High-magnesium igneous associations give new insights into geodynamic evolution of the accretionary Central Asian Orogenic Belt

Much controversy exists regarding the evolution and termination of accretionary orogens where typical continent-continent collision features are lacking. The Central Asian Orogenic Belt (CAOB) thus provides a golden opportunity. Precise zircon dating identified two-stage Triassic magmatic events in the southeast CAOB. The Olenekian andesites show typical sanukitoids affinities and yield depleted Hf isotopic compositions and high δ18O values, indicating that they were originated by partial melting of mantle peridotites with input of 20% terrigenous sediments. The Norian diorites have geochemical characteristics comparable with high-Mg adakites. Their depleted isotopic compositions and high Th/Nb and Ba/Nb ratios conform to an origin that melting of delaminated mafic lower crust subsequently interacted with mantle materials. These two-stage abnormal Triassic high-Mg rocks archive hot slab window triggered by break-off  the Paleo-Asian oceanic slab and lower-crustal delamination related to collapse of the southeast CAOB. This study further benefits the reconstruction of East-Asian blocks in Pangea.