Zircon U–Pb geochronology, geochemistry and tectonic implications of Triassic A-type granites from southeastern Zhejiang, South China

Abstract

The late Permian–Triassic granites in southeastern China have important tectonic significance for the evolution of East Asia. A detailed study utilizing zircon U–Pb dating, major and trace element geochemistry, and zircon Hf isotope geochemistry for Dashuang and Jingju granites in Zhejiang Province, South China was performed. LA-ICP-MS zircon U–Pb analyses yielded ages of quartz-monzonite and monzogranite that are 234±3Ma and 231±3Ma, respectively, from the Dashuang pluton, while the LA-ICP-MS zircon U–Pb ages of monzogranite and K-feldspar granite from the Jingju pluton are 246±2Ma and 241±3Ma, respectively. These ages indicate that the magmatism event took place during the early Triassic to Mid-Triassic. The two granites have high contents of total alkalis (Na2O+K2O=7.73–10.24%), high-field-strength elements (e.g., Zr=215–471ppm, Y=25.8–36.5ppm, Nb=15–28ppm, and Zr+Nb+Ce+Y=293–849ppm) and rare earth elements (total REE=299–701ppm), as well as high Ga/Al ratios (10,000×Ga/Al=2.44–2.9). The zircon saturation temperatures were 800–837°C for the two granites, which suggests that they have the petrographic and geochemical characteristics of A-type granites. In-situ Hf isotopic analyses revealed that the two granites have εHf(t) values ranging from −20 to −6 and two-stage depleted mantle Hf model ages from 1.6Ga to 2.6Ga, which indicate that the two granite magmas were formed by the partial melting of Paleoproterozoic crust rocks in the Cathaysia Block. A series of A-type granites distributed in the coastal region probably defines an extensional environment in the early Triassic.