Magmatic response to slab breakoff of the Neo-Tethyan ocean: Constraints from Eocene diorites and gabbros in the southern Lhasa terrane, Tibet

The India-Asia collision holds significant implications for understanding continental collision dynamics. It is essentially controlled by various dynamic processes, including oceanic slab subduction, slab rollback, and slab breakoff, all of which are accompanied by extensive magmatic activities in convergent margins. However, the magmatic consequences of the Neo-Tethyan slab breakoff during the India-Asian collision remain ambiguous. In the middle segment of the southern Lhasa terrane, two episodes of magmatism (-55 Ma and - 50 Ma) have been preserved. In this study, we present whole-rock major and trace elements, zircon U-Pb ages, and whole-rock SrNd-Hf isotopes of the - 55 Ma Gaga pluton and conduct a comprehensive comparison with the two episodes of magmatic mixing events. The Gaga host diorites were formed by the partial melting of the Neo-Tethyan oceanic slab and underwent amphibole fractional crystallization. The Gaga gabbroic enclaves, which are associated with magma mixing, originated from lithosphere mantle metasomatized by subduction-related fluids. Notably, the Gaga pluton exhibits more prominent arc-type characteristics of Nb-Ta and Zr-Hf negative anomalies than the 50 Ma intrusive rocks in adjacent areas. The genetic transition of Eocene magmatic rocks indicates that the - 55 Ma Gaga pluton formed during the initial stage of slab breakoff, and the - 50 Ma magmatic pluton was produced during the late stage of slab breakoff.