Petrogenesis of Late Early Cretaceous high-silica granites from theBangong–Nujiang suture zone, Central Tibet

Abstract

High-silica granites are common in collisional orogenic belts. However, the petrogenesis of such granites and the geodynamic processes associated with their emplacement are debated. In this study, we present zircon U–Pb ages and Hf isotopes, and whole-rock geochemical and Sr–Nd isotopic compositions for biotite granites from the Jiang Co area, in the Bangong–Nujiang suture zone, central Tibet. Zircon U–Pb dating reveals that the Jiang Co granites were emplaced during the late Early Cretaceous (ca.114 Ma). These granites are characterized by high and variable SiO2 (69.7–78.2 wt%) and K2O (4.7–8.4 wt%), and low MgO (0.09–1.25 wt%) and P2O5 (< 0.1 wt%) contents. The samples are enriched in Rb, Th, U, and Pb, and depleted in Ba, Nb, Ta, Sr, Ti, and Eu. These geochemical features suggest that the Jiang Co granites underwent extensive fractional crystallization. The high and variable initial 87Sr/86Sr (0.7066 to 0.7095) ratios and consistent εNd(t) (–8.2 to –9.2) values, along with TNd2DM ages of 1.66 to 1.58 Ga, and negative to positive zircon εHf(t) (−7.5 to +1.8) values, imply that these granites were formed by partial melting of the mature ancient crustal materials with minor involvement of mantle materials. Along with contemporaneous magmatic activity, we suggest that the Jiang Co granites were mainly generated by crustal melting in a post-collisional setting resulting from slab breakoff after the Lhasa–Qiangtang collision, and subsequently underwent a high degree fractional crystallization.