Geochronology, Geochemistry and Tectonic Significance of Buziwannan Late Triassic I-Type Granites in the West Kunlun Orogen Belt, Northwest China

Abstract

Аbstract The formation of the West Kunlun Orogenic Belt, which is located at the northwest margin of the Greater Tibetan Plateau, is associated with the evolution of Proto- and Paleo-Tethys Oceans from the early-Paleozoic to the Triassic and characterized by intensive magmatism and extensive metallogenesis. This paper presents a detailed study of the LA-ICP-MS zircon U-Pb chronology, zircon Hf isotope geochemistry, and major and trace element composition of the Buziwannan granodiorite and monzogranite in the South Kunlun Terrane. The LA-ICP-MS zircon U-Pb dating shows that the two granites were emplaced in the late Triassic (231.8–226.8 Ma), and are high-K calc-alkaline I-type granodiorite and monzogranite in composition. Zircons from the Buziwannan granites have εHf(T) values ranging from –3.83 to 2.96, with corresponding two-stage Hf model ages (T(DM2)) of 1075–1505 Ma, suggesting the importance of juvenile material in the magma source. The Buziwannan granites show enrichments in LREEs, Rb, Th, U and Hf, and depletions in Sr, Nb, Ta and Eu. The contradiction between continental signature and mantle-like isotopes indicates the granites were generated by the partial melting of residual subducting slab with input of sediments. The ratios of Rb/Sr and (La/Yb) N increase from the granodiorite to monzogranite, while the Sr, Dy and Er values decrease. These data are attributed to fractional crystallization processes. Fractionation of plagioclase, hornblende and biotite produced the variations in major elements and Rb, Sr and Ba, and the distribution of REEs are mainly controlled by fractionation of hornblende and subordinately of zircon. Together with the regional geology, the data infer that the Buziwannan granites and corresponding mineralization formed in a transitional setting from syn-collision to post-collision during the Late Triassic.