Petrogenesis and metallogenic implications of the rapakivi granites: evidence from in-situ geochemical data of minerals

The complex mineral assemblages and zonal texture in granites might be the outcome of multiple batches of magma convergence and mixing. Here, we delineate the genesis of the  adakitic granite porphyry in the Gangdese belt, southern Tibet and analyze the chamber dynamic process of rapakivi feldspars based on the mineral assemblage and mineral compositions. The influences of magma mixing and post-magma alteration on the composition of granite porphyry and its metallogenic potential were examined by in-situ geochemical data of minerals and whole-rock geochemical data. The Quxu granite porphyry has high contents of SiO₂ (68.14–69.36 wt%) and K₂O (3.08–3.42 wt%), high ratios of Sr/Y (180.04–202.15) and (La/Yb)_N (33.89–43.66) and low contents of Y (4.21–4.68 ppm) and Yb_N (1.97–2.29), characterized by of high-K adatikic granite. The granite porphyry has positive zircon ε_Hf(t) values ranging from 6.27 to 11.55 and apatite ε_Nd(t) values ranging from -0.65 to 1.06, and low plagioclase (⁸⁷Sr/⁸⁶Sr)_i ratios from 0.704037 to 0.705084, as it originated from a juvenile crust source. Mafic microgranular enclaves (MMEs) in the granite porphyry are regarded as wall rock fragments due to their angular shape and relative older crystallization age of 48.6 Ma, while felsic microgranular enclaves (FMEs) are defined as the magma schlieren because its crystallization age and major- and trace-element contents are consistent with the host granite porphyry. The granite porphyry undergoes magma mixing with more mafic magma in the magma chamber, resulting in the magma being slightly enriched in TiO₂, MnO, MgO, and P₂O₅ and depleted in CaO content and Sr-Nd isotopes. The magma mixing also supplies heat for the formation of the rapakivi texture of K-feldspar megacrysts in the magma chamber. Additionally, the high oxygen fugacity (ΔFMQ = 1.46) and water content of granite porphyry and its adakitic characteristics indicate its favorable metallogenic potential. This study provides new insights into the petrogenesis and metallogenic potentials of adakitic rapakivi granites in the Gangdese belt.