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 Sigma REE concentration of 136-148 ppm. Hf isotope analysis showed that the epsilon Hf(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 Sigma REE concentration of 168-298 ppm. They showed more depleted Hf isotopic composition with epsilon Hf(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).