Permian-Triassic Magmatism Along the Truong Son Zone in SE Asia and its Paleotethyan Tectonic Implications

The Truong Son zone is the largest tectonic-magmatic zone along the northern margin of the Indochina Block. The abundant Late Paleozoic-Early Mesozoic igneous rocks in the zone were suggested to be formed during the tectonic evolution of the Song Ma Paleotethyan branch/back-arc basin and subsequent collision between the Indochina and South China blocks. This paper reports our comprehensive study of the collected Permian-Triassic felsic igneous rocks along the zone and discusses their petrogenesis and the tectonic evolution of the Song Ma branch/back-arc basin. Our results show that the Early Permian granitoids originated from the newly underplated mafic rocks, while the late Early Permian-Late Permian granitoids and rhyolites originated from a mixed source of the Paleo to Meso-Proterozoic metaigneous and metagreywacke components. Formation of these Permian felsic igneous rocks were related to the subduction of the Song Ma branch/back-arc basin beneath the Indochina Block. The Triassic granitoids with negative εHf(t) and εNd(t) values were derived from a mixed source of the Paleo to Meso-Proterozoic metaigneous and metasedimentary rocks. In contrast, the Late Triassic granitoids with positive εHf(t) values were originated from a juvenile mafic crust with a component of metagreywacke source. These data suggested that the Song Ma Paleotethyan branch/back-arc basin finally closed at ~250 Ma, which followed by the collision between the South China and the Indochina blocks. During the Middle to Late Triassic, the Truong Son zone transformed to the post-collisional stage. © 2022 Science Press. All rights reserved.