India-Asia collisional processes: constraints from the Eocene magmatic rocks in the southern Tibetan Plateau

The India-Asia collision has been one of the most significant geological events on Earth over the last 100 Ma. However, the understanding of this collisional process remains highly controversial. In this study, we examined Eocene magmatic rocks in the Qiongguo area, southern Tibet, using geochemistry, zircon U-Pb age, and Lu-Hf isotopic analysis. The results show that the Qiongguo magmatic rocks were formed at 52–48 Ma. The dacites were derived from partial melting of the lower crust, and the rhyolites formed by the mixing of mantle- and crust-derived magmas. The La/Yb, Sm/Yb, and Dy/Yb values are widely used for the quantitative inversion of crustal thickness. We found that the La/Yb, Sm/Yb, and Dy/Yb values of 70–40 Ma intermediate-acidic magmatic rocks in the southern Tibetan Plateau began to increase from ca. 58 Ma, reached a peak at ca. 52 Ma, and then slowly decreased, indicating that crustal thickening-thinning processes may have occurred during the Eocene and crustal thickening peaked at ca. 52 Ma. The values of the zircon U-Pb ages, zircon saturation temperatures, ε_Hf(t), and ε_Nd(t) of magmatic rocks in the southern Tibetan Plateau all peaked at ca. 52 Ma, indicating the occurrence of a significant tectono-thermal anomaly and mantle upwelling event, which may have been the result of the slab breakoff of the Neo-Tethyan oceanic lithosphere after the India-Asia collision. Combined with previous data, these data show that the initial India-Asia collision began no later than ca. 58 Ma, and the continental crust gradually thickened and peaked at ca. 52 Ma. Subsequently, owing to the obstruction of the negative buoyancy of the continental crust, the continental crust decoupled from the oceanic crust and slab breakoff occurred, resulting in a wide range of tectono-thermal anomalies and mantle upwelling events and changing the tectonic background of the orogenic belt from extrusion to extension.