Late Eocene palaeogeomorphology and uplift process of the Central Tibetan Plateau: Constraints from intermediate-felsic magmatic rocks

Abstract

The Eocene palaeogeomorphology and uplift process of the Central Tibetan Plateau including the Lhasa and Qiangtang terranes have been controversial. In this paper, we study Eocene rhyolites in Wuma area of the Northern Lhasa terrane on the Central Tibetan Plateau by zircon U-Pb dating, geochemistry, zircon Lu-Hf isotopes, and Sr-Nd isotopes. The results show that the ages of Wuma rhyolites are 40-39 Ma, and the petrogenesis of them is related to partial melting of lower crust. The La/Yb, Dy/Yb, Sm/Yb and zircon Eu/Eu* of lower-crust-derived intermediate-felsic magmatic rocks can well reflect crustal thickness and Moho depth. Based on statistics of intermediate-felsic magmatic rocks in the Central Tibetan Plateau at 47–37 Ma, we find that lower-crust-derived intermediate-felsic magmatic rocks of the Northern Lhasa terrane have lower La/Yb, Dy/Yb, Sm/Yb and zircon Eu/Eu* than those of the Southern-Central Lhasa and Qiangtang terranes, indicating that Northern Lhasa terrane had thinner crust than Southern-Central Lhasa and Qiangtang terranes during late Eocene. Further, according to Airy isostatic compensation, in which thick crusts correspond to high altitudes and thin crusts correspond to low altitudes, we speculate that the Northern Lhasa terrane had lower altitude, while the Southern-Central Lhasa and Qiangtang terranes had higher altitude during late Eocene; that is, the Central Tibetan Plateau had the late Eocene palaeogeomorphology of two mountains and one basin. In addition, through statistics of intermediate-felsic magmatic rocks in the Northern Lhasa terrane and near its suture zones on both sides since 100 Ma, we find that La/Yb, Dy/Yb and Sm/Yb of these intermediate-felsic magmatic rocks increase at 40-34 Ma, indicating that the crust of the Northern Lhasa terrane have thickened and the surface may have been uplifted during late Eocene. Based on that there are abundant late Eocene lower-crust-derived intermediate-felsic magmatic rocks in the Central Tibetan Plateau and the late Eocene palaeogeomorphology of Central Tibetan Plateau is two mountains and one basin, and other evidences, we speculate that the late Eocene uplift of Northern Lhasa terrane may have been related to lower crustal channel flow.