Identification of a new source for the Triassic Langjiexue Group: Evidence from a gabbro-diorite complex in the Gangdese magmatic belt and zircon microstructures from sandstones in the Tethyan Himalaya, southern Tibet

Considerable debate persists as to the Triassic paleogeographic framework of the Neotethys and the origin of the Late Triassic Langjiexue Group in the Tethyan Himalaya. Triassic magmatic rocks in the Gangdese belt and Late Triassic Langjiexue sediments play a pivotal role in addressing these issues. Geochronological, petrological, and geochemical analyses have been performed on the Middle Triassic gabbro-diorite complex (with crystallization ages of ca. 244-238 Ma) from the Gangdese belt. These plutonic rocks are characterized by relatively low MgO and high Al2O3 contents, calc-alkaline trends, and depletion of Nb, Ta, and Ti, resembling low-MgO high-alumina basalts or basaltic andesites. These plutonic rocks exhibit depleted whole-rock epsilon(Nd(t)) values of similar to+5 and zircon epsilon(Hf(t)) values peaking at similar to+14. These features resemble those of rocks in a subduction-related arc setting. We also completed detrital zircon U-Pb dating and microstructure analysis for the sandstones of the Langjiexue Group in the Tethyan Himalaya. Zircon grains with ages >300 Ma are dominated by preweathered and weathered surfaces as well as fairly rounded to completely rounded scales, indicating a high degree of polycyclicity. In contrast, 300-200 Ma ones are characterized by fresh surfaces and completely unrounded to poorly rounded scales, indicating nearby sources. Collectively, our data, combined with published results, support that the subduction initiation of the Neotethys began no later than the Middle Triassic. Arc-affinity magmatic rocks supplied some materials to the Langjiexue Group. This scenario sheds new light on the provenance of the Langjiexue Group and the Triassic paleogeography of the Neotethyan realm.