LA-ICP-MS Zircon U-Pb Dating and Geochemical Characteristics of Granodiorite in the Xiangyang Coal Mine,Gansu Province

After carrying out the LA-ICP-MS zircon U-Pb dating with high precision, the results show that the granodiorite in the Xiangyang coal mine was formed at 445~438Ma, which belongs to the Late Ordovician epoch.Geochemical analysis shows that these rocks belong to high potassium calcium alkaline series, and the major elements have the geochemical characteristics of high silicon, rich alkali and calcium.The light rare earth elements are enriched while the heavy rare earth elements are relatively depleted, with weak negative Eu anomalies, displaying the fractional crystallization of plagioclase in the magma evolution.The largeion lithophile elements (such as Rb, K, La and Th) are relatively enriched, while the high field strength elements (including U, Nb, Ta, Ti, P and Sr) are relatively depleted.The discriminant result of source region suggests that these rocks belong to S-type granite.The ratios of trace elements, such as Nb/U (6.19~9.06), Nb/Ta (9.8~11.1), Zr/Hf (31.5~34) and La/Nb (2.29~4.49), are very close to the average values of basic upper crust, further indicating that these granodiorite were mainly formed by the partial melting of the crust.The Southern Qilian area has experienced very strong magmatic activities in Caledonian period, thus it's believed that the Xiangyang coal mine was formed in the volcano arc environment through analyzing its tectonic environment.