Petrogenesis and geodynamic significance of late Early Jurassic complex in Qinsai, Hainan Island, South China Sea

Abstract

Whole-rock geochemical and U–Pb–Hf isotopic data were used to investigate the age, petrogenesis, and tectonic setting of a suite of gabbro–diorite–monzonite–syenite rock exposed in Qinsai area, Hainan Island. Zircon U–Pb dating shows that the gabbro and monzonite, respectively, crystallized at 179.5 ± 2.7 and 179.0 ± 2.3 Ma. The suite consists of shoshonitic alkaline intrusive rocks with SiO2 content ranging from 41.7 to 63.4 wt%, with high U, K, and Pb and low Nb, Ta, and Ti. The suite is enriched in light rare earth elements relative to heavy rare earth elements. The gabbro has no obvious Eu anomaly but a positive Eu anomaly is evident in intermediate rocks. Zircon εHf( t) values are positive and have a wide variation (gabbro εHf( t) = +5.2 ± 8.2, monzonite εHf( t) = +4.7 ± 7.1). Gabbro and syenite are both likely derived from an enriched lithospheric mantle that was metasomatized by subduction fluids. Syenite evolved from the differentiation of gabbro, monzonite, and diorite. Collectively, these intrusive rocks formed in an extensional regime within an arc formed by subduction of the ancient Pacific plate beneath the Asian margin, which was accompanied by extensional thinning of the lithosphere.