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

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.