Triassic oceanic subduction in northwestern West Qinling Mountain: Evidence from Mg andesite and high-Mg andesite

Since the Triassic, tectono-magmatic activity in West Qinling Mountains has been very intense and the intermediate-acid intrusive rocks are distributed in a NW-trending arc. A lot of effective work has been done by previous researchers, but research on volcanic rocks with less exposure remain insufficient. Petrological, geochemical and chronologic analysis was used for andesite in Maixiu County and Tongren County of Qinghai Province and Ganjia and Dewulu areas in Xiahe County of Gansu Province. The results show that andesite belongs to calc-alkaline series and shows the characteristics of potassium-rich volcanic rocks. Maixiu andesite contains both high-magnesium andesite and magnesian andesite, while Ganga and Deuru andesite belongs to magnesian andesite, being two typical types of volcanic rocks formed by oceanic crust subduction. The formation of andesite in West Qinling area occurred early in the East and late in the west. From west to east, andesite in West Qinling Mountain presents the following characteristics:1) The values of SiO₂ and alkali decrease; 2) The formation of andesite took place earlier; 3) The values of K₂O, Na₂O, ∑REE increase; 4) The values of MF, FL, DI, La/Sm is larger; 5) The values of SI, Mg^# decrease, with significant linear features. The magmatic arc assemblage of Xiahe-cooperative magnesian andesite (mafic diorite) is composed of Tongren-Hezuo magnesian magmatic arc assemblage and Longwu gorge ophiolite belt in the north. Polarity indicates that the oceanic crust plate represented by Longwu gorge ophiolite subducted southward in the Triassic in the northwestern part of West Qinling Mountain.