Tourmaline and Mica Chemistry of the Wangxianling Granitoids, South China: Implications for Petrogenesis of Highly Fractionated Granite

Both fractional crystallization and fluid-melt-crystal interaction are involved in the formation of highly fractionated granites. This paper assessed those two processes using geochemistry of muscovite and tourmaline and bulk-rock chemistry of multi-phase Wangxianling granitoids, South China. Compositional variations suggest the coarse-grained muscovite granite is produced from fractional crystallization of the two-mica granite whereas the fine-grained muscovite granite represents a distinct magma pulse. Progressive fractionation of quartz, feldspar and biotite leads to elevated boron and aluminum content in melt which promoted muscovite and tourmaline to crystallize, which promotes two-mica granite evolving towards tourmaline-bearing muscovite granite. Fluid-melt-crystal interaction occurred at the magmatic-hydrothermal transitional stage and resulted in the textural and chemical zonings of tourmaline and muscovite in finegrained muscovite granite. The rims of both tourmaline and muscovite are characterized by the enrichment of fluid mobile elements such as Li, Mn, Cs and Zn and heavier δ¹¹B values of the tourmaline rims (–15.0‰ to –13.6‰) compared to cores (–15.7‰ to –14.3‰). Meanwhile, significant M-type REE tetrad effects (TE_1,3 = 1.07–1.18) and low K/Rb ratios (48–52) also correspond to fluid-melt-crystal interaction. This study shows zoned muscovite and tourmaline can be excellent tracers of fractional crystallization and late-stage fluid-melt-crystal interaction in highly evolved magmatic systems.