The origin of the Baluntai diorite in Central Tianshan Mountains, Xinjiang, and its geological significance

Abstract: Studies of petrology and geochemistry indicate that the Baluntai diorite is the high-K calc-alkaline diorite. The rocks, which have the data Na2O+K2O=4.14%-7.05%, Al2O3 ＝14.62%-17.35%, (La/Yb)N with an average of 10.46, are characterized by enrichment of large ion lithophile elements (LILE, such as K, Rb, Ba, and Pb) and light REE but depletion of high field strong elements (HFSE, like Nb, Ta, and Ti)，and were probably produced in an island arc settings. LA-ICP-MS zircon U-Pb dating shows that the Baluntai diorite was formed at (430.1±1.1)Ma. In-situ zircon Hf analyses show that zircons from Baluntai diorite have εHf(t) values from -12.7 to –7.6 and tDM2 model ages of 1895 Ma to 2219 Ma. Combined with Hf isotope analytical results and regional geological data, it is held that the Baluntai diorite resulted from partial melting of the Tianshan crystalline basement in response to northward subduction of South Tianshan Ocean during the Early Silurian. The isotopic ages of the Baluntai diorite body represent the upper age limit of the Wuwamen ophiolite.