Acta Petrologica et Mineralogica

The Oligocene to Miocene adakitic rocks are widely distributed in the southern Lhasa terrane, and their petrogenesis is crucial for understanding the post-collisional evolution of the Gangdese magmatic arc. Here, we conducted petrological, geochemical, zircon U-Pb geochronological, and in-situ zircon Hf isotopic studies of the Early Miocene biotite granites from the Quxu area in the middle segment of the Gangdese arc. The studied four biotite granites are classified as calc-alkaline, metaluminous to weak peraluminous granitoids, and show geochemical affinities to adakites, characterized by high SiO₂, Al₂O₃, K₂O and Sr contents, low Y and Yb contents, enrichment in LREE and depletion in HREE, and high Sr/Y (165~278) and (La/Yb)_N (26.6~39.7) ratios. Zircon U-Pb dating yields identical ages of 21~19 Ma, which are interpreted as the crystallization ages of the magmatic rocks. Zircon εHf(t) values ranges from -0.9~+12.7 and are mostly positive, reflecting variable contributions from juvenile and ancient crustal materials. The crustal thickness of the Gangdese arc in Early Miocene is estimated to be approximately 70~80 km based on the Sr/Y and (La/Yb)_N ratios. Our presented new data, combined with previous data, supports the proposition that the Gangdese magmtic arc underwent significant Cenozoic crustal thickening resulted from the tectonic shortening during and after the india-Asia continental collision, as well as the addition of mantle-derived magmas. We proposed that the breakoff of the deeply subducted Indian continent and/or delamination of the thickened Gangdese lithosphere resulted in the upwelling of asthenosphere and partial melting of the thickened lower crust, leading to the widespread occurrence of post-collisional adakitic rocks.

Gangdese magmatic arc; granite; adakitic rocks; Miocene; crustal thickening