Zircon U-Pb ages and geochemical features of the diorite porphyrite in Gajia Formation in the Nagqu area, Tibet.

A suite of diorite porphyrites exists in Middle-Late Triassic Gajia Formation of the Nagqu area, Tibet. The authors selected samples for zircon U-Pb dating by using LA-ICP-MS system, and obtained an age of 115.4±0.6Ma, belonging to Early Cretaceous and being consistent with ages of Duoni Formation and Zelong Group. The SiO2 average concentration is 57.96%, with the enrichment of Rb and Th and loss of Nb, P and Ti elements, exhibiting the characteristics of island arc granitoid. The diorite porphyrite and upper crust have similar spidergrams. The εHf(t) values are in the range of 2.0~4.1, with the mean value of 3.0, suggesting that magma came from new crust. Based on the comprehensive analysis of geochemistry and tectonic environment, the authors hold that the diorite porphyrite formed in a collision environment of the Lhasa block and Qiangtang terrane. At 115Ma, the subduction of the oceanic crust was over, and the evolution entered into the continental collision stage. Due to the collision between the Qiangtang terrane and the Lhasa terrane, the crust thickened, a large quantity of magma concentrated in shallow magmatic chambers and continuously heated the thickened crust heating, resulting in the partial melting of the overlying crust.