Definition and geological significance of the Gacangjian volcanic arc north of Dongqiao, Tibet

Abstract：Many researchers have confirmed that there occurred southward subduction of the small Bangong Co-Nujiang ocean basin in the Late Jurassic. Recently a sequence of Late Jurassic island-arc volcanic rocks has been found at the northern side of the Dongqiao ophiolite zone in the central part of the Bangong Co-Nujiang belt. This sequence of volcanic rocks, named the Gacangjian volcanic rocks, is distinguished from the Dongqiao ophiolite zone by its weak deformation in its inner part and its geochemical characteristics indicates that it formed in an island arc environment. The evolution of the volcanic rocks was characterized by eruption of tholeiitic-calc-alkaline magma through predominance of calc-alkaline volcanism to eruption of arc shoshonitic magma accompanied by Nb-rich arc basalt at the late stage, suggesting a mature arc. All of these demonstrate that northward subduction had taken place within the Bangong Co-Nujiang ocean basin by the Late Jurassic. This discovery has great significance for the reconstruction of the complete tectonic evolution of the Bangong Co-Nujiang belt and establishment of the tectonic framework of the Qinghai-Tibet Plateau.