Characteristics of Volcanic Rocks in Zongkequ Region, Zhiduo County, Qinghai Province

The volcanic rocks, located in Zongkequ region, Zhiduo County, Qinghai province, are mainly composed by andesite, dacite and rhyolite, with SiO₂, Al₂O₃ and Na₂O+K₂O contents of 33.48%~74.39%, 12.51%~20.10% and 2.21%~7.77%, respectively. Affected by the late magmatic activity, this set of volcanic rock shows the silicification characteristics, which caused the increasing of SiO₂ content and the decreasing of Al₂O₃ and Na₂O+K₂O contents. And these rocks belong to theorogenic Calc-alkaline series, and these basic volcanic rocks are tholeiitic basalts, with higher potassium content. The total amount of rare earth elements (∑REE) is 63.4%~332.9%, the total amount of light rare earth elements (∑LREE) is higher than heavy rare earth elements (∑HREE), and there is an obvious fraction between heavy and light rare-earth element. The Eu anomaly of intermediate-basic volcanic rocks is not significant, but the acidic volcanic rocks show obvious negative anomaly. These volcanic rocks are rich in Th, K, La and Zr elements, but loss in Nb, P, Sr and Ti elements. The discriminant diagrams of major elements, rare earth elements and trace element indicate that these rocks are typical Calc-alkaline rocks, which serve as the products of plate subduction-related magmatic arc belt. And thses research results can provide new evidences for defining the subduction polarity and tectonic environment of colorful ophioliticmelange belt. In this paper, the obtained Zircon U-Pbage of Andesite is (226.6±4.9)Ma, which belongs tothe Late Triassic.