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

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 epsilon(Hf)(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 & DEG;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.