Petrogenesis of rhyolite at Kalatage in the Eastern Tianshan, Northwest China: Evidences from geochemistry, zircon U–Pb geochronology, and Hf isotopes

Abstract

The Central Asian Orogenic Belt was one of the most important sites for juvenile crustal growth during the Phanerozoic worldwide, and Eastern Tianshan, Northwest China, located in southern Central Asian Orogenic Belt, is one of the key areas for unravelling the accretionary processes and continental growth. Zircon U–Pb geochronological, Hf isotopic, and whole‐rock geochemical analyses are reported for the Upper Carboniferous Qishan Formation rhyolites from the Kalatage area in the middle of the Harlik–Dananhu arc, Eastern Tianshan, to investigate its petrogenesis and geodynamic setting. Zircon U–Pb ages obtained by laser‐ablation inductively coupled mass spectrometry (LA‐ICP‐MS) indicated that zircon crystallization age of the rhyolite was 299.1 ± 2.1 Ma. The rhyolites are classified as subalkaline and high‐K calc‐alkaline series with A/CNK values mainly lower than 1.10. The REE patterns exhibit right inclined curves with negative Eu anomalies, and the trace element spider diagrams show depletions in Nb, Ta, and Eu, which is consistent with the geochemical characteristics of the island arc calc‐alkaline magma suffered fractional crystallization. In situ zircons Hf isotopic analyses yielded positive initial εHf(t) values ranging from 8.0 to 11.9 and the two‐stage Hf isotope crustal model ages (TDMC) of 554 to 807 Ma. It indicated that the rhyolite was derived from remelting of juvenile crust. The geochemical data for the rhyolites indicate that they were probably generated in a suprasubduction zone setting. It is proposed that the North Tianshan oceanic crust subducted northward beneath the Harlik–Dananhu arc during the Late Upper Carboniferous, and the rhyolites were derived from remelting of juvenile crust and generated in a suprasubduction zone setting.