The tectonic setting and timing of the Ayishan Group in the Lajishan area from the South Qilian tectonic belt are of great significance for understanding the evolution of this tectonic belt. In recent years, we have identified an assemblage of andesite, rhyolite, ignimbrite, sandstone and coarse sandstone in the Ayishan Group by geological mapping. These intermediate-acid volcanic rocks are interbedded in sedimentary rocks. The integrated study of field characteristic, petrography, zircon U-Pb geochronology and whole-rock geochemistry are conducted on the rhyolites from the Ayishan Group. LA-ICP-MS zircon U-Pb dating yields a Late Orodovician age of 447±4 Ma for the rhyolite. These rocks exhibit high SiO2 （69.6%～76.5%） content and aluminum saturation index （A/CNK = 0.85～1.10）, belonging to calc-alkaline to high K calc-alkaline series. They are characterized by enrichment of LREEs, LILE, such as Rb, Ba, and depletion of HREEs and HFSEs such as Nb, Ta, Ti. The extremely high whole-rock δ18O values of rhyolites indicate that their source rocks are dominated by sedimentary rocks. Combined with previous studies, it is inferred that the Ayishan rhyolites were formed by partial melting of the subducted continental sediments during Late Ordovician.
Sun Yi Niu Manlan Li Xiucai Wu Qi Cai Qianru Yuan Xiaoyu Wen Fengling Li Chen. Geochemical characteristics and geochronology of the rhyolites from Ayishan Group in the Lajishan area and its tectonic significance[J]. Chinese Journal of Geology, 2019, 54(3): 1016-1030.