Middle-Late Triassic southward-younging granitoids: Tectonic transition from subduction to collision in the Eastern Tianshan-Beishan Orogen, NW China

To constrain the closure mechanism and time of the Paleo-Asian Ocean, we report new geochronological and geochemical data for Triassic granites along a NW-SE corri-dor from Eastern Tianshan to Beishan, NW China. Seven granites have U-Pb ages that young southwards from 245 Ma to 234 Ma in the Kanguer accretionary complex, to 237 Ma to 234 Ma in the eastern Central Tianshan block, to 229 Ma to 223 Ma in the Liuyuan accretionary complex. Gran-ites in the Kanguer accretionary complex formed by fractional crystallization and are peraluminous, high-K, calc-alkaline, and crust-derived. They have very low MgO (Mg# = 6-9), Cr, and Ni contents, and their high epsilon Nd(t) (+3.40) and epsilon Hf(t) (+4.49 to +11.91) isotopes indicate that the Dananhu arc crust was juvenile. The Huaniushan pluton in the Liuyuan accretionary complex displays the geochemical signatures of both A1- and A2 -type granites (Y/Nb = 0.32-3.39). All other granites in the Central Tianshan block and Liuyuan accretionary complex are alumi-nous A2-types with high K2O + Na2O, Al, rare earth elements (REE), Zr + Nb + Y, Ga, Fe/Mg, and Y/Nb and remarkable depletions of Eu, Ba, Nb, Ta, Sr, P, and Ti. They have a broad range of MgO (Mg# = 9-59), Cr, and Ni contents, Isr (0.70741-0.70945) values, negative epsilon Nd (t) (-2.98 to -1.14), and low to moderate epsilon Hf(t) (-1.22 to +7.78), which sug-gests a mixture of mantle and crustal com-ponents. These 245-223 Ma granitoids show marked Nb-Ta depletions that point to a subduction origin. Notable enrichments in Nd-Hf isotopes of Late Triassic granites are likely an indication of collision. Integration with previous data enables us to conclude that the delamination of an oceanic slab and mantle upwelling induced partial melting of thickened arc crust during a tectonic transi-tion from a multiple supra-subduction mar-gin to a collisional setting in the Late Triassic.