Late Cretaceous adakitic and A-type granitoids in Chanang, southern Tibet: Implications for Neo-Tethyan slab rollback

Abstract

The co-existence of Cretaceous adakitic and A-type granitoids in southern Tibet can offer fundamental constraints on subduction of the Neo-Tethyan oceanic plate and associated crustal growth processes, if their petrogenetic relationships are understood. Here, we report whole-rock geochemistry, zircon U-Pb geochronology, in situ zircon Hf isotopes and zircon trace elements from the Chanang intrusive suite, including the Zeyu and Cuojielin plutons. The Cuojielin pluton is composed of A-type quartz syenites with high K2O + Na2O, FeOT, and Zr contents and distinct negative Eu anomalies, consistent with fractional crystallization of mafic magma in a reducing, low-pressure, high-temperature environment. The adakitic Zeyu pluton, by contrast, has a strikingly high density of mafic enclaves. The elemental and isotopic signatures of the Zeyu mafic enclaves indicate a hybrid origin by mixing of magmas derived from the mantle wedge and from juvenile lower crust. The adakitic Zeyu host rocks, with low MgO, Cr, and Ni contents, show typical characteristics which originate from residual or fractionated amphibole of juvenile lower crust. The co-existence of aluminous A2-type granites and adakitic rocks indicates an anomalous high-temperature and relatively H2O-rich environment. The synchronous regional occurrence of calc-alkaline magmatism, charnockites, granulite-facies metamorphism, back-arc extension, and generation of the two studied types of Chanang intrusive rocks can best be explained in the context of an episode of rollback of the Neo-Tethyan slab in the late Cretaceous.