Melt source and evolution of I-type granitoids in the SE Tibetan Plateau: Late Cretaceous magmatism and mineralization driven by collision-induced transtensional tectonics

Abstract

We report new whole-rock geochemical and Sr–Nd–Pb isotope and zircon U–Pb age and Hf isotope data of the Hongshan intrusive suite in the Triassic Yidun Terrane, eastern Tibet. These data allow us to explore the possible causative links between the magmatism and the coeval Cu–Mo mineralization in the region. The Hongshan intrusive rocks have SiO2 of 65.06–73.60wt.%, K2O of 3.17–6.41wt.%, and P2O5 of 0.11–0.39wt.%, enriched in Rb, Th, and U, and depleted in Ba, Sr, P, Ti, Nb, and Eu. These rocks are of high-K calc-alkaline to shoshonite series, showing geochemical signatures of metaluminous to slightly peraluminous I-type granite. Magmatic zircons separated from four samples yielded weighted mean 206Pb/238U ages of 79±0.7Ma, 78±0.5Ma, 77±0.8Ma, and 76±0.8Ma. Low MgO (0.42–1.47wt%), low HREE and Y, varying εHf(t) (−9.5 to −2.2), and negative εNd(t) (−7.7 to −5.8) suggest that magmas of the late Cretaceous Hongshan plutons were most likely generated by partial melting and mixing of ~20% juvenile lower crust-derived melts, represented by the ca. 215Ma basaltic andesite from the southern Yidun Terrane, with ancient basement-derived melts represented by the Baoshan S-type granitic melts from the Zhongza Block. We consider that partial melting processes are capable of removing chalcophile elements (such as Cu) and leaving siderophile metals (such as Mo) as residue in the lower crust of the Yidun Terrane, consequently inducing porphyry Cu–Mo mineralization. This consideration enables us to propose that the Triassic subduction-modified, copper-rich lithosphere was crucial for the giant copper mineralization that occurred in the Yidun Terrane during the late Cretaceous. Lithospheric-scale, transtensional faulting, developed as a result of collision-induced escape tectonics in SE Tibet, triggered asthenospheric upwelling, which in turn caused intra-plate extension and magmatism during the late Cretaceous, forming the Hongshan and coeval I-type granitoids within the Yidun Arc.