 62.7 638.8 36.2 12T 1189 20107 5.6 0.2342 6.9 0.0331 6.8 0.98 210.2 14.0 213.7 13.3 251.7 28.5 210.2 14.0 13C 1138 6590 1.0 0.9036 6.1 0.0943 5.3 0.88 580.8 29.6 653.7 29.2 913.9 59.7 580.8 29.6 14C 1827 23787 1.4 0.7387 8.4 0.0837 8.3 0.99 518.0 41.5 561.6 36.2 742.7 19.3 518.0 41.5 15C 391 2572 1.4 0.9310 6.8 0.0936 5.3 0.78 576.8 29.2 668.1 33.4 989.7 87.7 576.8 29.2 16 866 22114 0.7 1.2963 2.1 0.1365 1.5 0.72 824.7 11.5 844.1 11.8 895.4 29.3 824.7 11.5 17T 1126 26909 7.7 0.1928 3.7 0.0281 3.2 0.86 178.8 5.6 179.0 6.0 181.8 43.3 178.8 5.6 18 925 19172 1.0 0.8553 3.4 0.0935 2.9 0.84 576.0 15.8 627.5 15.9 818.0 38.5 576.0 15.8 19C 178 14251 1.3 0.9455 3.6 0.1066 3.5 0.96 652.9 21.6 675.7 17.9 752.5 22.6 652.9 21.6 20C 1211 3035 1.0 0.9967 11.6 0.0960 8.0 0.70 590.7 45.4 702.1 58.6 1077.2 166.9 590.7 45.4 21C 555 22277 1.2 1.0692 2.9 0.1176 2.7 0.91 716.8 18.1 738.4 15.4 804.4 25.3 716.8 18.1 22C 148 11050 1.5 0.9237 7.7 0.1004 6.7 0.87 616.5 39.7 664.3 37.7 830.3 78.3 616.5 39.7 23B 490 19643 0.6 0.8070 4.1 0.0901 4.0 0.97 556.0 21.1 600.8 18.6 773.6 22.2 556.0 21.1 23C 436 41116 1.9 1.1853 1.8 0.1298 1.2 0.70 786.6 9.1 793.8 9.7 814.1 26.2 786.6 9.1 24C 521 29236 1.2 0.8077 2.2 0.0910 2.0 0.87 561.6 10.5 601.2 10.1 753.2 23.0 561.6 10.5 25B 874 3326 2.4 0.4281 5.6 0.0478 5.4 0.96 300.9 15.9 361.8 17.1 773.9 31.8 300.9 15.9 25C 1322 13605 0.8 1.1515 4.6 0.1242 4.4 0.95 754.4 31.3 777.9 25.0 845.9 28.7 754.4 31.3 26 893 65606 1.2 1.0673 2.5 0.1176 2.2 0.85 716.8 14.7 737.4 13.4 800.7 27.9 716.8 14.7 27C 245 22289 4.2 0.7129 9.9 0.0816 9.3 0.94 505.9 45.2 546.5 41.9 719.6 72.7 505.9 45.2 27T 879 77814 1.5 1.1613 2.0 0.1265 1.7 0.89 768.0 12.6 782.6 10.7 824.2 19.0 768.0 12.6 28 1361 25624 1.7 0.4851 3.8 0.0575 3.5 0.90 360.4 12.1 401.6 12.7 645.8 35.3 360.4 12.1 29 475 40628 1.4 1.2419 2.0 0.1338 1.0 0.51 809.7 7.6 819.7 11.1 846.9 35.4 809.7 7.6 30 778 21358 0.9 1.0714 6.4 0.1155 5.8 0.91 704.9 38.7 739.4 33.5 845.5 55.3 704.9 38.7 32T 986 61327 1.4 1.1775 2.5 0.1272 2.2 0.87 771.7 16.0 790.1 13.9 842.4 25.8 771.7 16.0 33T 1438 75909 1.1 1.1260 1.6 0.1225 1.5 0.91 745.1 10.2 765.8 8.6 826.7 14.2 745.1 10.2 34 1276 3425 1.2 0.7988 6.7 0.0815 5.2 0.78 505.3 25.1 596.2 30.0 958.8 85.6 505.3 25.1 36 1375 55224 1.7 0.7249 6.0 0.0817 6.0 0.99 506.5 29.1 553.5 25.8 752.1 20.3 506.5 29.1
Orthogneiss JG061504-1 01 251 23740 1.4 1.1567 3.1 0.1278 2.0 0.64 775.1 14.5 780.4 16.9 795.7 49.9 775.1 14.5 03 1063 10816 2.3 0.7089 1.2 0.0862 0.7 0.61 533.1 3.6 544.1 4.9 590.4 20.0 533.1 3.6 04 300 2755 0.3 0.6680 6.1 0.0812 1.1 0.19 503.3 5.5 519.5 24.8 591.2 130.3 503.3 5.5 06 1362 13880 114.2 0.2053 2.4 0.0295 1.2 0.49 187.4 2.1 189.6 4.1 216.5 47.8 187.4 2.1 07t 768 18708 1.7 0.7104 2.6 0.0874 2.3 0.87 540.2 11.8 545.0 11.1 565.0 28.6 540.2 11.8 07t2 738 8804 2.4 0.6924 1.8 0.0855 0.9 0.49 529.1 4.5 534.2 7.5 556.2 34.3 529.1 4.5 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 313 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
09c 502 12778 1.5 1.2813 1.7 0.1370 0.7 0.43 827.8 5.6 837.4 9.6 863.2 31.4 827.8 5.6 10 466 10641 2.0 0.8258 9.7 0.0931 9.2 0.96 574.0 50.7 611.3 44.4 751.9 60.0 574.0 50.7 11 544 46871 3.5 0.6992 2.2 0.0866 1.8 0.79 535.1 9.0 538.3 9.2 551.7 29.4 535.1 9.0 12 356 24705 0.4 0.6834 1.7 0.0859 0.9 0.50 531.2 4.5 528.8 7.2 518.5 33.2 531.2 4.5 14 416 21498 0.6 0.6180 5.3 0.0788 4.8 0.92 489.1 22.8 488.6 20.4 486.2 45.5 489.1 22.8 16 507 21896 0.5 0.6566 1.5 0.0844 0.8 0.56 522.5 4.1 512.5 5.9 468.3 26.8 522.5 4.1 17 711 18372 0.6 0.6708 4.7 0.0809 1.5 0.31 501.4 7.0 521.2 19.2 608.9 96.9 501.4 7.0 21 698 27793 3.3 0.4658 5.5 0.0609 5.1 0.93 381.4 18.9 388.3 17.7 429.4 45.5 381.4 18.9 22 289 9378 0.4 0.6893 3.5 0.0870 0.7 0.20 537.8 3.6 532.4 14.6 509.1 76.1 537.8 3.6 28 564 2073 21.2 0.4551 16.7 0.0527 3.1 0.18 331.4 9.9 380.9 52.9 694.1 351.0 331.4 9.9
Orthogneiss PK97-6-4-1B 01 710 46403 1.8 0.6304 6.3 0.0796 5.8 0.92 494.0 27.7 496.4 24.8 507.2 53.8 494.0 27.7 03 349 8546 2.2 0.4152 6.2 0.0530 5.1 0.83 332.9 16.6 352.6 18.5 484.3 77.3 332.9 16.6 04 696 36334 1.6 0.5140 2.3 0.0659 2.0 0.86 411.4 8.0 421.1 8.0 475.0 26.5 411.4 8.0 05 826 37512 1.9 0.6106 2.5 0.0774 2.4 0.95 480.6 11.1 484.0 9.7 500.0 16.8 480.6 11.1 06 1211 3629 1.6 0.6230 6.6 0.0710 5.8 0.88 441.9 24.8 491.7 25.8 730.9 67.0 441.9 24.8 07 348 19334 1.5 0.6285 3.3 0.0798 2.5 0.74 494.8 11.7 495.2 13.0 496.6 49.2 494.8 11.7 08 125 767 2.0 0.8050 10.5 0.0726 4.1 0.39 451.5 18.1 599.6 47.7 1208.5 191.0 451.5 18.1 09 190 14772 1.8 0.6317 3.7 0.0836 3.2 0.86 517.4 15.9 497.2 14.7 405.1 43.4 517.4 15.9 10C 473 27629 1.8 0.6159 8.0 0.0763 2.9 0.37 473.9 13.5 487.3 30.9 550.5 162.1 473.9 13.5 10T 639 41913 2.0 0.6129 2.6 0.0776 2.4 0.92 481.9 10.9 485.4 9.9 501.8 21.8 481.9 10.9 11 549 21325 1.8 0.5961 6.5 0.0764 6.3 0.96 474.8 28.7 474.7 24.6 474.4 37.9 474.8 28.7 12 227 19250 1.8 0.6266 2.3 0.0806 1.6 0.71 499.9 7.7 494.0 8.8 466.6 35.2 499.9 7.7 13 383 18759 1.4 0.6504 3.9 0.0808 1.2 0.31 500.6 5.9 508.7 15.8 545.3 81.9 500.6 5.9 14 528 12746 1.7 0.6987 5.5 0.0899 4.7 0.84 554.7 24.8 538.0 23.2 467.7 66.2 554.7 24.8 15 170 14553 1.6 0.6581 3.4 0.0830 1.8 0.52 513.8 8.7 513.4 13.6 511.9 63.2 513.8 8.7 16C 2912 3110 0.6 0.4940 8.7 0.0560 5.0 0.57 351.2 17.1 407.6 29.3 741.1 151.5 351.2 17.1 16T 667 3995 1.5 0.7314 6.3 0.0823 2.7 0.43 509.8 13.1 557.4 26.9 756.9 119.6 509.8 13.1 17 544 10925 1.5 0.5997 5.7 0.0746 4.9 0.86 463.8 22.1 477.0 21.8 541.1 63.2 463.8 22.1 18 1427 1027 2.6 0.5324 14.0 0.0493 4.9 0.35 310.1 14.9 433.4 49.3 1156.3 260.2 310.1 14.9 19 298 14191 1.7 0.6804 3.7 0.0856 2.5 0.67 529.4 12.7 527.0 15.3 516.5 60.6 529.4 12.7 20 208 18732 1.6 0.6274 2.8 0.0812 1.8 0.65 503.5 8.8 494.5 10.9 452.7 47.1 503.5 8.8 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 314 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
Orthogneiss JG061604-1 01 161 4536 0.7 0.5780 3.6 0.0788 2.3 0.65 488.8 11.0 463.2 13.3 338.0 61.1 488.8 11.0 02 162 3107 0.6 0.5878 4.0 0.0760 1.3 0.32 472.3 5.9 469.4 15.1 455.3 84.4 472.3 5.9 03 399 2277 1.0 0.4075 6.6 0.0526 4.1 0.61 330.7 13.1 347.0 19.5 457.8 116.5 330.7 13.1 04c 327 4212 54.3 0.1819 7.8 0.0287 1.4 0.18 182.1 2.6 169.7 12.1 -1.0 184.2 182.1 2.6 04t 553 5646 49.9 0.1884 2.8 0.0275 0.9 0.31 175.1 1.5 175.2 4.6 177.5 63.3 175.1 1.5 05e 443 5803 1.1 0.5494 9.6 0.0672 4.0 0.42 419.5 16.4 444.6 34.5 576.6 189.4 419.5 16.4 06 311 2984 35.3 0.1837 8.3 0.0275 2.6 0.31 174.7 4.4 171.3 13.0 124.4 185.1 174.7 4.4 07 429 5706 14.8 0.1857 7.9 0.0283 2.7 0.34 180.0 4.8 173.0 12.6 78.6 177.9 180.0 4.8 08 830 10912 53.2 0.1872 4.4 0.0286 1.2 0.27 181.5 2.1 174.3 7.0 77.2 99.7 181.5 2.1 09 511 7751 39.2 0.1819 4.9 0.0275 0.9 0.19 175.0 1.6 169.7 7.7 96.3 115.0 175.0 1.6 10c 224 1297 0.9 0.4168 9.3 0.0521 2.3 0.25 327.3 7.4 353.8 27.9 531.3 198.6 327.3 7.4 10t 290 11561 0.7 0.6520 2.7 0.0845 0.9 0.35 522.8 4.7 509.7 10.6 451.4 55.1 522.8 4.7 11c 405 22387 1.0 0.5514 7.3 0.0719 6.5 0.88 447.6 28.1 445.9 26.5 437.2 76.1 447.6 28.1 11t 427 10364 35.8 0.1919 5.9 0.0292 0.7 0.12 185.7 1.3 178.3 9.6 81.2 139.0 185.7 1.3 12 326 3348 0.6 0.4397 8.3 0.0571 6.9 0.83 358.1 24.0 370.0 25.7 445.4 102.0 358.1 24.0 13c 629 8708 1.0 0.5050 12.3 0.0638 12.0 0.97 398.6 46.3 415.1 41.9 507.7 60.6 398.6 46.3 13t 325 15444 1.5 0.5082 3.1 0.0642 1.0 0.31 401.3 3.7 417.2 10.6 506.0 64.6 401.3 3.7 14 175 7634 0.8 0.5950 5.0 0.0779 1.4 0.28 483.3 6.6 474.1 18.9 429.8 106.7 483.3 6.6 15c 341 7384 1.0 0.3449 14.4 0.0468 12.6 0.87 294.7 36.2 300.9 37.5 349.4 159.3 294.7 36.2 15t 581 11864 14.4 0.1866 4.2 0.0272 1.1 0.27 172.9 1.9 173.7 6.7 184.2 93.7 172.9 1.9 16 251 5906 49.9 0.1893 10.2 0.0282 1.7 0.16 179.2 2.9 176.0 16.5 133.8 236.7 179.2 2.9 17 775 14382 61.2 0.1936 5.0 0.0285 3.7 0.74 181.1 6.6 179.7 8.2 161.5 78.5 181.1 6.6 18 365 26666 0.8 0.6389 3.4 0.0810 2.6 0.76 502.0 12.4 501.6 13.4 500.1 48.2 502.0 12.4 19 468 11114 15.0 0.1900 4.7 0.0291 0.8 0.17 184.8 1.4 176.6 7.6 68.1 110.2 184.8 1.4 20 210 4845 49.8 0.2018 7.9 0.0301 0.7 0.09 191.1 1.4 186.6 13.4 130.2 185.0 191.1 1.4 21c 645 26177 0.9 0.5893 1.8 0.0748 0.7 0.40 465.2 3.3 470.4 6.9 496.3 37.2 465.2 3.3 21t 461 12015 0.8 0.5272 3.0 0.0684 1.1 0.37 426.2 4.6 430.0 10.6 450.0 62.5 426.2 4.6 22 269 10435 0.9 0.4603 4.4 0.0613 1.9 0.43 383.8 7.0 384.5 14.0 388.6 88.4 383.8 7.0 23 337 11045 2.5 0.3167 10.7 0.0435 9.9 0.93 274.2 26.6 279.3 26.0 322.0 89.7 274.2 26.6 24 184 12167 0.9 0.7629 4.8 0.0958 0.8 0.17 589.9 4.7 575.7 21.2 520.1 104.2 589.9 4.7 25 571 14545 56.5 0.1937 4.1 0.0286 2.0 0.50 182.0 3.7 179.8 6.8 150.8 83.5 182.0 3.7 26c 282 10382 59.9 0.1978 5.3 0.0287 0.7 0.13 182.2 1.3 183.3 8.9 198.0 122.8 182.2 1.3 27 132 2240 0.8 0.7447 28.5 0.0811 5.2 0.18 502.4 25.1 565.1 124.3 826.4 596.7 502.4 25.1 28 319 5598 0.7 0.6383 6.9 0.0792 1.0 0.14 491.5 4.6 501.2 27.3 546.0 149.5 491.5 4.6 29 344 10588 52.7 0.1935 6.1 0.0288 0.8 0.13 182.8 1.4 179.6 10.0 137.8 141.4 182.8 1.4 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 315 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
Orthogneiss PK97-6-4-1A 01 471 14669 0.9 0.5985 3.4 0.0735 2.9 0.86 457.1 12.9 476.3 13.0 570.0 37.9 457.1 12.9 02 133 5153 1.0 0.6749 9.4 0.0837 3.6 0.38 518.1 17.7 523.7 38.3 547.8 189.5 518.1 17.7 04 175 7256 2.2 0.5978 2.7 0.0753 2.0 0.73 468.1 9.0 475.8 10.4 513.3 40.6 468.1 9.0 05 415 24215 1.2 0.6591 7.5 0.0821 7.2 0.97 508.5 35.3 514.0 30.1 538.7 41.4 508.5 35.3 06 1339 970 19.0 0.3130 5.4 0.0315 4.1 0.75 200.2 8.1 276.5 13.2 985.3 73.1 200.2 8.1 07 726 26669 1.1 0.6453 1.7 0.0804 1.2 0.68 498.7 5.6 505.6 6.8 536.7 27.1 498.7 5.6 08 399 9460 1.8 0.5783 8.3 0.0714 4.4 0.52 444.8 18.8 463.3 31.0 556.2 154.8 444.8 18.8 09 230 10450 2.2 0.5007 9.4 0.0665 9.2 0.98 414.9 36.9 412.1 31.9 396.5 46.9 414.9 36.9 10 132 6112 1.1 0.6333 5.0 0.0796 1.5 0.30 493.6 7.2 498.1 19.8 518.9 105.3 493.6 7.2 11 219 5375 1.3 0.5958 2.9 0.0717 1.5 0.51 446.2 6.4 474.6 11.0 614.2 53.6 446.2 6.4 12 216 6397 1.3 0.5795 7.9 0.0774 7.6 0.97 480.3 35.3 464.1 29.4 384.8 45.4 480.3 35.3 13 484 7556 6.1 0.2349 6.4 0.0334 5.7 0.89 211.8 11.8 214.2 12.4 240.5 68.0 211.8 11.8 14 227 4007 1.2 0.6883 11.0 0.0779 2.4 0.22 483.8 11.4 531.8 45.4 743.5 226.5 483.8 11.4 15 302 9874 1.2 0.6069 4.0 0.0754 2.4 0.61 468.5 11.0 481.6 15.2 544.6 68.7 468.5 11.0 16 393 6590 0.7 0.6719 8.7 0.0788 4.4 0.50 489.3 20.6 521.9 35.6 667.2 161.4 489.3 20.6 17 139 3940 1.1 0.6790 7.7 0.0837 6.2 0.80 518.2 30.6 526.2 31.4 561.0 99.2 518.2 30.6 18 528 26875 1.2 0.6261 2.8 0.0786 1.6 0.56 488.0 7.5 493.7 11.1 520.2 51.7 488.0 7.5 19 474 25490 1.3 0.6332 1.7 0.0798 1.4 0.83 494.8 6.7 498.1 6.7 512.9 20.6 494.8 6.7 21 433 13518 1.3 0.6731 3.0 0.0850 2.3 0.77 525.9 11.6 522.6 12.1 508.2 41.7 525.9 11.6 22 386 9674 1.8 0.8299 3.7 0.0781 3.0 0.80 484.6 13.8 613.6 17.0 1123.7 44.0 484.6 13.8 23 411 10480 1.7 0.5872 8.6 0.0719 8.2 0.95 447.9 35.3 469.1 32.4 574.2 59.6 447.9 35.3 24 458 10239 1.3 0.6311 2.7 0.0767 1.6 0.59 476.2 7.4 496.8 10.7 593.1 47.4 476.2 7.4 25 688 1408 1.5 0.5776 7.2 0.0614 6.0 0.83 384.3 22.2 462.9 26.8 874.7 84.2 384.3 22.2 26 666 24597 1.0 0.5335 4.8 0.0659 4.3 0.90 411.4 17.1 434.1 16.9 556.8 45.8 411.4 17.1 27 949 13843 17.1 0.1967 3.5 0.0295 3.2 0.91 187.6 5.9 182.3 5.8 113.4 34.8 187.6 5.9
Orthogneiss JG053104-2 010R 329 6789 1.2 1.1988 3.8 0.1284 1.6 0.41 778.9 11.5 800.0 21.2 859.3 72.6 778.9 11.5 01C 828 24428 1.2 0.6365 2.9 0.0792 1.0 0.34 491.3 4.7 500.1 11.4 540.9 59.6 491.3 4.7 01R 527 19951 1.2 0.6555 3.3 0.0819 1.2 0.35 507.2 5.7 511.9 13.3 532.7 67.8 507.2 5.7 02C 633 11402 0.8 1.1768 4.0 0.1244 2.7 0.68 755.8 19.3 789.8 21.7 887.0 59.9 755.8 19.3 02T 451 14854 5.4 0.5900 5.1 0.0682 1.4 0.27 425.5 5.8 470.9 19.3 698.2 104.8 425.5 5.8 03C 404 17986 0.9 1.2330 2.9 0.1312 1.6 0.54 794.5 11.8 815.7 16.5 874.2 51.4 794.5 11.8 03R 436 16151 1.1 1.3333 3.0 0.1419 2.3 0.75 855.5 18.3 860.3 17.6 872.7 41.2 855.5 18.3 04T 121 2858 3.2 0.7213 13.8 0.0766 5.4 0.39 475.5 24.6 551.4 58.8 878.8 263.9 475.5 24.6 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 316 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
05C 562 18117 1.5 1.2477 6.9 0.1314 6.5 0.95 795.7 48.9 822.3 38.9 895.0 45.9 795.7 48.9 05R 637 3019 1.0 1.2302 8.4 0.1247 1.0 0.12 757.3 7.5 814.4 46.9 973.8 169.7 757.3 7.5 06C 364 11360 1.5 1.1389 2.9 0.1251 2.1 0.73 759.7 15.0 772.0 15.5 807.8 41.2 759.7 15.0 06R 644 14390 1.9 1.1592 2.9 0.1261 1.5 0.52 765.7 11.0 781.6 16.0 827.0 52.5 765.7 11.0 07C 177 7018 1.2 1.3462 6.7 0.1351 2.3 0.34 817.2 17.6 865.9 39.2 992.8 128.8 817.2 17.6 07R 150 4029 1.3 1.2527 8.4 0.1286 1.6 0.19 779.9 11.7 824.6 47.5 947.1 169.4 779.9 11.7 08R 626 2023 2.0 0.6985 8.0 0.0861 2.2 0.28 532.5 11.4 537.9 33.3 560.7 167.2 532.5 11.4 09C 539 7293 1.5 0.6066 4.6 0.0756 2.7 0.59 470.1 12.3 481.4 17.6 535.9 80.9 470.1 12.3 09R 548 3456 1.7 0.6369 9.8 0.0771 1.2 0.12 478.6 5.5 500.4 38.9 601.2 212.0 478.6 5.5 10C 360 10394 1.0 1.1995 2.6 0.1289 1.2 0.45 781.6 8.8 800.4 14.6 852.9 48.9 781.6 8.8 11C 394 26682 1.3 1.2579 2.2 0.1368 1.0 0.46 826.8 7.9 826.9 12.5 827.4 40.8 826.8 7.9 11R 436 16915 1.5 1.2579 3.2 0.1346 1.4 0.44 814.0 10.7 827.0 17.9 862.1 58.8 814.0 10.7 12C 337 25423 1.8 1.3316 3.5 0.1420 2.4 0.69 856.0 19.5 859.6 20.5 868.6 53.0 856.0 19.5 12R 315 37777 1.6 1.3056 3.1 0.1414 1.1 0.36 852.4 9.0 848.2 17.9 837.0 60.6 852.4 9.0 13R 443 15826 1.5 1.2899 2.6 0.1360 1.5 0.57 822.0 11.4 841.2 14.9 892.3 44.3 822.0 11.4 14C 807 12463 1.9 0.6220 3.4 0.0754 1.6 0.45 468.5 7.0 491.1 13.4 597.8 66.3 468.5 7.0 14R 322 5574 2.6 0.5022 11.3 0.0636 3.5 0.31 397.3 13.5 413.2 38.5 503.2 238.2 397.3 13.5 15C 395 1883 1.2 1.4383 12.2 0.1361 2.3 0.19 822.6 17.7 905.0 73.4 1111.8 240.6 822.6 17.7 15R 377 13146 1.5 1.3036 1.8 0.1397 0.7 0.39 843.0 5.5 847.3 10.4 858.5 34.5 843.0 5.5 16C 451 7751 1.5 1.4053 5.1 0.1434 1.8 0.35 863.7 14.5 891.2 30.3 960.0 97.7 863.7 14.5 16R 461 3495 2.4 0.7242 15.0 0.0791 13.8 0.92 490.5 65.4 553.2 64.1 820.2 121.9 490.5 65.4 17R 213 5131 4.7 0.4358 17.9 0.0473 15.1 0.84 297.7 44.0 367.3 55.3 833.9 200.5 297.7 44.0 18C 291 9018 1.5 1.5501 3.7 0.1546 1.9 0.51 926.5 16.2 950.5 22.8 1006.5 64.6 926.5 16.2 18R 1123 34467 1.0 0.7856 28.0 0.0854 27.5 0.98 528.1 139.6 588.7 125.8 829.7 109.8 528.1 139.6 19C 694 29586 1.6 1.2932 1.3 0.1341 0.8 0.64 811.0 6.3 842.7 7.5 927.3 20.7 811.0 6.3 19R 431 23055 1.7 1.3269 5.0 0.1392 4.0 0.80 840.2 31.3 857.5 28.9 902.7 62.0 840.2 31.3 20C 294 16364 2.0 1.2065 6.4 0.1279 1.8 0.28 775.6 13.2 803.6 35.3 881.8 126.1 775.6 13.2 21C 886 31479 0.9 1.2089 4.0 0.1270 3.8 0.95 771.0 27.6 804.7 22.3 899.1 26.9 771.0 27.6 21R 439 34094 1.6 1.2416 3.7 0.1318 2.5 0.68 798.3 19.0 819.6 20.8 877.9 56.0 798.3 19.0
Orthogneiss JG063004-1 01C 1043 14734 12.8 0.1934 6.1 0.0283 1.9 0.30 180.0 3.3 179.5 10.0 172.7 135.8 180.0 3.3 01R 893 11938 19.4 0.1958 8.7 0.0290 0.9 0.10 184.5 1.7 181.6 14.4 143.1 202.9 184.5 1.7 03 365 5190 35.8 0.1789 13.4 0.0281 1.9 0.14 178.9 3.3 167.1 20.6 2.0 320.1 178.9 3.3 05 3714 9099 77.9 0.1920 3.7 0.0282 2.2 0.60 179.1 3.9 178.3 6.0 168.3 68.7 179.1 3.9 06T 387 4393 24.9 0.1891 19.7 0.0266 2.0 0.10 169.5 3.3 175.9 31.9 262.3 454.6 169.5 3.3 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 317 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
07 53 267 57.6 0.0824 87.3 0.0270 7.9 0.09 172.0 13.4 80.4 67.6 -2220.0 1277.7 172.0 13.4 08 822 6752 17.2 0.1852 4.8 0.0275 1.6 0.32 175.0 2.7 172.5 7.6 138.4 106.3 175.0 2.7 09 254 1075 16.8 0.2865 15.1 0.0421 4.4 0.29 265.6 11.5 255.8 34.1 167.4 337.8 265.6 11.5 11 49 469 75.7 0.2486 69.8 0.0267 9.5 0.14 169.7 15.9 225.5 142.0 856.5 1662.5 169.7 15.9 12 138 1631 32.2 0.2229 24.9 0.0320 2.7 0.11 203.3 5.5 204.3 46.1 216.5 580.0 203.3 5.5 13 925 6520 15.7 0.1817 8.9 0.0277 1.5 0.17 175.9 2.5 169.5 13.9 81.4 208.3 175.9 2.5 14 799 5430 19.3 0.1889 7.8 0.0280 1.0 0.13 177.9 1.8 175.7 12.5 145.2 180.8 177.9 1.8 15 1345 23033 28.8 0.1907 4.1 0.0287 2.3 0.57 182.2 4.2 177.3 6.7 111.7 80.6 182.2 4.2 16 1029 10544 22.5 0.1837 5.7 0.0284 1.9 0.32 180.7 3.3 171.2 9.0 42.1 129.1 180.7 3.3 17R 815 11477 19.9 0.2067 5.0 0.0296 1.6 0.32 188.2 3.0 190.8 8.7 222.2 109.2 188.2 3.0 18 61 1267 20.3 0.2927 43.1 0.0290 7.0 0.16 184.0 12.7 260.6 99.4 1022.0 902.2 184.0 12.7 19 1391 39990 39.6 0.1887 4.8 0.0278 2.6 0.53 176.8 4.5 175.5 7.8 158.6 96.0 176.8 4.5 22 1188 8972 30.0 0.1942 9.6 0.0277 1.8 0.19 176.4 3.2 180.2 15.9 231.1 219.1 176.4 3.2 2C 47 388 67.3 0.3270 88.7 0.0271 7.5 0.08 172.7 12.7 287.3 225.6 1368.5 8.4 172.7 12.7 B1 1227 19568 45.6 0.2018 2.6 0.0295 2.1 0.81 187.5 3.9 186.7 4.5 175.8 36.3 187.5 3.9 B2 1386 34576 29.2 0.1922 2.7 0.0283 2.3 0.83 179.6 4.1 178.5 4.5 164.2 35.5 179.6 4.1 B3 946 13632 25.1 0.2047 1.9 0.0295 1.5 0.76 187.2 2.7 189.1 3.3 212.6 29.0 187.2 2.7 B4 1281 30474 27.8 0.1967 1.9 0.0287 1.3 0.71 182.2 2.4 182.3 3.1 183.7 30.5 182.2 2.4 B5 763 34610 3.5 1.4639 4.5 0.1341 4.1 0.92 811.3 31.6 915.6 27.3 1176.2 35.8 811.3 31.6 B6 38 1470 61.6 0.1923 18.5 0.0276 9.0 0.48 175.3 15.5 178.6 30.4 221.9 377.8 175.3 15.5 B6T 185 1185 37.4 0.2151 13.5 0.0275 3.2 0.24 175.2 5.5 197.8 24.3 477.1 290.8 175.2 5.5 B7 564 25235 15.5 0.1916 2.7 0.0276 1.7 0.65 175.3 3.0 178.0 4.4 213.4 47.8 175.3 3.0 B9 28 3387 56.5 0.2482 27.3 0.0297 8.4 0.31 188.7 15.6 225.1 55.2 625.2 569.2 188.7 15.6
Paragneiss JG061504-4 01 597 82630 1.5 4.0917 1.6 0.2817 1.0 0.64 1599.8 14.5 1652.6 13.1 1720.5 22.8 1720.5 22.8 02 299 7215 1.8 3.7755 4.6 0.2710 4.1 0.89 1546.0 55.9 1587.5 36.6 1643.2 38.3 1643.2 38.3 03 151 25090 1.6 2.0461 1.7 0.1884 1.2 0.73 1112.7 12.5 1131.0 11.4 1166.3 22.6 1166.3 22.6 04 197 15895 0.7 1.1624 4.1 0.1256 3.8 0.93 762.8 27.2 783.1 22.1 841.2 30.1 762.8 27.2 05 154 5024 0.8 1.4887 5.0 0.1522 1.5 0.31 913.4 13.1 925.8 30.5 955.4 97.6 913.4 13.1 05t1 492 21967 12.3 0.1980 7.3 0.0273 2.8 0.38 173.6 4.8 183.5 12.3 312.6 154.1 173.6 4.8 06 98 15309 1.3 4.1344 2.2 0.2863 2.0 0.93 1623.1 28.8 1661.1 17.7 1709.5 15.0 1709.5 15.0 07 513 17530 1.9 1.1179 1.6 0.1233 1.5 0.93 749.5 10.3 762.0 8.4 798.8 12.3 749.5 10.3 08 217 61871 0.6 9.4811 1.9 0.4172 1.1 0.60 2247.9 21.5 2385.7 17.5 2505.6 25.7 2505.6 25.7 09 243 25649 0.7 1.2827 1.4 0.1372 1.0 0.73 828.7 7.8 838.1 7.8 862.9 19.2 828.7 7.8 10 376 20935 3.0 0.6608 3.3 0.0752 3.1 0.94 467.3 13.9 515.1 13.2 733.2 23.4 467.3 13.9 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 318 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
10t1 808 86708 13.8 0.2054 3.8 0.0284 2.0 0.52 180.8 3.5 189.7 6.6 301.9 73.8 180.8 3.5 11 160 20559 1.2 1.2216 3.2 0.1320 2.2 0.68 799.3 16.6 810.5 18.1 841.5 49.5 799.3 16.6 11t1 235 23693 1.7 1.2747 4.7 0.1413 2.9 0.62 851.9 23.2 834.5 26.6 788.4 77.0 851.9 23.2 12 531 58166 0.7 3.6742 1.4 0.2684 1.1 0.78 1532.9 15.1 1565.8 11.3 1610.4 16.6 1610.4 16.6 13 147 22655 1.1 1.2606 2.0 0.1353 1.2 0.60 818.0 9.1 828.2 11.1 855.7 32.4 818.0 9.1 14 26 13410 1.2 13.1298 1.8 0.5028 1.0 0.55 2625.7 21.6 2689.0 17.1 2737.0 24.8 2737.0 24.8 15 263 25031 0.4 1.5111 2.1 0.1550 1.4 0.66 929.1 12.1 934.9 12.8 948.5 32.1 929.1 12.1 16 90 19734 1.7 6.8180 2.0 0.3378 1.7 0.84 1876.1 27.8 2088.1 18.1 2304.0 19.3 2304.0 19.3 16t1 615 7843 10.3 0.1813 4.0 0.0281 2.5 0.62 178.8 4.3 169.2 6.2 36.4 74.6 178.8 4.3 17 656 48492 0.9 1.5137 1.6 0.1547 1.5 0.93 927.2 13.1 935.9 9.9 956.5 11.9 927.2 13.1 18X 144 17829 1.4 5.0264 4.6 0.3112 4.4 0.96 1746.5 66.7 1823.8 38.7 1913.2 24.3 1913.2 24.3 19 420 40564 1.6 1.4400 1.4 0.1504 1.2 0.83 903.4 9.9 905.7 8.5 911.3 16.5 903.4 9.9 20 699 84673 1.7 3.4106 1.3 0.2483 1.0 0.80 1429.8 12.8 1506.8 9.9 1616.8 14.2 1616.8 14.2 22 625 156689 1.2 9.4482 1.7 0.4148 1.2 0.70 2236.9 22.9 2382.5 16.0 2509.6 21.0 2509.6 21.0 23 343 21386 0.8 1.0897 9.2 0.1194 8.7 0.95 727.4 59.9 748.4 48.6 811.6 60.3 727.4 59.9 24 194 15064 0.6 1.4612 2.2 0.1496 1.7 0.77 898.6 14.5 914.5 13.5 953.1 29.3 898.6 14.5 25 175 20470 1.2 1.3317 2.5 0.1440 2.2 0.88 867.3 17.5 859.6 14.2 839.8 24.3 867.3 17.5 26 677 49738 0.9 1.2181 2.4 0.1326 1.9 0.79 802.6 14.6 808.9 13.6 826.3 31.1 802.6 14.6 27 247 29007 1.1 1.6252 1.6 0.1642 1.4 0.87 980.1 12.9 980.0 10.2 979.8 16.3 980.1 12.9 28 200 23338 1.8 2.8027 1.3 0.2321 1.0 0.74 1345.6 12.1 1356.3 10.1 1373.1 17.4 1373.1 17.4 29 129 40905 1.3 9.6724 2.2 0.4277 2.0 0.93 2295.3 39.0 2404.1 20.0 2497.5 13.5 2497.5 13.5 30 239 51917 1.0 4.0783 1.6 0.2790 1.0 0.64 1586.5 14.1 1650.0 12.7 1731.8 21.8 1731.8 21.8 31c 197 28746 2.2 3.2887 9.5 0.2347 8.9 0.95 1359.0 109.6 1478.4 73.7 1654.1 56.8 1654.1 56.8 31t1 1051 7405 9.9 0.1888 5.7 0.0283 1.5 0.26 179.7 2.6 175.6 9.2 121.2 130.2 179.7 2.6 31t2 750 7826 10.2 0.1927 3.4 0.0274 1.5 0.43 174.3 2.5 178.9 5.6 239.7 71.4 174.3 2.5 32t1 330 31997 1.8 1.2327 2.9 0.1315 1.2 0.43 796.6 9.1 815.6 16.1 867.6 53.8 796.6 9.1 33c 106 165386 1.5 1.3006 4.8 0.1335 2.3 0.47 807.8 17.1 846.0 27.8 947.4 87.9 807.8 17.1 34c 301 1162314 1.7 9.6153 2.6 0.4283 1.7 0.64 2298.3 32.3 2398.6 24.0 2485.0 33.7 2485.0 33.7 34c2 440 303994 2.1 9.1957 2.3 0.4043 2.0 0.87 2188.7 37.2 2357.7 21.1 2507.3 19.0 2507.3 19.0 34c3 477 286250 2.9 8.5739 3.6 0.3834 3.4 0.96 2092.3 61.6 2293.8 32.7 2478.5 16.9 2478.5 16.9 34t1 525 11365 16.0 0.2084 7.5 0.0273 3.3 0.44 173.9 5.6 192.3 13.1 424.0 149.9 173.9 5.6 34t2 241 38975 2.2 8.5649 5.8 0.3927 5.1 0.87 2135.2 92.3 2292.8 53.1 2436.5 48.5 2436.5 48.5 35c 512 226956 1.5 4.3677 3.0 0.2977 2.6 0.88 1679.7 39.2 1706.2 24.8 1739.0 25.9 1739.0 25.9 35t1 800 30468 11.6 0.1952 4.7 0.0275 2.0 0.42 174.7 3.4 181.0 7.8 264.2 97.9 174.7 3.4 35t2 812 53797 10.9 0.2018 2.9 0.0283 1.6 0.54 179.8 2.8 186.7 4.9 273.9 55.8 179.8 2.8 36c 259 107604 0.8 3.0608 2.6 0.2444 2.2 0.82 1409.8 27.3 1422.9 20.1 1442.7 28.6 1442.7 28.6 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 319 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
36t1 354 131109 5.3 2.2304 3.1 0.1811 2.5 0.82 1073.1 25.1 1190.6 21.7 1410.8 33.9 1410.8 33.9 37c 266 183644 1.1 1.2847 3.4 0.1364 1.3 0.37 824.4 9.8 839.0 19.4 877.7 65.4 824.4 9.8 37t1 91 11713 1.7 1.1306 7.0 0.1196 3.8 0.53 728.0 25.9 768.1 38.0 886.2 123.3 728.0 25.9 38c 238 223276 1.0 10.3756 3.1 0.4513 1.7 0.57 2401.0 34.9 2468.9 28.3 2525.2 42.2 2525.2 42.2 38t1 811 34025 11.9 0.1952 3.3 0.0280 2.0 0.63 178.3 3.6 181.0 5.4 217.2 58.5 178.3 3.6 39c 299 67215 1.5 1.1253 8.1 0.1210 7.6 0.94 736.5 52.8 765.5 43.5 851.2 57.6 736.5 52.8 39t1 651 26161 11.7 0.1986 7.0 0.0271 3.2 0.46 172.2 5.4 183.9 11.7 337.2 140.3 172.2 5.4 40t1 514 1529 9.9 0.5245 36.6 0.0289 5.6 0.15 183.8 10.1 428.2 128.4 2118.2 657.2 41c 191 76329 1.4 1.5459 3.1 0.1542 2.5 0.81 924.2 21.6 948.8 19.2 1006.4 37.2 924.2 21.6 42c 186 36742 1.7 3.9855 3.3 0.2810 2.2 0.68 1596.2 31.6 1631.2 26.9 1676.7 45.1 1676.7 45.1 42t1 854 22889 14.8 0.2000 4.2 0.0269 3.0 0.70 171.0 5.0 185.1 7.2 369.6 68.8 171.0 5.0 43c 197 59035 0.5 1.6810 1.6 0.1658 1.2 0.77 988.7 11.3 1001.3 10.2 1029.2 20.6 1029.2 20.6 43t1 565 35817 14.5 0.1899 3.9 0.0276 1.8 0.48 175.6 3.2 176.5 6.2 189.1 78.9 175.6 3.2
Quartzite JG062505-3 01 315 75354 1.4 5.0405 1.9 0.3184 1.7 0.88 1781.7 26.0 1826.1 16.0 1877.2 16.1 1877.2 16.1 02 170 9896 0.9 0.6038 3.3 0.0811 2.0 0.60 502.8 9.5 479.6 12.5 370.1 59.1 502.8 9.5 03 304 29765 0.9 1.2093 9.3 0.1286 8.9 0.95 779.9 65.1 804.9 51.7 874.8 58.3 779.9 65.1 04 97 17349 0.6 2.6868 3.0 0.2317 2.2 0.73 1343.1 26.5 1324.8 22.2 1295.3 39.9 1295.3 39.9 05 638 33360 0.9 2.1866 2.8 0.1946 2.5 0.88 1146.3 26.2 1176.8 19.7 1233.3 26.2 1233.3 26.2 06 404 47094 0.7 3.6249 7.0 0.2360 7.0 0.99 1365.6 85.8 1555.0 56.1 1822.7 16.9 1822.7 16.9 07 395 95740 4.3 5.3718 1.6 0.3371 1.2 0.77 1872.5 20.2 1880.4 13.8 1889.0 18.5 1889.0 18.5 08 387 39668 1.4 1.6820 4.4 0.1698 3.9 0.88 1011.0 36.3 1001.7 28.0 981.6 42.4 1011.0 36.3 09 638 91598 1.1 3.0785 2.1 0.2460 1.8 0.86 1418.0 22.5 1427.3 15.9 1441.3 20.4 1441.3 20.4 10 443 60537 1.5 1.5897 1.9 0.1599 1.5 0.79 956.3 13.0 966.2 11.6 988.6 23.4 956.3 13.0 11 427 42242 1.3 1.5740 2.3 0.1564 1.7 0.75 937.0 15.3 960.0 14.5 1013.1 31.3 937.0 15.3 12 1022 79431 2.5 1.1923 1.6 0.1282 1.0 0.63 777.7 7.3 797.0 8.7 851.3 25.4 777.7 7.3 13 505 52436 3.1 1.5707 3.0 0.1560 2.7 0.89 934.6 23.3 958.7 18.5 1014.5 27.0 934.6 23.3 14 533 39939 1.2 1.4777 1.5 0.1517 1.1 0.76 910.8 9.4 921.3 8.9 946.5 19.6 910.8 9.4 15 333 68933 0.6 9.6617 1.4 0.4248 1.2 0.85 2282.1 23.6 2403.1 13.3 2507.2 12.8 2507.2 12.8 16 190 32628 2.8 5.2795 2.6 0.3327 2.1 0.84 1851.3 34.4 1865.6 21.8 1881.4 25.3 1881.4 25.3 17 114 19238 0.6 4.3494 2.2 0.2983 1.9 0.89 1682.7 28.5 1702.8 17.8 1727.6 17.9 1727.6 17.9 18 294 40637 1.0 3.8571 1.9 0.2796 1.4 0.71 1589.3 19.2 1604.8 15.5 1625.1 25.3 1625.1 25.3 19 763 59226 1.9 1.5881 2.1 0.1603 1.0 0.49 958.6 8.9 965.5 12.8 981.4 36.5 958.6 8.9 20 319 69131 1.5 4.7504 2.5 0.3002 1.4 0.58 1692.1 21.3 1776.2 20.6 1876.4 35.9 1876.4 35.9 21 143 6644 0.6 0.6071 11.0 0.0786 7.6 0.69 487.6 35.7 481.7 42.2 453.9 176.7 487.6 35.7 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 320 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
22 294 30957 0.9 2.1836 2.6 0.1972 1.0 0.38 1160.1 10.6 1175.8 18.4 1204.8 48.3 1204.8 48.3 23 94 10404 0.4 1.4748 3.6 0.1542 2.4 0.67 924.4 21.0 920.1 21.8 909.8 54.9 924.4 21.0 24 115 18167 1.0 2.4919 1.8 0.2155 1.4 0.75 1257.9 15.5 1269.7 13.1 1289.8 23.4 1289.8 23.4 25 245 39713 3.5 5.0880 2.5 0.3178 1.8 0.73 1778.7 28.5 1834.1 21.3 1897.5 30.8 1897.5 30.8 26 564 49204 1.5 1.5388 3.1 0.1557 2.7 0.87 933.0 23.4 946.0 19.0 976.4 31.0 933.0 23.4 27 188 35425 0.8 5.0049 6.1 0.3079 3.5 0.58 1730.6 53.6 1820.2 51.9 1924.3 89.7 1924.3 89.7 28 554 48118 0.9 1.6123 1.9 0.1614 1.5 0.81 964.7 13.6 975.0 11.7 998.3 22.4 964.7 13.6 29 265 29280 2.0 2.5413 5.6 0.2049 5.4 0.96 1201.7 58.9 1284.0 40.6 1424.3 28.2 1424.3 28.2 30 203 30786 1.6 2.0077 1.8 0.1859 1.2 0.70 1099.2 12.4 1118.1 11.9 1155.2 24.9 1155.2 24.9 31 623 36676 3.1 1.6078 3.7 0.1633 2.7 0.75 975.3 24.8 973.3 22.9 968.6 49.2 975.3 24.8 32 187 17498 0.9 1.2500 1.7 0.1371 1.0 0.60 828.4 8.0 823.4 9.7 809.8 28.9 828.4 8.0 33 176 37339 0.8 10.1411 1.4 0.4496 1.0 0.71 2393.4 20.0 2447.7 13.1 2493.2 16.9 2493.2 16.9 34 368 21512 1.6 0.6508 3.7 0.0825 3.6 0.97 511.1 17.9 509.0 15.0 499.5 20.1 511.1 17.9 35 91 19814 1.2 10.2244 1.7 0.4517 1.3 0.78 2402.7 26.2 2455.3 15.6 2499.1 17.9 2499.1 17.9 36 417 29248 4.4 1.5608 1.2 0.1562 1.0 0.82 935.6 8.7 954.8 7.6 999.2 14.3 935.6 8.7 37 195 11686 2.6 9.5841 4.3 0.4300 3.2 0.74 2305.7 62.5 2395.6 40.0 2473.0 49.3 2473.0 49.3 38 585 53088 3.4 1.6734 2.2 0.1659 1.9 0.86 989.3 17.6 998.5 14.2 1018.7 22.9 1018.7 22.9 39 66 12592 1.4 10.7275 3.6 0.4270 3.5 0.96 2292.3 67.0 2499.8 33.7 2673.0 17.2 2673.0 17.2 40 824 57138 2.9 1.5002 3.2 0.1509 2.3 0.73 906.3 19.6 930.5 19.3 988.3 44.2 906.3 19.6 41 141 27436 1.0 6.3252 2.1 0.3669 1.8 0.84 2014.6 31.3 2021.9 18.8 2029.5 20.6 2029.5 20.6 42 239 17654 0.8 2.0089 8.1 0.1860 6.4 0.79 1099.8 64.4 1118.5 54.8 1155.1 98.5 1155.1 98.5 43 548 45096 6.9 1.6127 1.9 0.1615 1.6 0.82 965.1 14.1 975.1 12.0 997.7 22.2 965.1 14.1 44 91 12627 0.5 2.3823 1.4 0.2067 1.0 0.70 1211.4 11.0 1237.3 10.2 1282.7 19.8 1282.7 19.8 45 356 40327 1.2 7.9193 3.7 0.3722 3.4 0.90 2039.8 58.7 2221.9 33.6 2394.2 27.6 2394.2 27.6 46 318 68633 2.1 10.4152 1.5 0.4538 1.0 0.68 2412.3 20.1 2472.4 13.7 2522.1 18.3 2522.1 18.3 47 452 42384 0.9 1.6161 1.7 0.1622 1.0 0.59 969.2 9.0 976.5 10.6 992.8 27.9 969.2 9.0 48 96 4533 1.2 0.5713 3.9 0.0776 1.7 0.44 481.6 8.0 458.8 14.5 346.4 80.0 481.6 8.0 49 108 5186 0.6 0.5820 3.5 0.0797 1.8 0.52 494.0 8.7 465.7 13.2 328.3 68.3 494.0 8.7 50 132 15904 0.9 2.1818 2.0 0.2011 1.8 0.91 1181.3 19.7 1175.3 13.9 1164.1 16.2 1164.1 16.2 51 132 35576 1.1 9.5829 1.4 0.4387 1.3 0.90 2344.9 25.0 2395.5 13.0 2438.8 10.5 2438.8 10.5 52 507 85686 1.9 11.1573 2.2 0.4521 2.1 0.95 2404.4 41.6 2536.4 20.3 2643.6 10.8 2643.6 10.8 53 147 8122 0.7 0.6206 3.2 0.0796 1.6 0.49 493.9 7.4 490.2 12.4 473.4 61.4 493.9 7.4 54 314 62509 2.7 3.1047 1.4 0.2468 1.0 0.71 1422.1 12.8 1433.9 10.9 1451.4 19.1 1451.4 19.1 55 791 89036 3.4 2.1448 1.9 0.1939 1.6 0.85 1142.7 16.8 1163.4 13.0 1202.1 19.3 1202.1 19.3 56 112 4922 0.8 0.5613 5.6 0.0755 4.4 0.78 469.3 19.7 452.4 20.5 367.5 79.3 469.3 19.7 57 113 10127 1.1 1.5774 1.7 0.1597 1.1 0.64 955.2 9.8 961.3 10.7 975.5 26.9 955.2 9.8 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 321 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
58 328 34518 2.2 1.4885 2.9 0.1474 2.2 0.76 886.6 18.1 925.7 17.4 1019.9 37.5 886.6 18.1 59 541 79411 3.0 4.6361 1.9 0.3078 1.3 0.68 1729.9 19.4 1755.8 15.7 1786.8 25.2 1786.8 25.2 60 355 27255 1.5 0.6574 6.9 0.0835 5.4 0.78 516.7 26.8 513.0 28.0 496.4 96.3 516.7 26.8 61 79 4142 0.7 2.5374 2.8 0.2143 1.5 0.53 1251.8 16.8 1282.8 20.2 1335.1 45.3 1335.1 45.3 62 204 16953 0.6 1.8812 3.7 0.1741 2.9 0.79 1034.9 27.9 1074.5 24.5 1155.8 44.8 1155.8 44.8 63 640 56612 0.9 3.1831 4.1 0.2257 3.5 0.87 1311.7 42.0 1453.1 31.5 1666.4 37.6 1666.4 37.6 64 104 5820 0.9 0.6042 5.4 0.0793 3.6 0.67 492.2 17.2 479.9 20.6 421.2 89.1 492.2 17.2 65 474 51562 4.4 1.6332 1.5 0.1632 1.0 0.68 974.4 9.0 983.1 9.3 1002.4 22.2 974.4 9.0 66 208 18141 0.7 1.5001 6.1 0.1473 5.3 0.87 885.9 44.2 930.4 37.3 1037.6 60.3 885.9 44.2 67 1222 95533 9.7 1.5844 2.3 0.1590 2.1 0.90 951.3 18.4 964.1 14.4 993.4 20.5 951.3 18.4 68 331 78649 0.8 10.6322 2.0 0.4589 1.0 0.51 2434.5 20.3 2491.5 18.2 2538.3 28.3 2538.3 28.3 69 391 30351 1.5 1.2396 1.9 0.1344 1.0 0.52 812.8 7.6 818.7 10.8 834.6 34.2 812.8 7.6 70 332 60699 0.6 10.0840 2.3 0.4472 1.9 0.79 2382.8 37.0 2442.5 21.7 2492.6 24.3 2492.6 24.3 71 451 64448 1.7 4.4427 2.0 0.2971 1.5 0.75 1677.1 21.7 1720.3 16.2 1773.4 23.4 1773.4 23.4 72 92 12629 0.8 1.5955 3.9 0.1610 3.3 0.84 962.1 29.4 968.5 24.4 982.9 43.2 962.1 29.4 73 261 25804 1.1 1.4771 2.4 0.1499 1.5 0.63 900.4 12.6 921.0 14.4 970.8 37.7 900.4 12.6 74 592 85892 1.6 3.2981 1.6 0.2526 1.1 0.69 1451.8 14.7 1480.6 12.8 1522.1 22.4 1522.1 22.4 75 275 47440 1.7 4.9770 3.2 0.3113 3.1 0.95 1747.3 47.1 1815.4 27.3 1894.6 17.6 1894.6 17.6 76 270 19526 0.6 1.2974 1.8 0.1368 1.0 0.56 826.6 7.9 844.6 10.3 892.2 30.8 826.6 7.9 77 710 99923 1.4 3.8045 1.4 0.2779 1.0 0.74 1580.9 14.0 1593.7 10.9 1610.7 17.1 1610.7 17.1 78 167 42061 0.7 10.4948 2.3 0.4613 2.2 0.95 2445.3 44.1 2479.5 21.1 2507.6 12.0 2507.6 12.0 79 261 19436 1.8 2.0385 3.0 0.1860 2.5 0.85 1099.4 25.5 1128.5 20.1 1184.7 30.4 1184.7 30.4 80 238 24217 0.8 1.6095 2.8 0.1610 1.8 0.66 962.6 16.4 973.9 17.4 999.5 42.1 962.6 16.4 81 526 85491 1.4 4.8211 2.5 0.3076 2.1 0.85 1729.0 32.3 1788.6 21.2 1858.8 24.2 1858.8 24.2 82 915 41226 1.3 0.6376 5.9 0.0803 4.7 0.80 497.9 22.7 500.8 23.5 514.2 78.9 497.9 22.7 83 335 37891 1.3 3.5948 3.0 0.2627 2.5 0.83 1503.8 33.4 1548.4 23.7 1609.7 30.8 1609.7 30.8 84 810 18051 2.6 1.5898 3.5 0.1580 2.7 0.75 945.6 23.4 966.2 22.0 1013.4 47.0 945.6 23.4 85 202 25817 1.2 2.0837 1.8 0.1905 1.5 0.84 1124.0 15.5 1143.5 12.4 1180.5 19.5 1180.5 19.5 86 333 39092 0.9 1.4016 2.1 0.1455 1.3 0.61 875.5 10.5 889.6 12.6 924.8 34.6 875.5 10.5 87 221 53420 0.9 5.3246 1.9 0.3344 1.0 0.54 1859.5 16.2 1872.8 15.9 1887.6 28.3 1887.6 28.3 88 289 54411 1.5 3.8360 1.6 0.2825 1.0 0.62 1603.9 14.5 1600.3 13.3 1595.6 24.1 1595.6 24.1 89 489 43926 3.5 1.5150 1.2 0.1542 1.0 0.83 924.2 8.6 936.5 7.4 965.3 14.0 924.2 8.6 90 181 46003 1.4 5.3185 1.5 0.3365 1.0 0.65 1869.8 16.2 1871.8 13.2 1874.1 21.3 1874.1 21.3 91 549 108409 2.8 4.7505 2.0 0.3086 1.6 0.78 1733.8 24.2 1776.2 17.1 1826.4 23.0 1826.4 23.0 92 117 4868 1.2 0.6110 4.7 0.0798 2.8 0.60 494.9 13.3 484.2 17.9 433.9 83.0 494.9 13.3 93 473 113880 1.1 10.0392 1.5 0.4431 1.2 0.80 2364.3 24.1 2438.4 14.0 2500.8 15.3 2500.8 15.3 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 322 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
94 42 1997 0.6 0.5579 8.2 0.0762 2.7 0.33 473.2 12.5 450.2 30.0 333.9 176.6 473.2 12.5 95 305 24803 0.6 1.6038 2.1 0.1608 1.4 0.70 961.0 12.8 971.7 12.9 995.8 30.0 961.0 12.8 96 63 7291 0.5 1.9943 2.0 0.1896 1.6 0.81 1119.2 16.6 1113.6 13.5 1102.6 23.6 1102.6 23.6 97 193 26288 1.1 4.3570 5.8 0.2968 3.8 0.66 1675.3 56.7 1704.2 48.2 1740.0 80.4 1740.0 80.4 98 403 64509 0.6 4.3755 1.3 0.2981 1.0 0.76 1681.9 14.8 1707.7 10.9 1739.6 15.6 1739.6 15.6 100 844 46504 1.2 1.6268 3.8 0.1629 3.6 0.95 973.0 32.7 980.6 24.1 997.8 25.4 973.0 32.7
Quartzite AP061304-A 01 101 803 1.7 1.9035 10.0 0.1632 3.4 0.34 974.4 31.0 1082.3 66.4 1306.5 182.1 02 88 6028 0.7 2.0687 4.5 0.1869 0.9 0.20 1104.8 9.3 1138.5 31.0 1203.4 87.3 1203.4 87.3 03 710 91258 1.5 1.5334 1.3 0.1529 1.1 0.85 917.3 9.7 943.9 8.2 1006.4 14.3 917.3 9.7 04 257 10683 1.3 2.0543 1.9 0.1906 1.1 0.59 1124.5 11.7 1133.7 13.0 1151.5 30.5 1151.5 30.5 05 241 19678 0.8 2.6356 1.8 0.2195 1.5 0.82 1279.1 17.2 1310.6 13.3 1362.5 20.0 1362.5 20.0 06 128 30502 1.6 2.7953 2.7 0.2294 2.4 0.86 1331.2 28.5 1354.3 20.5 1390.9 26.6 1390.9 26.6 07 176 91288 1.1 11.2376 1.0 0.4492 0.8 0.74 2391.8 15.2 2543.0 9.6 2666.0 11.6 2666.0 11.6 08 529 97913 2.1 1.0704 1.4 0.1190 0.8 0.54 725.0 5.1 738.9 7.3 781.4 24.7 725.0 5.1 09 496 80837 6.7 1.3042 1.7 0.1385 0.8 0.46 836.0 6.0 847.6 9.6 878.0 30.7 836.0 6.0 12 371 6899 1.5 0.9933 4.8 0.1130 4.2 0.86 689.9 27.2 700.4 24.3 734.2 51.3 689.9 27.2 13 618 13182 1.5 5.9792 8.6 0.2981 8.1 0.95 1682.1 120.4 1972.8 74.8 2293.1 47.5 2293.1 47.5 14 491 67081 1.1 1.5946 1.8 0.1625 1.2 0.64 970.4 10.5 968.1 11.3 962.7 28.5 970.4 10.5 15 395 41951 1.9 1.4744 2.1 0.1511 0.7 0.33 907.0 5.9 919.9 12.9 951.1 41.2 907.0 5.9 18 124 35454 0.7 1.8920 2.2 0.1853 1.2 0.53 1096.0 11.6 1078.3 14.5 1042.6 37.3 19 172 2941 1.7 1.4067 7.9 0.1476 1.9 0.25 887.8 16.2 891.8 46.7 901.7 157.3 887.8 16.2 20 490 50446 0.7 1.6361 1.2 0.1632 1.0 0.79 974.4 8.7 984.2 7.7 1006.0 15.2 974.4 8.7 21 709 83091 10.2 1.1409 1.2 0.1266 0.9 0.80 768.3 6.7 772.9 6.3 786.3 14.8 768.3 6.7 22 69 9117 0.6 1.4695 7.2 0.1541 0.7 0.10 923.9 6.1 917.9 43.5 903.6 147.7 923.9 6.1 23 52 17323 0.2 10.6044 2.6 0.4671 1.6 0.60 2470.9 32.7 2489.1 24.5 2504.0 35.4 2504.0 35.4 24 199 5121 0.9 0.6176 7.3 0.0823 2.1 0.29 509.9 10.5 488.4 28.4 388.7 157.4 509.9 10.5 25 14 1563 1.5 12.0969 7.4 0.4871 1.3 0.18 2558.0 28.1 2611.9 69.6 2654.0 121.1 2654.0 121.1 26 170 17028 0.7 1.0689 3.5 0.1222 1.1 0.30 743.2 7.4 738.2 18.3 723.0 70.4 743.2 7.4 27 739 153048 1.7 3.1859 2.7 0.2418 2.6 0.96 1395.8 32.1 1453.8 20.5 1539.4 13.2 1539.4 13.2 28 261 24269 2.0 1.2792 3.2 0.1339 2.8 0.89 809.9 21.5 836.5 18.2 907.7 30.1 809.9 21.5 29 51 12448 0.9 1.9229 7.1 0.1772 1.6 0.23 1051.7 15.7 1089.1 47.4 1164.6 137.0 30 190 62200 2.1 5.3135 1.2 0.3293 0.9 0.80 1835.0 15.0 1871.0 10.0 1911.3 12.6 1911.3 12.6 31 66 51945 0.9 10.4575 1.7 0.4649 1.2 0.74 2461.0 25.4 2476.2 15.5 2488.6 18.9 2488.6 18.9 32 77 19951 1.5 2.1012 5.4 0.1972 1.3 0.23 1160.2 13.3 1149.2 37.2 1128.4 104.7 1128.4 104.7 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 323 Spectrometry Isotopic ratios Apparent ages (Ma)
Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Best age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
33 271 67360 1.1 1.2077 2.3 0.1332 1.4 0.58 806.1 10.3 804.1 12.9 798.7 39.5 806.1 10.3 34 127 12354 0.5 2.0367 4.7 0.1935 2.2 0.46 1140.5 22.6 1127.8 32.2 1103.6 84.0 1103.6 84.0 35 70 1680 0.8 0.6825 19.7 0.0895 1.7 0.09 552.5 9.0 528.3 81.1 425.1 440.6 552.5 9.0 36 261 149233 1.0 9.0885 2.2 0.4109 2.1 0.95 2219.0 40.0 2347.0 20.5 2460.1 11.8 2460.1 11.8 37 24 3972 2.2 1.9894 9.5 0.1801 1.9 0.20 1067.5 19.1 1111.9 64.1 1199.8 183.2 38 281 7157 1.2 1.5559 6.8 0.1509 1.3 0.20 905.9 11.4 952.8 42.3 1062.9 135.1 905.9 11.4 40 357 12024 11.0 1.5496 4.5 0.1525 3.6 0.79 914.8 30.5 950.3 27.9 1033.5 56.2 914.8 30.5 41 128 7693 0.3 0.6657 5.4 0.0830 1.9 0.36 513.8 9.4 518.1 21.8 537.2 109.7 513.8 9.4 42 238 4647 1.1 0.9237 4.4 0.1097 1.5 0.34 671.1 9.5 664.3 21.6 641.4 89.8 671.1 9.5 43 139 75826 1.3 13.0214 1.8 0.5001 1.7 0.92 2614.1 36.5 2681.2 17.3 2732.2 11.5 2732.2 11.5 44 540 144222 0.8 13.2450 1.0 0.4889 0.7 0.71 2565.9 14.8 2697.3 9.3 2797.2 11.5 2797.2 11.5 46 181 49994 0.7 1.5870 3.6 0.1609 0.7 0.19 962.0 6.3 965.1 22.4 972.3 71.9 962.0 6.3 47 54 7080 1.2 2.0321 5.1 0.1819 0.7 0.14 1077.3 7.0 1126.3 34.6 1222.2 99.0 1222.2 99.0 48 677 3571 2.6 0.8357 6.4 0.0961 1.2 0.19 591.4 6.7 616.8 29.4 711.1 132.8 591.4 6.7 49 152 13695 1.3 1.2196 4.3 0.1365 0.9 0.20 825.0 6.6 809.6 24.2 767.6 89.7 825.0 6.6 50 46 5700 0.4 1.5027 9.1 0.1671 1.1 0.12 996.2 9.8 931.5 55.4 781.3 189.8 996.2 9.8 51 144 36024 0.8 2.6252 2.1 0.2260 0.9 0.43 1313.6 10.7 1307.7 15.5 1298.0 37.0 1298.0 37.0 52 734 74445 2.7 2.1933 1.4 0.1974 1.3 0.87 1161.1 13.4 1178.9 10.1 1211.7 13.8 1211.7 13.8 55 162 49238 1.6 24.2362 1.0 0.6092 0.7 0.71 3066.8 17.1 3277.9 9.7 3409.7 10.9 3409.7 10.9 56 389 101138 1.2 9.7343 1.0 0.4296 0.7 0.71 2303.9 13.6 2410.0 9.1 2500.8 11.8 2500.8 11.8 58 299 20097 1.3 2.0711 1.4 0.1892 0.7 0.49 1117.3 7.2 1139.3 9.7 1181.4 24.5 1181.4 24.5 59 244 11015 1.2 1.2856 2.9 0.1389 1.0 0.34 838.4 7.7 839.3 16.3 841.7 55.9 838.4 7.7 60 73 4723 0.7 0.5897 8.3 0.0721 1.7 0.21 448.9 7.4 470.7 31.2 578.6 176.2 448.9 7.4 61 137 15083 0.5 1.2424 4.8 0.1292 2.4 0.50 783.2 17.8 820.0 26.9 921.0 85.0 783.2 17.8 62 227 13284 1.5 3.6961 1.8 0.2682 0.7 0.38 1531.6 9.6 1570.5 14.6 1623.2 31.5 1623.2 31.5 63 178 70936 1.6 10.5128 1.2 0.4499 1.0 0.81 2394.9 19.2 2481.0 11.0 2552.3 11.8 2552.3 11.8 64 128 2661 1.4 0.7038 10.8 0.0901 0.9 0.09 555.9 4.9 541.1 45.4 479.0 238.9 555.9 4.9 65 53 17055 0.9 14.4033 2.4 0.5251 1.0 0.43 2721.0 22.7 2776.6 22.4 2817.4 34.7 2817.4 34.7 66 210 52534 1.1 2.1486 2.1 0.1901 0.7 0.34 1121.8 7.5 1164.6 14.8 1245.0 39.4 1245.0 39.4 67 56 3539 0.7 0.7525 12.2 0.0884 1.3 0.11 546.2 6.9 569.7 53.1 664.6 259.9 546.2 6.9 68 39 900 0.6 3.3784 15.4 0.2753 2.6 0.17 1567.8 35.9 1499.4 121.5 1404.0 293.1 69 184 91594 2.4 9.6454 2.3 0.4142 2.2 0.95 2234.3 41.2 2401.5 21.1 2546.5 11.8 2546.5 11.8 70 596 81718 8.4 0.6626 2.4 0.0822 0.7 0.29 509.5 3.4 516.2 9.6 546.2 49.7 509.5 3.4 71 164 32997 2.4 5.9001 7.4 0.2983 6.5 0.88 1682.9 96.6 1961.2 64.2 2269.3 59.9 2269.3 59.9 72 47 24662 0.4 11.3694 2.3 0.4787 1.3 0.54 2521.4 26.5 2553.9 21.9 2579.8 32.8 2579.8 32.8 73 143 14862 1.6 3.2680 3.1 0.2373 1.4 0.43 1372.7 16.7 1473.5 24.4 1621.7 52.6 1621.7 52.6 Table 2. (Cont’d) Amdo basement U-Pb Zircon Geochronologic Analyses by Laser-Ablation Multicollector ICP Mass 324 Spectrometry Isotopic ratios Apparent ages (Ma)
Best Spot U 206Pb U/Th 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± age ± ID (ppm) 204Pb 235U (%) 238U (%) corr. 238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma) (Ma)
74 320 13261 0.5 0.7793 4.2 0.0890 1.0 0.23 549.3 5.1 585.1 18.7 726.4 87.1 549.3 5.1 75 56 13307 3.2 1.3384 8.3 0.1418 1.7 0.20 854.7 13.2 862.5 48.4 882.7 169.0 854.7 13.2 76 315 9717 2.3 2.2886 4.4 0.1872 3.1 0.71 1106.3 31.4 1208.8 30.9 1396.8 59.2 1396.8 59.2 77 68 5729 1.5 13.0184 2.8 0.4910 2.3 0.79 2575.0 47.8 2681.0 26.8 2761.9 28.5 2761.9 28.5 78 153 61747 2.1 14.5884 3.6 0.4879 2.5 0.68 2561.7 51.8 2788.8 34.2 2957.5 42.6 2957.5 42.6 79 27 4046 0.4 0.8420 36.2 0.0830 2.9 0.08 514.1 14.2 620.2 169.6 1029.2 754.2 80 125 25412 1.0 1.6954 2.9 0.1617 1.2 0.40 966.2 10.5 1006.8 18.6 1096.2 53.3 1096.2 53.3 81 135 23638 1.0 1.8421 2.8 0.1720 2.2 0.76 1023.2 20.4 1060.6 18.6 1138.4 36.3 1138.4 36.3 82 68 3077 0.4 0.7882 16.1 0.0862 2.1 0.13 533.3 10.8 590.2 72.3 815.2 336.2 533.3 10.8 83 38 14639 0.8 10.7864 2.6 0.4565 0.9 0.33 2423.9 17.4 2504.9 24.0 2571.2 40.7 2571.2 40.7 84 69 11630 1.0 4.0276 3.6 0.2815 1.1 0.30 1598.8 15.2 1639.8 29.6 1692.7 64.0 1692.7 64.0 85 258 34662 3.7 1.5388 5.9 0.1504 5.3 0.89 903.2 44.2 946.0 36.4 1047.0 55.0 903.2 44.2 86 177 14551 0.5 1.9771 2.8 0.1766 0.9 0.33 1048.5 8.9 1107.7 19.0 1226.0 52.3 1226.0 52.3 87 121 37721 0.7 1.5506 4.0 0.1482 0.8 0.21 891.0 6.8 950.7 24.5 1091.6 77.9 891.0 6.8 88 72 2427 0.9 3.0704 6.1 0.2153 2.8 0.46 1256.9 32.3 1425.3 46.8 1686.7 100.0 1686.7 100.0 89 249 45270 1.7 1.6927 2.0 0.1600 1.3 0.67 956.6 11.9 1005.8 12.7 1114.5 29.5 1114.5 29.5 90 89 10076 2.2 2.0756 5.0 0.1880 0.8 0.17 1110.6 8.7 1140.8 34.0 1198.7 96.4 1198.7 96.4 91 127 9992 1.5 1.5725 4.6 0.1538 1.7 0.37 922.0 14.7 959.4 28.4 1046.3 85.6 922.0 14.7 92 546 2996 3.2 1.5037 5.3 0.1354 4.2 0.78 818.5 32.2 931.9 32.6 1210.6 65.4 94 142 22106 1.0 1.2759 5.8 0.1302 4.2 0.74 788.9 31.5 835.0 32.8 960.0 79.7 788.9 31.5 95 94 19470 0.5 2.1789 2.8 0.1889 0.9 0.32 1115.6 9.3 1174.3 19.8 1284.1 52.4 1284.1 52.4 96 366 80206 2.1 1.5748 2.0 0.1567 1.6 0.82 938.5 14.2 960.3 12.4 1010.5 23.3 938.5 14.2 98 230 6272 4.6 1.6662 3.4 0.1650 0.7 0.22 984.4 6.8 995.7 21.8 1020.7 68.0 1020.7 68.0 99 115 6349 1.0 0.7830 4.6 0.0897 0.8 0.18 554.0 4.5 587.2 20.5 717.6 96.0 554.0 4.5 100 280 4208 8.1 0.6051 9.3 0.0716 6.7 0.72 446.0 29.0 480.5 35.6 648.8 138.3 446.0 29.0 101 434 1642 1.4 1.6366 11.5 0.1682 1.3 0.11 1002.2 11.9 984.4 72.4 944.9 234.1 1002.2 11.9 102 147 11967 0.8 0.8148 5.7 0.0957 0.8 0.14 589.1 4.4 605.2 26.1 665.8 121.5 589.1 4.4 103 93 1113 1.1 0.6346 20.9 0.0829 1.7 0.08 513.7 8.6 499.0 82.7 432.1 469.5 513.7 8.6