Characteristics and genesis of ultrabasic and basic rocks in the Kuzi Mountain on the eastern margin of Southern Tianshan, Xinjiang

Abstract：Researches on the ultrabasic and basic rock belt in the Kuzi Mountain on the eastern margin of eastern Southern Tianshan in Xinjiang have been very insufficient; however, the genesis and the formation of the geological setting of this rock belt is of important significance for the tectonic framework of Southern Tianshan. This belt consists of wehrlite，diabase and gabbro. The olivines in wehrlite are forsterite, whose Fo is about 93-96; the accessory mineral chrome spinel has Cr#=Cr/(Cr+Al)×100 values of 75.6-98.8 and Mg#=Mg/(Mg+Fe)×100 values of 1.8-41.4, suggesting a high chromium, low magnesium and Alaskan-type complexes in the orogenic belt. The pyroxenes in diabase are augites, whose Mg#=Mg/(Mg+Fe)×100 values show a continuous trend of evolution in decreasing order, reflecting differentiation of magma; The feldspars are composed of labradorite and andesine. The pyroxenes in gabbro are augites, the Ab values of feldspars are above 95, indicating albite. The values of K2O and TiO2 are high, and LILE content is obviously higher than HFSE content. It is considered that they are intrusions of the same orogenic belt as the wehrlite. Isotopic chronology suggests that the magmatic event might have continued from Carboniferous to Permian (331-286 Ma).