Crustal melting and flow beneath northern Tibet: Evidence from mid-Miocene to Quaternary strongly peraluminous rhyolites in the southern Kunlun Range
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One of the major geophysical discoveries concerning the Tibetan Plateau is the existence of unusually weak layers in the mid- to lower crust, a characteristic widely interpreted as the result of crustal melting. This interpretation, however, remains highly contentious, particularly when applied to northern Tibet where crustally derived magmatic rocks are scarce. Here we report the finding of tourmaline-bearing mica and biotite rhyolites in the Bukadaban-Malanshan area, southern Kunlun Range, near the northern margin of the Tibetan Plateau. Zircon U-Pb and whole-rock or mineral 40Ar-39Ar analyses suggest that these rocks erupted between 9.0 and 1.5 Ma. These rocks are geochemically similar to Himalayan leucogranites (interpreted as crustal melts), with strongly peraluminous compositions, high SiO2 contents (69.0-76.0 wt %), and clear negative Eu, Ba and Sr anomalies. They have low εNd (-5.8 to -8.6) and high 87Sr/86Sr (0.7125-0.7178), 206Pb/204Pb (18.59-18.70), 207Pb/204Pb (15.49-15.63) and 208Pb/204Pb (38.31-38.74) isotopic compositions as well as magmatic zircon εHf (-0.7 to -5.0) compositions similar to those of global marine sediments and Proterozoic-Triassic sedimentary rocks in northern Tibet. We suggest that the Bukadaban-Malanshan rhyolites were generated by dehydration melting of metasedimentary rocks at 0.5-1.2GPa and 740-863°C. Our data not only confirm the occurrence of a partially molten zone in the mid- to lower crust beneath northern Tibet but also constrain the crustal melting to have existed from middle Miocene to Quaternary times. Adopting the crustal flow model, we further argue the importance of outward flowing of the melt-weakened crust in the formation of crustal inflation, surface uplift, and earthquakes along the northern margin of the Tibetan Plateau.
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