Triassic to Cretaceous magmatic record in SE Vietnam and southern Borneo: the southern limit of the Paleo-Pacific subduction

The eastern margin of Asia formed an active Andean-type margin from the Triassic until the Cretaceous. SE Vietnam and West Borneo were located at the southern end of this subduction zone. Triassic igneous rocks in Indochina are usually interpreted to be related to the closure of the Paleo-Tethys. However, in West Borneo Triassic I-type granitoids and associated fore-arc volcaniclastic turbidites can be attributed to west-directed subduction of the Paleo-Pacific. The arc can be traced through the South China Sea to SE Vietnam and Hainan with magmatism continuing into the Jurassic, although with a diminished intensity. Thus, West Borneo can be correlated with the Indochina basement. With the beginning of the Cretaceous and docking of the Banda block (which consists of SW Borneo) with West Borneo, large Paleo-Pacific subduction-related magmatic provinces formed in southern Borneo and SE Vietnam. Lower Cretaceous I-type granitoids are thereby dominant in Borneo, while in SE Vietnam Upper Cretaceous I-type rocks are more abundant. At around 80 to 85 Ma magmatism changed from I-type to A-/S-type affinities, suggesting cessation of the Paleo-Pacific subduction and a switch to post-orogenic magmatism that lasted until the end of the Cretaceous. The Mesozoic igneous record of the West Borneo and SE Vietnam provinces is remarkably similar and indicates a common tectonic history associated with the various stages of Paleo-Pacific subduction beneath eastern Asia.