HR: 16:30h
AN: T14C-03    [Abstracts]
TI: U-Pb Basement and Detrital Zircon Geochronology of the Lhasa and Qiangtang Terranes in Tibet
AU: Kapp, P
EM: pkapp@email.arizona.edu
AF: Department of Geosciences, University of Arizona, 1040 E 4th St., Tucson, AZ 85721, United States
AU: * Pullen, A
EM: apullen@email.arizona.edu
AF: Department of Geosciences, University of Arizona, 1040 E 4th St., Tucson, AZ 85721, United States
AU: Gehrels, G E
EM: ggehrels@email.arizona.edu
AF: Department of Geosciences, University of Arizona, 1040 E 4th St., Tucson, AZ 85721, United States
AU: Ding, L
EM: dinglin@mail.igcas.ac.cn
AF: Institute of Tibetan Plateau Research, Chinese Academy of Sciences, P.O. Box 2871, Beijing, Heb 100085, China
AB: U-Pb ages of >3500 zircons from Paleozoic sandstones and basement exposures in the Qiangtang and Lhasa terranes of Tibet provide new constraints on the pre-Mesozoic geological framework and paleogeography of Tibetan terranes. 1. Carboniferous-Permian strata exposed to the north and south of the E-W trending high- pressure central Qiangtang metamorphic belt (CQMB) show no obvious differences in their detrital zircon age spectra. These results, together with stratigraphic and sedimentologic similarities, indicate that the CQMB does not separate strata of Cathaysian affinity in the north from that of Gondwana affinity in the south as previously suggested. 2. Upper Paleozoic strata in the southern Qiangtang terrane are locally exposed structurally above kyanite/silliminate-bearing orthogneisses and metasedimentary assemblages. The orthogneisses yielded crystallization ages in the 470-480 Ma range and metasandstones show minimum ages of ~540 Ma. We suggest that these metamorphic rocks represent early Ordovician and older basement of the Qiangtang terrane which was deformed and metamorphosed during an early- to mid-Paleozoic orogeny and then unconformably overlain by upper Paleozoic strata. 3. In the central Lhasa terrane, an exposure of ~509 Ma granitic basement is unconformably overlain by a straight section of Cambrian (?) through Permian strata. Detrital zircon age spectra determined for the Paleozoic strata are distinct from those of Qiangtang strata, but strikingly similar to Tethyan strata of the Tibetan Himalaya. These results are unexpected because it is generally thought the Lhasa and Qiangtang terranes were separated by a relatively narrow and short-lived Meso-Tethys Ocean whereas the Lhasa terrane and Tethyan Himalaya were separated by the large and long-lived Neo-Tethys Ocean. 4. The youngest ages of detrital zircons in all Paleozoic sandstones are significantly older (>100 Ma) than their depositional ages, consistent with their deposition in passive margin settings. Our and previous results suggest that Tibetan basement rocks are largely Neo-Proterozoic to Ordovican in age. In addition, the Paleozoic cover sequence sandstones are immature and dominated by <1.0 Ga detritus, indicating little input from older continental sources. Consequently, we suggest that the present Tibetan Plateau is made up mainly, if not entirely, of juvenile terranes. In contrast, it is bounded by blocks that include significantly older (and by inference rheologically stronger) basement. Hence, the spatial extent of the Tibetan Plateau was likely predetermined by initial conditions.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 8038 Regional crustal structure
DE: 8110 Continental tectonics: general (0905)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 9320 Asia
SC: Tectonophysics [T]
MN: 2007 Fall Meeting