Mesozoic continental crustal rejuvenation of South China: Insights from zircon Hf[sbnd]O isotopes of early Jurassic gabbros, syenites and A-type granites

Abstract

Whole rock geochemistry and Sr-Nd-Hf isotope, zircon UPb and HfO isotope data are reported for the early Jurassic gabbro-syenite-alkali-feldspar granite in the inland of South China, in order to investigate their sources, petrogenesis and tectonic implication, and to further trace the rejuvenation of continental crust of South China. The zircon UPb dating constrains the crystallization ages of 198–195 Ma for syenites, and of about 190 Ma for alkali-feldspar granites and gabbros, respectively. The early Jurassic gabbros have low SiO2 contents and high MgO, TiO2, Cr, V and Ni concentrations, with weak negative Nb and Ta anomalies, variable whole rock initial 87Sr/86Sr ratios (0.7064–0.7117), εNd(t) values (−1.5 to +0.6), εHf(t) values (−1.6 to +5.7) and zircon εHf(t) values (+3.4 to +6.4) and mantle-like δ18O values (+4.9 to +6.2‰). They are inferred to be derived from partial melting of a fertile lithospheric mantle that had been metasomatized by subduction-related fluids/melts, with subsequent limited crustal assimilation and fractional crystallization. The early Jurassic syenites are high-K to shoshonitic rocks with weakly negative Nb and Ta anomalies, strongly negative Sr, P and Ti anomalies, unradiogenic Sr and radiogenic Nd and Hf isotopic compositions (εNd(t) up to +5.3 and εHf(t) up to +11.5), indicating that they were generated by low-percentage partial melting of an amphibole-bearing lithospheric mantle, with subsequent crustal assimilation and extensive crystal fractionation and/or accumulation. The geochemical features of the early Jurassic K-feldspar granites suggest that they were mainly generated by mixing between mantle-derived mafic magma and crustal-derived felsic magma with extensive feldspar-dominant fractional crystallization. The early Jurassic magmatism was possibly triggered by asthenospheric upwelling following delamination of the flat-subducted Paleo-Pacific plate beneath South China, which provided heat to partial melting of above lithospheric mantle and continental crust. The underplating of depleted mantle-derived magmas changed the ancient crustal structures and modified the components of continental crust, indicating lithospheric rejuvenation during early Jurassic in South China.