U-Pb Isotope Geochronology of Syntectonic Granites from Hainan Island, South China: Constraints on Tectonic Evolution of the Eastern Paleo-Tethys Ocean

Since the recognition of the Indosinian orogeny in northern Vietnam, Triassic orogens have been widely identified around the western and southwestern boundaries of the South China block. The paleo-Tethys sutures stretch from west to southeast, from Jinshajiang-Ailaoshan via NE Vietnam to Hainan Island; these sutures exhibit and develop voluminous Permian-early Triassic magmatism and numerous high-strain shear zones. As ophiolites related to the paleo-Tethys are lacking on Hainan Island, the eastward extension of the Indosinian orogeny and subduction and closure time of the paleo-Tethys Ocean on Hainan Island remain controversial. Here, an integrated kinematic and geochronological study has been conducted on two shear zones, called the Xiaomei and Mangsan shear zones. U-Pb zircon dating yields an age of early Triassic (252–251 Myr) for Xiaomei syntectonic granites which formed in the same tectonic setting and presented the similar nanoparticles to the Indosinian granites from Ailaoshan ductile shear zone, including the strawberry-like and flower-like nanoparticles. The NE-trending Mangsan shear zone represented by the gneissic granites with middle Permian ages (264–262 Myr) formed in the same tectonic setting as the Wuzhi-shan granites that were proposed as I-type granites. These middle Permian gneissic granites with arc affinity may represent an arc setting related to the NW subduction of the paleo-Tethys. The analyses reveal that granites with late Triassic ages (235–232 Myr) in the Xiaomei shear zone have the characteristics of A-type granites. The late Triassic extensional events on Hainan Island may be related to the subduction of the paleo-Pacific Plate beneath the East Asian continent.