Abstract:

We present a case study of the Early Paleozoic orebearing gneiss granite in the Tianhudong iron-molybdenum ore deposit in the central Tianshan in an attempt to provide new insights for the early Paleozoic tectonic setting and tectonic evolution of the central Tianshan. LA-ICP-MS zircon U-Pb dating yields （445.3±4.6） Ma for the studied granite. The granite is composed mainly of biotite, amphibole, quartz, plagioclase and K-feldspar. Geochemical analyses show the granite belongs to subaluminous calc-alkaline granite. The granite is generally enriched in large ion lithophile elements (LILEs) such as Rb, Ba and LREEs, but depleted in typical high field strength elements (HFSEs) such as Nb, Ta, Ti, Y and HREEs, consistent with the geochemical characteristics of typical arc igneous rock. The zircon Hf isotope compositions (ε_Hf(445 Ma)) of the studied granite vary from -6.30 to -1.77, and the T_DM2 ranges from 1.538 to 1.825 Ga, indicating the source rocks of the granite contain significant crustal materials. Integrating all the available data, we suggest that the granite is produced by melting of the mainly crustal protolith during a subduction process. Combined with the previous studies and our recent study results, it is suggested that the central Tianshan zone was a magmatic arc tectonic environment during the Early Paleozoic. The mechanism of the Early Paleozoic central Tianshan arc can be ascribed to the southern subduction of the ancient Tianshan ocean located between the Tuha block and Tarim block.

Key words: zircon U-Pb dating, Hf isotope, geochemistry, granites, central Tianshan, Tianhudong iron-molybdenum ore deposit