Tectonic Evolution of the Hainan Island, South China: Geochronological and Geochemical Constraints from Late Permian to Early Triassic Basalts

The tectonic evolution of Hainan Island during the Late Permian–Early Triassic period is still unclear. This study identified two types of basalts on the island and presented detailed geochronology, whole-rock geochemistry, and Hf isotope data of the Late Permian–Early Triassic basalts. U-Pb dating results indicated that baddeleyites and zircons of one sample from Group 1 basalts had formation ages of 256 ± 3 Ma and 255 ± 3 Ma, respectively, and two samples from Group 2 gave formation ages of 241 ± 2 Ma and 240 ± 3 Ma, respectively. Both groups are characterized by negative anomalies of Nb, Ta, and Ti, and enrichment in Ba, Th, U, and K. Group 1 belongs to sub-alkaline basalt and exhibited SiO2 contents ranging from 50.50% to 51.05%, with ΣREE concentration of 136–148 ppm. Hf isotope analysis showed that the εHf(t) values of baddeleyites and zircons were −10.56 to −4.70 and −14.94 to −6.95, respectively. Group 2 belongs to alkaline basalt and had a higher SiO2 content of 52.48%–55.49% and ΣREE concentration of 168–298 ppm. They showed more depleted Hf isotopic composition with εHf(t) values ranging from −2.82 to +4.74. These data indicate that the source area of Group 1 was an enriched mantle, likely derived from partial melting of spinel lherzolite mantle, and was modified by subduction-derived fluids. Group 2 was derived from depleted mantle, most likely originating from partial melting of garnet + spinel lherzolite mantle. They were contaminated by crustal materials and metasomatized by subduction-derived fluids with a certain degree of fractional crystallization. Comprehensive analysis suggests that Group 1 samples likely formed in an island arc tectonic setting, while Group 2 formed in a continental intraplate extensional (or initial rift) tectonic setting. Their formation was mainly controlled by the Paleo-Tethys tectonic domain. Group 1 basalts implied that subduction of the Paleo-Tethys oceanic crust lasted at least in the late Permian (ca. 255 Ma). Group 2 basalts revealed that the intra-plate extensional (or initial rift) stage occurred in the middle Triassic (ca. 240 Ma).