Zircon U-Pb Age and Geochemistry of Yamusi Granodiorite in the Eastern Part of the Qilian Orogen, China

Abstract

Yamusi granodiorite in the eastern part of the Qilian Orogen consists mainly of gneissic granodiorite. Researchers have studied other nearby rock masses, and many studies, such as those focusing on the zircon U-Pb age chronology and geochemistry of Yamusi granodiorite, still need to be completed. We obtained a new LA–ICP–MS zircon U-Pb age of 480.3 ± 1.3 Ma for Yamusi granodiorite, which suggested that it was formed during the early Ordovician period. The whole-rock geochemical data show that this granodiorite is relatively rich in Na and poor in K (K2O/Na2O = 0.40–0.73). The granodiorite is metaluminous–weakly peraluminous and can be classified as medium-K calc-alkaline granite. It yields high Sr/Y ratios (35.17–53.78) and low Yb (<18 ppm) and Y (1.8 ppm) contents, an Mg# value of <45, and high La/Y ratios (2.9–13.4, mean = 5.76). The trace element compositions of the granodiorite are characterized by positive large-ion lithophile elements (LILEs; e.g., Cs, Rb, and Ba) and negative high-field-strength element (HFSE; e.g., Nb, Ta, and Ti) anomalies, similar to arc magmatic rocks. There is clear fractionation between the light and heavy rare earth elements (REEs), with (La/Yb)N ratios of 1.77–9.03 (mean = 3.88). The petrogenesis research suggests that the granodiorite originated mainly from the partial melting of the mafic lower crust, with a minor mantle-derived component. Based on the regional geological setting, we suggest that the Yamusi granodiorite was formed during the northward subduction of the Proto-Tethyan oceanic crust to form an intracontinental arc.