Zircon U-Pb age, geochemical characteristics, and tectonic implications of the early Permian ultrabasic dykes in the Harlik Mountain, east Tianshan, Xinjiang

Objective During multiple stages of magmatic activity in the Harlik Mountains of the east Tianshan, extensive granitic and mafic dyke swarms were formed. This study aims to constrain the closure time of the Paleo-Asian Ocean in this area and to elucidate the tectonic environment in which ultrabasic dykes were formed. Methods This study reports petrography, geochemistry, and zircon U-Pb ages of a particular type of ultrabasic dyke swarms (amphibolites) in Qincheng, Hami. Results The amphibolites have SiO2 contents from 39.00% to 45.48%, TiO2 contents from 1.60% to 3.01%, and Mg# values from 50 to 60. They are relatively weakly enriched in light rare earth elements (La/Yb)N = 1.34~2.25) and show no Eu anomalies (δEu = 0.76~1.12). They are enriched in large ion lithophile elements (LILEs: Rb, Ba, K, Sr) and depleted in high field strength elements (HFSEs: Nb, Zr, Hf). The amphibolites belong to the alkaline rock series. LA-MC-ICP-MS zircon U-Pb dating reveals that these three amphibolites crystallized at 298.4±1.7 Ma, 297.7±1.6 Ma, and 295.5±1.6 Ma, respectively. Whole rock Sr-Nd analysis reveals initial 87Sr/86Sr ratios from 0.7047 to 0.7051 and εNd(t) values from −2.63 to 1.81. Conclusion Based on the comprehensive analysis of regional geology, chronology, geochemistry, and Sr-Nd isotopes, it is suggested that the primitive magma of the early Permian amphibolites in Qincheng may have been created by partial melting of the lithospheric mantle, and these rocks were formed in a post-collisional extensional tectonic setting. It is furthermore implied that the Paleo-Asian Ocean in the Harlik Mountains region closed before the early Permian.