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The history of Indian continental subduction beneath Asian plate remains unclear. Miocene ultrapotassic rocks in southern Tibet, with extremely enriched isotopes, have often been used to trace mantle metasomatism and geodynamic processes associated with Indian continental subduction. These rocks, however, may have been contaminated by Lhasa ancient crust. Uncertainties on primary ultrapotassic magmas obscure their mantle sources. Here we report on first mafic igneous enclaves in Cenozoic lavas of southern Tibet. They consist principally of clinopyroxene, phlogopite, and sanidine and have a zircon U-Pb age of 21.5 +/- 0.3 Ma. Mineral and bulk-rock geochemical characteristics indicate their crystallization from primary ultrapotassic magmas. Bulk-rock Sr-Nd, clinopyroxene Sr, and zircon O isotopes demonstrate their isotopically enriched character. Combined with precollisional basalts, we suggest that the mantle source of the enclaves was enriched by the subducted Indian continent. This study implies that the Indian continent had subducted beneath central Lhasa no later than early Miocene.

Plain Language Summary India-Eurasia plate convergence has been one of the most important Cenozoic geological events on Earth. The Indian continent should now lie beneath central Lhasa block of southern Tibet, as the geophysical data showed. However, we cannot actually determine when the Indian continent reached its present position. The Cenozoic mantle-derived ultrapotassic rocks in central Lhasa, characterized by extremely enriched isotopic compositions, have often been used in attempts to trace mantle metasomatism and the geodynamic processes associated with Indian continental subduction. However, these rocks were likely contaminated by Lhasa ancient crustal materials. Uncertainties on primary ultrapotassic magmas and their origins have limited our ability to directly characterize their mantle sources. We have recently identified the early Miocene mafic enclaves in central Lhasa. Our study indicates that these enclaves likely originated in basic ultrapotassic magmas. The bulk-rock Sr-Nd, in situ clinopyroxene Sr, and zircon O isotope compositions differ from those of precollisional basalts, further demonstrating the isotopically enriched character of the primary magmas and suggesting that their mantle source was metasomatized by the subducted Indian continent. This study shows that the Indian continent could likely have reached its present position (central Lhasa block) no later than the early Miocene.