Petrogenesis of Late Cretaceous gabbronorites in southwestern Lhasa Terrane, Tibetan Plateau, China: Sediment melt-mantle interaction and magmatic flare-up in response to Neo-Tethys slab roll-back

Abstract

The geodynamic processes responsible for the Late Cretaceous magmatic flare-up in the southern Lhasa Terrane, Tibetan Plateau, have been a matter of debate. In this study, we have reported on newly identified Late Cretaceous gabbronorites in the Dajiacuo Area of the southwestern Lhasa Terrane. These rocks show ophitic textures comprising plagioclase, orthopyroxene, and clinopyroxene, while hydrous minerals, such as amphibole and biotite, are not observed. They display geochemical affinities for high-Mg andesites (HMAs) given their SiO2 (53.5–59.7 wt. %), Mg# (56.8–59.9), and Cr (up to 178 ppm). Combined with their high Th/Yb and low Ba/La ratios and similar Sr–Nd–Hf isotopes to those of the Neo-Tethyan sediments, it is likely that the Dajiacuo gabbronorites were formed by the interaction between sediment-derived melts and the mantle. The Dajiacuo gabbronorites have similar Zr/Hf ratios to the Neo-Tethyan sediments and present no negative Zr–Hf anomalies. This indicates that the zircons in the subducted sediments may have been fully destroyed. This requires specific dynamic conditions to provide a hot subduction geothermal gradient. This is consistent with the high-temperature affinity of the parent magma inferred from Ti-in-zircon thermometry and the presence of orthopyroxene. Based on this, coupled with the synthesis of existing tectonomagmatic data for the southern Tibetan Plateau, we propose that the Late Cretaceous magmatic flare-up in the southern Lhasa Terrane was likely triggered by the roll-back of the northwards subducted Neo-Tethyan lithosphere.