Granulite facies xenoliths from the Yuhuashan complex, central Jiangxi, South China: constraints on Late Palaeozoic orogeny and middle-lower crust components

The Cretaceous Yuhuashan igneous complex contains abundant xenoliths of high-grade metamorphic rocks, with the assemblage garnet ± hypersthene + biotite + plagioclase + K-feldspar + quartz. The biotite in these samples has high TiO₂ (>3.5%), indicating high-T metamorphism (623-778 °C). P-T calculations for two felsic granulites indicate that the peak metamorphism took place at 880-887 °C and 0.64-0.70 GPa, in the low pressure/high temperature (LP-HT) granulite facies. Phase equilibrium modelling gives equilibrium conditions for the peak assemblage of a felsic granulite of >0.6 GPa and >840 °C, consistent with the P-T calculations, and identifies an anticlockwise P-T-t path. LA-ICPMS U-Pb dating of metamorphic and detrital zircon from one xenolith reveals that the granulite facies metamorphism took place at 273.6 ± 2.2 Ma, and the protolith was a sedimentary rock deposited later than 683 Ma. This represents the first Late Palaeozoic (Variscan) granulite facies event identified in the South China Block (SCB). Coupled with other geological observations, the LP-HT metamorphic conditions and anticlockwise P-T-t path suggest that Variscan metamorphism probably occurred in a post-orogenic or intraplate extensional tectonic setting associated with the input of external heat, related to the underplating of mantle-derived magma. Based on P-T estimates and the comparison of the protolith composition with mid- to low-grade metamorphic rocks in the area, it is suggested that the mid-lower crust under the Xiangshan-Yuhuashan area consists mainly of these felsic granulites and gneisses, whose protoliths were probably subducted to these depths during the Early Palaeozoic orogeny in the SCB, and underwent two episodes of metamorphism during Early Palaeozoic and Late Palaeozoic time.