Petrogenesis of biotite granite with transitional I-A-type affinities: Implications for continental crust generation

Abstract

The I-S-A-type classification scheme for granites has been widely used to identify potential sources and the tectonic setting of granitic magmatism. Recent geochemical studies have shown that most granites are derived from a mixture of mantle and crustal sources, and that granitic magmatism contribute to generation and differentiation of the continental crust. However, the origin of granites with transitional affinities remains unclear, along with their role in the growth and differentiation of the continental crust. In this contribution, the ca. 91 Ma Bozhushan biotite granites in the southwestern Cathaysia Block, China, are characterized by high abundance of high-field-strength elements (HFSEs), with Zr + Nb + Ce + Y > 350 ppm and 10000*Ga/Al > 2.6, which are features typical of A-type granites. However, biotites in the Bozhushan granites are compositionally distinct from those of typical A- and S-type granites. Decreasing P2O5 contents with increasing SiO2 contents are consistent with an I-type rather than a S-type granite. Petrographic observations and geochemical data suggest that the Bozhushan granites have transitional I-A-type affinities. The Bozhushan granites contain remarkably evolved isotopic compositions, with εNd(t) = −9.2 to −8.0 and εHf(T) = −9.8 to −7.3, which are slightly lower than values for Late Cretaceous Baiyunshan alkaline igneous rocks. The Bozhushan granites have the low Y/Nb and Yb/Ta ratios, which are similar to the Baiyunshan alkaline igneous rocks, and continental alkaline basalts from the East African Rift and eastern China, ocean island basalts, and A1-type granitoids. Geochemical data indicate that the Bozhushan granites were likely generated by high-temperature melting of a hybrid source containing underplated alkaline igneous rocks and Precambrian basement rocks. Although the Bozhushan granites have crustal isotopic compositions, our data indicate that these granites also represent the formation of new crust. Thus, granites with transitional I-A-type affinities, such as the Bozhushan granite, have a role in the generation of continental crust.