Generation Of Coeval Metaluminous And Muscovite-bearing Peraluminous Granitoids In The Same Composite Pluton In West Qinling

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 understanding of granite petrogenesis,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 242–243 Ma in West Qinling,NE Tibetan Plateau.The Middle Triassic granitoids in the GCP can be divided petrographically and geochemically into two groups.Group ? samples are characterized by slightly higher K₂O/Na₂O ratios and metaluminous with lower Zr-saturation temperatures?740–756 ??.The synmagmatic zircons from this group exhibit relatively low contents of Hf?9312–13407 ppm?with lower U/Yb ratios?1.36–3.68?.By contrast,group II samples have,to some extent,lower K₂O/Na₂O ratios and mostly strongly peraluminous compositions with marginally higher Zr-saturation temperatures?762–778 ??.The synmagmatic zircons in this group show relatively high concentrations of Hf?99235–153456 ppm?along with higher U/Yb ratios?1.19–4.78?.These differences indicate that the two groups of zircons were crystallized from different batches of magmas.Both groups share similar trace element and Sr–Nd–Pb–Hf isotopic compositions,pointing to a middle-to lower-crustal origin with an amphibolitic residue.The metaluminous granitoids are interpreted as partial melting of amphibolites in the middle-to-lower crust beneath West Qinling,whereas the muscovite-bearing peraluminous granodiorites are generated by partial melting of a mixed source comprising amphibolites and metagranitoids?metatonalites or metagreywackes?.The hydrous basaltic melts derived from partial melting of a metasomatized mantle wedge source was also incorporated into the magma mush.It is inferred that disparate magma sources,entrained peritectic assemblages,and various degrees of antecryst recycling collectively exert a main control on the granitic magma chemistry of the GCP.Consequently,our data suggest that muscovite-bearing strongly peraluminous granitoids,which were widely regarded as the products of recycled metasedimentary rocks,can also originate from an amphibolite-dominated source,with some involvement of metagranitoids,metatonalites,or metagreywackes.