Characteristics and Tectonic Significance of the Volcanic Rock in Ordovician Koumenzi Formation, North Qilian Orogen

The Ordovician Koumenzi formation located in south Wusongtala of Qilian county exists as a tectonic slice confined by ductile and brittle faults, and enclosed in surrounding strata. The lower unit of this formation is composed of a set of intermediate-mafic volcanic layers bearing a clear magmatic eruption sequence, with lithologies of andesite, basalt and less dacitic, andesitic and basaltic pyroclastics. Field investigation and geochemical analyses indicate that the andesite is high aluminum and sodium alkaline volcanic rock; its trace element and the REE characteristics exhibits oceanic island-arc basalt features (VAB). Basalt combination in general belongs to poor aluminum and high sodium alkaline basalt series, which bears similarities to contaminated oceanic basalt(MORB) in terms of trace elements and the REE characteristics. The volcanic rocks of Koumenzi formation have two distinct lithogeochemical characteristics. Analysis on the tectonic setting show dual attributes of contaminated ocean basin and island arc. The occurrence of oceanic basalt and island arc volcanic rock implicate subduction event of oceanic crust of north Qilian in Middle-Late Ordovician. The volcanic activity record provides evidence for tectonic evolution of North Qilian in Early Paleozoic.