Geochemical Signals of Coexisting Magma Mixing and Fractional Crystallization Processes in the Arc Setting: Case Study of Wulan Intrusive Suite in the NE Tibet Plateau

To understand the fundamental processes for the generation of granitic plutons and compositional diversity of continental crust, we investigate the Wulan intrusive suite in the North Wulan terrane (NWT) along the NE Tibet Plateau. The intrusive suite consists of three-stage igneous rocks including stage I gabbro and granite emplaced at 258-254 Ma, stage II gabbro, diorite, and tonalite emplaced at 250-245 Ma, and stage III gabbro, granodiorite, and leucogranite emplaced at 242-241 Ma. Geochemical analyses and thermodynamic modeling suggest that gabbros were derived from partial melting of heterogeneous lithospheric mantle domains, and they were involved in the formation of coeval intermediate to granitic pluton/batholith. MMEs with variable compositions hosted in granitic pluton do not always represent the products that were generated by magma mixing between injected mafic melts and host granite. In contrast, some low-Si varieties may be generated by crystal fractionation of injected mafic magmas. The hybrid compositions of diorite also indicate a complex petrogenetic process characterized by fractionation of basaltic magmas followed by replenishment of mantle-derived mafic melts. These lines of evidence suggest that both fractional crystallization and magma mixing may concurrently occur in the construction of hybrid intermediate to granitic rocks. In addition, the injected mafic melts, together with newly formed MMEs, would induce compaction and convection of granitic crystal mush, thereby promoting melt extraction to form highly evolved leucogranites. These complex petrogenetic processes and their different combinations play an important role in the construction of the hybrid continental crust in the arc setting.