Geochemical characteristics of the Yinaigou Early Paleozoic Caotangou Group volcanic rocks in Tianshui of West Qinling Mountains and their geological significance

Abstract：There are volcanic rocks of Caotangou Group in Yinaigou, Tianshui. The main rock types of these metamorphic volcanic rocks are metabasalt, metaandesite, metadacite and metarhyolite. These metamorphic volcanic rocks of the study area basically belong to tholeiitic basalt, whose primary magma was derived from an enriched mantle source. The metamorphic volcanic rocks of Caotangou Group are characterized by high content of LILE (Cs, Ba, Th and U) and low content of HFSE (Nb, Ta, P, Ti and Y). The tectonic environment discrimination by trace elements reveals that metamorphic volcanic rocks were formed in an island arc setting above the subduction zone. Combined with the chronological data, the authors hold that the Tianshui-Wushan oceanic basin already existed in Early Paleozoic and began subducting then, leading to the formation of Caotangou Group island arc in Yinaigou, which was similar to things of Danfeng Group in west Qinling from the viewpoints of rock assemblage, forming time and geochemical characteristics. The results obtained by the authors provide evidence for the study of Tianshui-Wushan tectonic zone which served as the west extensional part of the Shangdan paleo-suture zone in Early Paleozoic.