Crust-mantle mixing and crustal reworking of southern Tibet during Indian continental subduction: Evidence from Miocene high-silica potassic rocks in Central Lhasa block

Abstract

Collisional zones are commonly considered as important regions for crustal reworking, but the reworking mechanism remains debated. The well-known Himalayan-southern Tibetan orogen, built by India-Asia collision and convergence, has the thickest continental crust on Earth and is therefore an ideal region for studying crustal reworking during collisional orogenesis. Here we revisit the Miocene high-silica potassic rocks (trachytes) in the Konglong area of the central Lhasa block, southern Tibet. Integrated studies of geochronology, mineral compositions, bulk-rock major- and trace-element geochemistry, and Sr-Nd-Pb-Hf-O isotopes unequivocally indicate that the Konglong trachytes formed by mixing between enriched mantle-derived ultrapotassic and thickened ancient crust-derived magmas. Combined with post-collisional magma mixing recently identified in the southern Lhasa block, we suggest that magma underplating and subsequent mantle-crust interaction (i.e., the matter and energy transfer from the mantle to the crust) has been a common and important crustal reworking process in southern Tibet during Indian continental subduction. This process may be related to Indian plate flat subduction and subsequent foundering during the post-collisional stage. In combination with the nature of Cenozoic magmatism in the Himalaya block, we suggest that in addition to partial melting of the subducted continental crust, magma underplating and subsequent crust-mantle mixing beneath the obducted continent has also played an important role in crustal reworking of the collisional zone.