Slow crystal settling controls the diversity of high-silica granites of the Late Cretaceous Shengsi Pluton at northeastern tip of southeast China

Abstract

The intrinsic mechanism on geochemical diversity of high-silica (>70 wt% SiO2) granites (HSGs) still remains unclear. In this paper, we focus on the magmatic differentiation of the Shengsi Pluton at the northeastern tip of SE China by an integrated study of geochronology, whole-rock geochemistry and mineral chemistry of zircon and apatite. The Shengsi granites crystallized at ca. 97 Ma, and can be divided into early medium-grained phase (MGP) and late fine-grained phase (FGP). The FGP granites have higher SiO2 (75.0–77.1 wt%) contents and lower amount of plagioclase, apatite and titanite. The MGP granites likely represent the primary granitic melts and they show transitional geochemical features between I- and A-type granites. In contrast, the FGP granites may represent the melts differentiated from the MGP magma mush and they are characterized by continuously varied to strongly depleted middle rare earth elements (MREEs). Geochemical modeling demonstrates that the varied REEs of the FGP granites are likely controlled by the crystallization/accumulation of titanite, plagioclase and minor zircon during magmatic differentiation. Such a crystal fractionation process in HSGs could be explained by slow “crystal settling” (mainly titanite) in crystal mush induced by dripping solid–liquid mixtures from the magma roof. This work exemplifies the effective crystal fractionation of >70 wt% SiO2 melts to generate >75 wt% SiO2 melts in the present coastal area of SE China, and it also highlights the role of slow settling of early crystallized minerals in crystal mush on geochemical diversity of HSGs.