Neoproterozoic granitic activities in the Xingdi plutons at the Kuluketage block, NW China: Evidence from zircon U-Pb dating, geochemical and Sr-Nd-Hf isotopic analyses

摘要

Neoproterozoic igneous rocks are widely distributed in the Kuluketage block along the northern margin of the Tarim Craton. However, the published literature mainly focuses on the ca. 800 Ma adakitic granitoids in the area, with the granites that intrude the 735-760 Ma mafic-ultramafic rocks poorly studied. Here we report the ages, petrography and geochemistry of two granites in the Xingdi mafic-ultramafic rocks, in order to construct a new view of the non-adakitic younger granites. LA-ICP-MS zircon U-Pb dating provided weighted mean ~(206)Pb/~(238)U ages of 743.0 ±2.5 Ma for the No.Ⅰ granite (G1) and 739.0 ± 3.5 Ma for the No.Ⅱ granite (G2). A clear core-rim texture of similar age and a high zircon saturation temperature of ca. 849 ± 14 ℃ were observed for the No.Ⅰ granite; in contrast, G2 has no apparent core-rim texture but rather inherited older zircons and a lower zircon saturation temperature of ca. 763 ± 17℃. Geochemical analysis revealed that G1 is an alkaline A-type granite and G2 is a high-K calc-alkaline Ⅰ-type granite. Both granites share similar geochemical characteristics of arc-related magmatic rocks and enriched Sr-Nd-Hf isotopes, likely due to their enriched sources or mixing with enriched magma. Whereas G1 and its host mafic rocks form typical bimodal intrusions of the same age and similar Sr-Nd-Hf isotope compositions, G2 is younger than its host mafic rocks and its Sr-Nd-Hf isotope composition indicates a lower crust origin. Although they exhibit arc-related geochemical features, the two granites likely formed in a rift setting, as inferred from thier petrology, Sr-Nd-Hf isotopes and regional tectonic evolution.