Geochemical evidence for the Eocene surface uplift of the southern Lhasa subterrane, southern Tibet and the implications

Abstract

Whether the crust of southern Tibet has been thickened during the Eocene and if so, the mechanisms of crustal thickening in this region need to be further investigated. This study presents geochronological and geochemical data for Eocene (45–41 Ma) granitic plutons, including the Aima diorites in the eastern part and Huo'er pluton (granites and mafic enclaves) in the western part of the southern Lhasa subterrane. The Aima diorites show adakitic affinities, such as moderate SiO 2 contents and relatively high Sr/Y values, with depleted 87 Sr/ 86 Sr (i) (0.704039–0.704054), εNd (t) (+2.9 to + 3.3) isotopic compositions, indicating that they are derived from a juvenile continental crust. The Huo'er host granites have high SiO 2 and low MgO contents. Meanwhile, they are characterized by the enriched initial 87 Sr/ 86 Sr (i) (0.707899–0.708618), εNd (t) (from − 6.4 to − 5.3) and variable zircon εHf (t) (from − 6.8 to + 5.0) values. These geochemical characteristics and the presence of mafic enclaves demonstrate that the Huo'er host granites were the result of melting of crust (both ancient and juvenile) of southern Lhasa subterrane with the involvement of mantle-derived materials. The Huo'er mafic enclaves display an igneous texture and contain acicular apatite, suggesting that they are quenching of mafic magmas that have co-mingled with the host granites. The mafic enclaves have arc-like trace-element patterns, together with their enriched 87 Sr/ 86 Sr (i) (0.707608–0.707826), εNd (t) (from − 4.8 to − 4.6) and zircon εHf (t) (from − 4.0 to − 1.7) isotopes, revealing that they were derived from an enriched lithospheric mantle source. Taking the regional tectonic evolution into consideration, we propose that the Eocene Aima diorites and Huo'er pluton are the products of the last stage of magmatism related to break-off of the subducted Neo-Tethyan oceanic slab. Our study of these two plutons, along with results from previous studies, suggests that the crust of the southern Lhasa subterrane was thickened during the Eocene. The thickened crust was mainly caused by underplating of mafic magmas and, to a lesser extent, by thrust-related accretion of continental crust of the Indian terrane to the Lhasa terrane. We further propose that the Eocene thickened crust of southern Lhasa subterrane has triggered the surface uplift of this region during the Eocene, which may have non-negligible impact on the Cenozoic evolution of the Indian Monsoon system. • The Eocene Aima diorites and Huo'er pluton were in response to the slab break-off. • The crust of the southern Lhasa subterrane became thickened during the Eocene. • Magma underplating and accretion of Indian crust contributed to crustal thickening. • The Eocene surface uplift is synchronous with intensification of the Indian Monsoon.