Geochemistry and Sr-Nd isotope systematics of metabasites in the Tunchang area, Hainan Island, South China: implications for petrogenesis and tectonic setting

Abstract

The metabasites in Tunchang area, East-central Hainan Island, South China generally display strongly LREE-depleted patterns, extremely low HFSE concentrations and relatively high LILE contents; a few boninite-like samples have a concave-up, slightly LREE-enriched pattern. The low but variable Ti/V ratios between 6 and 20, and the high mg-numbers and Cr, Co, Ni contents, are indicative of plume-influenced intraoceanic arc magmas. It accord with this model are moderately high ɛNd(t) values ranging from +2.16 to +6.75, and 87Sr/86Sri values ranging from 0.70239 to 0.70824. These geochemical features imply that the protoliths for the Tunchang area metabasites are relics of an intra-oceanic island arc and the parental melts of these rocks were generated from a depleted, primitive mantle arc source rather than from a N-MORB source in a supra-subduction zone-like setting. The minor geochemical differences between the two identified groups of metabasites are due to a combined result of the variable degree of partial melting and the crystal fractionation of clinoproxene ± olivine ± plagioclase. The Tunchang area metabasites are distinctive in geochemical and isotopic compositions, and likely also in age from the Bangxi area metabasites in the northwestern part of Hainan Island. We assume that the NE–SW trending Baisha Fault Zone likely marks an early suture zone due to the collision of an Early Paleozoic intra-oceanic arc with the South China continent. In contrast, the Bangxi area metabasites more likely are formed in an extensional back-arc basin due to subduction of the Paleo-Tethys beneath the South China continental margin during the Late Paleozoic.