Two episodes of Mesozoic mafic magmatism in the Nansha Block: Tectonic transition from continental arc to back-arc basin

Abstract

It has been proposed that the proto-South China Sea (PSCS) existed in the southeastern continental margin of the South China Block (SCB) before the opening of the present South China Sea (SCS). However, the age of initial spreading and the geodynamics of the opening of the PSCS remain enigmatic. Here we present geochronological and geochemical data for diorite and diabase samples from Well NK-1, which was drilled into the Nansha Block in the Dangerous Grounds area of the southern SCS, to investigate their petrogenesis and their association with the formation of the PSCS. The Early Jurassic diorites (ca. 177 Ma) are geochemically similar to continental arc basalt, with enrichment in Rb, Ba, Pb, and Sr, and pronounced negative Nb, Ta, and Ti anomalies. They were originated from a mantle source metasomatized by subducted-sediment-derived melts in a continental arc setting related to subduction of the Paleo-Pacific plate beneath the SCB. The Early Cretaceous diabases (ca. 124 Ma) are compositionally similar to back-arc basin basalt, with enrichment in Rb, Ba, and Pb, and negligible Nb, Ta, Ti, Zr, and Hf anomalies. The mantle source of the diabases was metasomatized by aqueous fluids derived from the subducted slab in a back-arc basin associated with subduction of the Paleo-Pacific plate. Furthermore, the diabase samples are geochemically similar to mafic rocks and ophiolite complexes previously regarded as remnants of the PSCS oceanic crust. Therefore, the Early Cretaceous diabases in the Nansha Block are interpreted as representing coeval magmatism of the nascent ocean of the PSCS, indicating that the PSCS during the Early Cretaceous was a spreading back-arc basin associated with retreat of the subducting Paleo-Pacific plate.