Permo–Triassic granitoids, Hainan Island, link to Paleotethyan not Paleopacific tectonics

Abstract

Abstract Hainan Island may have lain at the boundary between the Tethyan and Pacific domains since the late Paleozoic. Permo–Triassic granitoids outcropped on the island provide insight into Mesozoic tectonic evolution and potential dynamics. Based on analyses of zircon U-Pb geochronological and Hf-O isotopic data, along with whole-rock elemental and Sr-Nd isotopic data, the Permo–Triassic granitoids are divided into three groups: Late Permian (256–252 Ma) gneissic granitoids (Group 1), Early–Middle Triassic (247–244 Ma) massive granitoids (Group 2), and Middle-Late Triassic (242–225 Ma) massive granitoids (Group 3). Groups 1 and 2 exhibit similar elemental and Sr-Nd isotopic compositions with εNd(t) values ranging from –6.71 to –3.25. Group 1 is characterized by gneissic foliation and calc-alkaline I-type geochemical affinities with negative Nb-Ta and Ti anomalies; it has low εHf(t) (–4.2∼+0.2) and high δ18O values (9.35–10.46‰), possibly related to derivation of a metabasite source in a continental arc setting. The Group 2 samples are peraluminous massive granitoids with slightly high A/CNK ratios of 1.00–1.31, δ18O values (8.85–11.75‰), and high Sr/Y ratios but low εHf(t) (–8.7∼+1.6) values. They may have originated from the mixed source of greywacke and metabasite, indicating a compressive tectonic setting. The Group 3 high-K calc–alkaline granitoids have high zircon temperatures (842–867 °C) and show the geochemical signatures of A-type granites. They are featured by slightly low whole-rock εNd(t) and zircon εHf(t) values, suggestive of the derivation from a metabasite–greywacke source in an extensional setting. In combination with other geochronological and geochemical data from Hainan Island, it is proposed that the Permian–Triassic igneous magmatism in Hainan Island reflects a continuous amalgamation process from back-arc consumption (272–252 Ma) to orogenic assembly (251–243 Ma) and orogenic collapse (242–225 Ma) between the South China and Indochina Blocks. Such a temporal–spatial pattern for the Hainan granitoids is consistent with that along the Truong Son, Jinshajiang–Ailaoshan–Song Ma tectonic zone, suggesting dominant control of Paleotethyan domain instead of Paleopacific tectonics.