Petrogenesis and tectonic significance of late Mesozoic granitic and adakitic rocks from inland South China: constraints from geochemistry, zircon U–Pb geochronology and Hf isotopes

Abstract

Widespread late Mesozoic Yanshanian magmatic rocks exposed in the South China Block are of critical importance for unravelling the geodynamic setting of palaeo-Pacific subduction. However, available information on magmatic rocks from inland South China is inadequate to constrain the petrogenesis and tectonic setting of late Mesozoic magmatism. This study focuses on the late Jurassic granitic batholiths and stocks exposed in NW Guangdong, and further inland areas at the boundary region of Hunan (Xiang), Hubei (E) and Jiangxi (G) (abbreviated as XEG). Zircon U–Pb geochronology reveals that the studied granitic rocks were emplaced during latest Jurassic at 153–145 Ma. Geochemically, the Daping and Taibao plutons in NW Guangdong exhibit features of adakite-like melts, such as high Sr, SiO₂, Al₂O₃ contents, and low Yb content, without an Eu anomaly. This suggests that these rocks were most likely derived from partial melting of a high Sr/Y source at a normal lower continental crustal depth (30–40 km). Combined with the data from the coeval A-type granites in the late Mesozoic Qinhang metallogenic zone, we suggest that the Daping and Taibao adakitic granites were probably generated in low pressure environments related with the extension of the Qinhang zone in response to the late Jurassic roll-back of the palaeo-Pacific slab. The XEG granites, characterized by peraluminous geochemical signatures and enrichment of light rare earth elements and negative εHf(t) values, are classified as highly fractionated I- and S-types, I-type to S-type, formed at an intraplate setting. If correct, this interpretation indicates that the slab roll-back of the palaeo-Pacific plate probably has not reached the XEG region, updating current models for the subduction of the palaeo-Pacific plate.