The Discussion To The Evolutionary Time Of SouthernTianshan Ocean

It has important significance to reflect the information of subduction in Baluntai area for exploring the ocean-continent evolution characteristics of South Tianshan ocean and contrasting Wuwamen ophiolite, Changawuzi ophiolite and Kumishen ophiolite.Based on detailed petrology, geochemistry studies and isotopic methods. We got the results as follows: LA-ICP-MS zircon U-Pb dating shows that the Baluntai basalt producing in Wuwamen ophioliteas was formed at 456.5Ma. The isotopic ages of the Baluntai basalt represents the age of the Wuwamen ophiolite and can be considered as theduration of South Tianshan ocean spreading. The Wuwamen ophiolite,Changawuzi ophiolite and Kumishen ophiolite have same period of formation time and represent the consistency of the evolution of the South Tianshan ocean. Studies of petrology and geochemistry indicate that the Baluntai diorite is the high-K calc-alkaline diorites. The rocks which have Na2O+K2O=4.14%-7.05%, Al2O3 =14.62%-17.35,(La/Yb)N with an average of 10.46, are characterized by enriching in large ion lithophile elements(LILE, e.g K,Rb,Ba,Pb) and light REE but depleting in high field strong elements(HFSE, e.g. Nb, Ta, Ti),originated in the island arc settings. LA-ICP-MS zircon U-Pb dating shows that the Baluntai diorite was formed at 430 Ma. In-situ zircon Hf analyses show that zircons from Baluntai diorite have εHf(t) values-12.7 to –7.6 and tDM2 model ages of 1895 Ma to 2219 Ma. Combined with Hf isotope test results and regional geological data, the Baluntai diorite was resulted in 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 represents the upper age limit of the Wuwamen ophiolite. The Baluntai quartz syenite is the product of extensional magmatism of the orogen, which indicates the post-collision extensional period of the Southern Central-Tianshan suture zone in the Late Carboniferous.