2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 10
Presentation Time: 10:45 AM

The Insight into the Basement of Central Tibet: Timing, Exhumation, and Extent of the Qiangtang Metamorphic Belt and Duguer Shan Gneiss


PULLEN, Alex, Department of Geosciences, University of Arizona, Gould-Simpson Building #77, Tucson, AZ 85721, KAPP, Paul, Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, GEHRELS, George E., Department of Geosciences, University of Arizona, Tucson, AZ 85721 and DING, Lin, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100029, apullen@email.arizona.edu

The Qiangtang metamorphic belt in central Tibet is a >450-km-long, east–west trending exposure of tectonic mélange including high–near-ultrahigh-pressure (near-UHP) blocks. Protoliths of the Qiangtang metamorphic belt are consistent with a derivation from upper Paleozoic Qiangtang continental margin strata and from rocks derived from an arc source within the Paleo-Tethys basin to the north. These rocks are exposed structurally beneath low-grade Carboniferous–Triassic strata in the footwalls of domal, Late Triassic–Early Jurassic low-angle normal faults. An epidote bearing granite from the mélange was determined to have a U–Pb zircon age of 364.2 ±5.4 Ma (2-sigma) and is interpreted to represent a portion of an arc terrane which developed within the Paleo-Tethys, or alternatively, as a fragment of Qiangtang basement. 40Ar/39Ar studies of white mica and amphibole indicate that the Qiangtang metamorphic belt was exhumed to mid-crustal levels within the Qiangtang terrane by Late Triassic–earliest Jurassic time, immediately following (near-U)HP metamorphism. We suggest that these relationships can be explained through the incorporation of Qiangtang continental margin into a tectonic mélange with an arc terrane of Paleo-Tethys affinity that was subducted to the south beneath the Qiangtang terrane in Middle–Late-Triassic time. Kyanite/sillimanite-bearing orthogneisses exposed structurally beneath upper Paleozoic Qiangtang strata farther to the south of the Qiangtang metamorphic belt yielded crystallization ages of ~470–480 Ma. These rocks are interpreted to indicate the southernmost extent of the Qiangtang metamorphic belt beneath the Qiangtang terrane.