Geochemical characteristics and zircon U-Pb ages of the granite in Xiji basin of Ningxia

In this paper, the authors studied geochemical characteristics and U-Pb age of the granite in Xiji basin of Ningxia in North Qilian Mountains and made a comparative research on the acid rocks of this area. The granite in the Xiji basin is a series of high potassium calc-alkaline rocks and shows sodium-rich and aluminium characteristics. LREE/HREE=10.89~11.93, LREE are relatively rich, LREE fractionation is obvious, and HREE earth distillation is not obvious with characteristics of the continental arc rock. The negative anomaly of Eu is not obvious, REE data for the granodiorite show strongly fractionated REE patterns (La_N/Yb_N=10.90~15.41). As for the composition of trace elements, the granite is rich in LILE Rb, Th, Pb and La, but poor in Sr, Ta, Nb, Ce and Ti, with weak enrichment of Nd, Sm and Dy. The curve of trace elements has the characteristics of orogenic granite with Nb negative anomaly, suggesting normal continental arc granite. The genetic type of granite in the Xiji basin is of I type, which formed in the continental arc environment and was the product of the collision of plates. Its magma was derived from the partial melting of the lower crust. U-Pb zircon dating by LA-ICP-MS shows that magma crystallization age is 434.3±9.0Ma, suggesting early adakite and a product of the magma intrusion of Caledonian period. The granite in the Xiji basin, the granodiorite in Nahuashan-Juewushan area of eastern North Qilian Mountain and the diorite of Laohushan area in Gansu Province are the same products as the northern Qilian magmatic arc belt and were formed by lava activity related to the plate subduction. It is thus proved that the Xiji basin belonged to the North Qilian orogenic belt indirectly.