Rare earth element tetrad effect and negative Ce anomalies of the granite porphyries in southern Qiangtang Terrane, central Tibet: New insights into the genesis of highly evolved granites

Abstract

Highly evolved granites are geochemically similar to the mature continental crust, and are sometimes associated with W-Sn-Nb-Ta and rare-metal mineralizations. These rocks have important implications for tracing the compositional evolution of continental crust and magma metallogenesis. They were commonly considered to have been produced by partial melting of crustal materials or by high-degree fractional crystallization.In this study, we report on high SiO2 granite porphyries in the Xiabie Co area of southern Qiangtang Terrane, central Tibet, which provide new insights into the petrogenesis of the highly evolved granites. Zircon dating results indicate that they were generated in the late Early Cretaceous (107 ± 1 Ma). They consist mainly of orthoclase, albite, and quartz, with accessory minerals including zircon, fluorite, and Fe-Ti oxides. Geochemically, they have high SiO2 but low MgO, Fe2O3T, CaO, and P2O5, and are enriched in Rb but depleted in Ba, Sr, Eu, and Ti, similar to highly evolved granitic rocks. They are characterized by distinct rare earth element (REE) tetrad effects and negative Ce anomalies. They also have slightly higher εNd (t) values (−0.99 to −0.36) than those of the adjacent early granites (−2.05 to −3.01), and relatively high and variable zircon εHf(t) values ranging from −0.3 to +5.4. Compared to typical highly evolved granites and petrological experiment data, we suggest that the Xiabie Co granite porphyries represent highly evolved melts produced by fractional crystallization. The interaction between fractionated melts and F-rich aqueous fluids played an important role in the genesis of their REE tetrads effect. Involvement of meta-sedimentary rocks that went through surficial chemical weathering into their source region is the predominant reason for the generation of negative Ce anomalies. This indicates that, in addition to fractional crystallization, the melt-fluid interactions and partial melting of source materials also play important roles in the generation of highly evolved granites.