Petrogenesis and tectonic setting of Mid-Triassic volcanic rocks in the East Kunlun orogenic belt, NW China: Insights from geochemistry, zircon U-Pb dating, and Hf isotopes

摘要

The Elashan magmatic belt is located at the eastern margin of the East Kunlun orogenic belt (E-KOB) in northwestern China, where voluminous magmatism occurred during the Late Permian to Late Triassic. Mid-Triassic volcanism produced the Xilikete Formation, which shows a geochemistry similar to that of highly fractionated I-type (HFI) volcanic rocks. These rocks are characterized by high SiO2 (73.82-74.97 wt%), differentiation index (DI; 91.01-94.69), and alkalis (e.g., high K2O + Na2O, with K2O/Na2O ratios greater than 1.0) and by enrichment in some large- ion lithophile elements (LILEs; e.g., Rb, K, and Pb) and depletion in other LILEs (e.g., Ba and Sr) as well as some high-field-strength elements (HFSEs; e.g., Nb, Ta, and Ti); these features confirm that they are HFI volcanic rocks. Zircon U-Pb ages indicate that the volcanic rocks were emplaced at similar to 239 Ma. These zircons have epsilon(Hf)(t) values ranging from -2.26 to +0.36, with two-stage model ages (T-DM2) of 1,441-1,221 Ma, indicating a magma source that involved partial melting of Mesoproterozoic lower crust accompanied by a minor juvenile mantle component. The similar to 240 Ma magmatism in the E-KOB was probably developed in response to subduction of the Palaeo-Tethys oceanic plate. Previous studies indicate that a tectonic transformation from subduction to continental collision occurred at similar to 240 Ma. Combining these data with regional geological observations, we conclude that the Daheba volcanic rocks formed in an active continental margin setting related to the subduction of the Palaeo-Tethys oceanic plate beneath the E-KOB.