Zircon U-Pb geochronology, Hf isotopes, and whole-rock geochemistry of Hongshuihe Early to Middle Triassic quartz diorites and granites in the Eastern Kunlun Orogen, NW China: Implication for petrogenesis and geodynamics

摘要

The Hongshuihe area of Eastern Kunlun Orogen is well-known for the BIF-type deposits. Some widespread intrusive rocks in the northern Hongshuihe area were previously thought to be emplaced in the Hercynian. This article presents LA-ICP-MS zircon U-Pb ages, with LA-MC-ICP-MS in situ Hf isotopes and major and trace element data for quartz diorites and granites of the Hongshuihe pluton. Two samples from the Hongshuihe quartz diorite and granite yielded a weighted mean zircon U-Pb age of 244 +/- 2 Ma (Anisian) and 246 +/- 1 Ma (Olenekian), respectively. The Hongshuihe subalkaline quartz diorite is metaluminous with the alumina saturation index values of 0.93-1.00, and high SiO2 contents of 59.67-65.51 wt.%, low Y values of 11.1-16.7 ppm, and low HREE (Yb = 1.08-1.83 ppm) that collectively suggest high-SiO2 adakite-like rocks. The Hongshuihe granites have slightly high alkali contents of 7.04-8.12 wt.%, alumina saturation index values of 1.01-1.03, low (Zr + Nb + Ce + Y) contents of 315.30-455.50 ppm, and high Fe2O3T/MgO ratios of 4.03-5.16, indicating fractionated I-type normal island arc magma. The zircon epsilon(Hf)(t) values from -4.0 to -1.1 for the Hongshuihe quartz diorite sample and values from -2.4 to +1.7 for the granite sample correspond to partial melting of Mesoproterozoic thickened lower crust and partial melting of juvenile mantle-derived mafic lower crust, respectively. We conclude that during the Early-Middle Triassic, the Eastern Kunlun area underwent a tectonic setting of the northward subduction of the Palaeo-Tethys oceanic slabs.