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

摘要

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 epsilon(Nd)(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 epsilon(Hf)(t) (-4.2 similar to i-0.2) and high delta O-18 values (9.35-10.46 parts per thousand), 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, delta O-18 values (8.85-11.75 parts per thousand), and high Sr/Y ratios but low epsilon(Hf)(t) (-8.7 similar to+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 degrees C) and show the geochemical signatures of A-type granites. They are featured by slightly low whole-rock epsilon(Nd)(t) and zircon epsilon(Hf)(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.