Petrogenisis of Lumanshan granites in Hualong area of southern Qilian Mountain: Constraints from geochemistry, zircon U-Pb geochronology and Hf isotope

Abstract: The pluton in Lumanshan of Hualong area, located in southern Qilian Mountain, is mainly composed of biotitic monzogranites. Zircon LA-ICP-MS dating yielded a concordant age of 452.9±1.8 Ma, indicating that the pluton was formed in Caledonian. Geochemical data show that the Lumanshan pluton is silica enriched in alkali. It is a weakly peraluminous granite and belongs to the high-K calc-alkaline series. In trace and REE aspects, the granites are enriched in Rb, Th, U, K, Pb and LREE and depleted in Ba, Sr, Ti, P with strong negative anomaly of Eu (δEu=0.06~0.55 with a mean value of 0.31). Integrated geological and geochemical data suggest that the Lumanshan pluton should be genetically ascribed to highly fractionated I-type granites. The Hf isotope shows that the εHf(t) values of the pluton range from -7.4 to 0.1, with tDM2 ages between 1425Ma and 1930Ma, indicating that depleted mantle materials had been involved in magma formation and the source region of granite probably included the Hualong Group. In combination with comprehensive analysis of the tectonic evolution, the authors hold that the Lumanshan pluton was formed in a subduction-collision environment between the Qaidam and South Qilian terrane and most likely generated by mixing of a depleted mantle-derived magma and an induced crustal-melted felsic magma in the deep crust; later, it suffered further differentiation during magma ascent.