Geochronology, geochemistry, and petrogenesis of late Permian to early Triassic mafic rocks from Darongshan, South China: Implications for ultrahigh-temperature metamorphism and S-type granite generation

Abstract

The role of the mantle in generating ultrahigh-temperature metamorphism and peraluminous S-type granites, and the extent of crust−mantle interaction are topics fundamental to our understanding of the Earth's evolution. In this study we present geochronological, geochemical, and Sr–Nd–Hf isotopic data for dolerites and mafic volcanic rocks from the Darongshan granite complex belt in western Cathaysia, South China. LA–ICP–MS U–Pb zircon analyses yielded magma crystallization ages of ca. 250–248 Ma for the dolerites, which are coeval with eruption of the mafic volcanic rocks, ultrahigh-temperature metamorphism, and emplacement of S-type granites in the Darongshan granite complex belt. The mafic volcanic rocks are high-K calc-alkaline or shoshonitic, enriched in Th, U, and light rare earth elements, and depleted in Nb, Ta and Ti. The dolerites are characterized by high Fe2O3tot (11.61–20.39 wt%) and TiO2 (1.62–3.17 wt%), and low MgO (1.73–4.38 wt%), Cr (2.8–10.8 ppm) and Ni (2.5–11.4 ppm). Isotopically, the mafic volcanic rocks have negative whole-rock εNd(t) values (−6.7 to −9.0) and high ISr values (0.71232 to 0.71767), which are slightly depleted compared with the dolerite samples (εNd(t) = −10.3 to −10.4 and ISr = 0.71796 to 0.71923). Zircons in the dolerites have εHf(t) values of −7.6 to −10.9. The mafic volcanic rocks are interpreted to have resulted from the partial melting of an enriched lithospheric mantle source with minor crustal contamination during ascent, whereas the dolerites formed by late-stage crystallization of enriched lithospheric mantle-derived magmas after fractionation of olivine and pyroxene. The formation of these mantle-derived mafic rocks may be attributed to transtension along a NE-trending strike-slip fault zone that was related to oblique subduction of the Paleo-Pacific plate beneath South China. Such underplated mafic magmas would provide sufficient heat for the generation of ultrahigh-temperature metamorphism and S-type granites, and act as a mafic end-member for S-type granite genesis.