First identification of postcollisional A-type magmatism in the Himalayan-Tibetan orogen

A-type magmatism is commonly generated in extensional settings. Cenozoic postcollisional extensional structures and magmatism widely occur in the well-known Himalayan-Tibetan orogen (HTO). So far, however, no Cenozoic A-type magmatism has been identified in the orogen. Here, we report on newly identified 24-23 Ma felsic lavas (trachytes + rhyolites) in the Konglong area of the Lhasa block, southern Tibet. They have diagnostic minerals (sodalites) and geochemical characteristics (high total-alkali, FeO/MgO, Zr, and Ga/Al, low CaO, and strongly negative Eu, Sr, and Ba anomalies), demonstrating their close affinity with A-type granitoids. Generation of the A-type lavas required high-temperature melting (up to 944 degrees C) of crustal source rocks in the pressure range of 0.5-1.2 GPa. Our study not only identifies the first postcollisional A-type magmatism in the HTO, but it also indicates an extension event contemporaneous with the onset of the Main Central thrust and South Tibetan detachment system, and development of the Kailas basin and ultrapotassic magmatism of the southern HTO. These events straddled the Himalaya and Lhasa blocks and can be reconciled with foundering of the subducted Indian plate. Thus, we suggest a tectonic model of Indian plate flat subduction and foundering beneath the western Lhasa block for the postcollisional evolution of the southern HTO.