LA-ICP-MS zircon U-Pb dating of Early Paleozoic Huangmenchuan granodiorite in Tianshui area of Gansu Province and its tectonic significance

Located in the northern Qilian orogenic belt of Tianshui region within Gansu Province, the Huangmenchuan granodiorite intruded into basic volcanic rocks of the Upper Ordovician Chenjiahe Group. The petrologic and geochemical characteristics show that this granodiorite was formed by emplacement of crust-mantle mixed type of magma. The results of LA-ICP-MS zircon U-Pb data of the granodiorite from Huangmenchuan show that the Huangmenchuan granodiorite was emplaced at 440.5Ma±4.4Ma, belonging to Lower Silurian. The element geochemical characteristics indicate that Huangmenchuan granodiorite belongs to middle-K cal-alkaline series with features of I-type granite. The granodiorite samples are located in Andeans-type continental margin granite area in the environment discrimination diagram. Based on regional geology, the authors hold that there existed a limited ocean basin in Early Paleozoic, and large quantities of arc-type magmatic rocks were produced by the northward subduction of the ocean crust in eastern Qilian orogenic belt. The study of the continental dynamics of the tectonic framework and the evolution history in the area between Qilian and Qinling orogenic belts is of great significance.