Generation of coeval metaluminous and muscovite-bearing peraluminous granitoids in the same composite pluton in West Qinling, NE Tibetan Plateau

Abstract

There is considerable controversy over the nature of (strongly) peraluminous granitoids and their relationship with coeval metaluminous granitoids. The origin of peraluminous granitoids is of great importance for an understanding of granite petrogenesis, as well as the formation and evolution of the continental crust. This study reveals the presence of contemporaneous metaluminous granitoids and muscovite-bearing peraluminous granitoids in the Guomaying composite pluton (GCP) at approximately 243–242 Ma in West Qinling in the NE Tibetan Plateau. The Middle Triassic granitoids in the GCP can be petrographically and geochemically divided into two groups. The group I samples are characterized by slightly higher K2O/Na2O ratios and are metaluminous with lower Zr-saturation temperatures (740–756 °C). In contrast, the group II samples have lower K2O/Na2O ratios and mostly strongly peraluminous compositions, with marginally higher Zr-saturation temperatures (762–778 °C). Both groups share similar trace elements and Sr–Nd–Pb–Hf isotopic compositions, pointing to middle to lower crustal origins with an amphibolitic residue. The metaluminous granitoids are interpreted as partial melting of amphibolites in the middle to lower crust beneath West Qinling, and the muscovite-bearing peraluminous granodiorites were also generated by partial melting of an amphibolitic source, with addition of a feldspar-rich component in the source. The hydrous basaltic melts derived from partial melting of a metasomatized mantle wedge source were also incorporated into the magma mush. It is inferred that disparate magma sources, entrained peritectic assemblages, and various degrees of antecryst recycling collectively exerted influence on the granitic magma chemistry of the GCP. Consequently, our data suggest that muscovite-bearing strongly peraluminous granitoids, which are widely regarded as the products of recycled metasedimentary rocks, can also originate from an amphibolite-dominated source with involvement of a feldspar-rich component.