Late Cretaceous‐Palaeocene arc magmatism in Bumeicun, Gangdese, southern Tibet: Products of slab rollback of Neo‐Tethyan Ocean?

Abstract

The most intense arc magmatism in the Gangdese arc occurred during the Palaeocene–Eocene, which is widespread across the entire southern Lhasa subterrane with Palaeocene–Eocene I‐type intrusive rocks and Linzizong volcanic successions. What drives the voluminous magmatism remains enigmatic. New geochronological and geochemical data on Late Cretaceous‐Palaeocene magmatic activity from Bumeicun, southern Tibet enables recognition of this distinct magmatic flare‐up event that we ascribe to Neo‐Tethyan Oceanic slab rollback. Bumeicun, located in the southern margin of Gangdese magmatic arc, exposes volcanic and plutonic rocks with close spatial–temporal links, and a wide range of silica content (53.0–74.7 wt.%). Zircon U–Pb dating result shows that the rocks form at 65.24 ± 0.77–67.65 ± 0.31 Ma. The Bumeicun igneous rocks are characterized by relative enrichment of large‐ion lithophile elements, depletion of high field‐strength elements (HFSEs) and enrichment of light rare earth elements, showing typical arc features. These rocks also show positive zircon εHf(t) value (+8.70 to+19.68), which are ascribed to the partial melting of arc mantle wedges. Followed by a succession of melt extraction and fractional crystallization, the coeval intermediate‐felsic volcanic and plutonic rocks in Bumeicun are formed. Previous researches note that Gangdese magmatic arc had a period of magmatic quiescence in the Late Cretaceous, and a subsequent burst of magmatism occurred in the P‐E. Our proposal of slab rollback model is consistent with the distinctly high zircon saturation temperatures (Tzr: 730–827°C) of Bumeicun magmatism. In addition, the slab rollback led to the Gangdese arc magmatism migration from north to south from the Cretaceous to the Eocene.