Origin of Granites from A'quedun Area in Qimantage Mountains: Constraints from Zircon U-Pb Dating, Geochemistry and Hf Isotope

The geological setting of granite formation in A'quedun area of Qimantage Mountains has important constraits in the collision-extension process in the Caledonian of East Kunlun. The A'quedun intrusive rocks mainly consist of granodiorite and tonalite. Zircon LA-ICP-MS U-Pb dating results show that granodiorite and tonalite were formed at 385±5 Ma and 393±5 Ma respectively, which should be the product of magmatism in the Middle Devonian. The granodiorites have low SiO₂ (63.56%-66.57%), Mg^# (0.31-0.35), K₂O/Na₂O ratios (0.49-0.66) and A/CNK (0.95-1.02), characterized by moderate LREE/HREE fractionation, strong LILE enrichment and depleted Nb-Ta, P, Ti and Eu depletion (δEu=0.38-0.98). Relative to the granodiorites, the tonalities have high contents of SiO₂ (67.93%-70.76%) and low Mg^# (0.22-0.33), stronger LREE/HREE fractionation, Nb-Ta, Ti and weakly Eu depletion (δEu=0.80-1.06). The tonalites have positive ε_Hf(t) values (+1.91 to +15.63), and broadly two stages Hf model ages (T_DM2(Hf)=0.53-1.28 Ga). Based on the regional geological data, the East Kunlun Qimantage area was in post-collision extension stage during the Middle Devonian. The A'quedun granitic rocks derived from mantle magma assimilation contamination ancient crust in a mechanism for the exhumation of the subducted slab. The granodiorite and tonalite melt pressure changes with time, showing that the crustal thickness is still in continuous thinning process during the Middle Devonian.