Petrogenesis of Xiaohaizi syenite complex from Bachu area,Tarim

Xiaohaizi syenite complex located within Tarim basin is an important part of Permian Tarim Large Igneous Provinces(LIPs).SIMS zircon U-Pb analysis yields Early Permian age of 279.7±2.0Ma,and coeval with diabase and quartz syenite porphyry dikes in the same area.Based on mineralogical composition,it can be categorized into fayalite syenite and amphibole syenite.The former consists mainly of alkali feldspar,fayalite,clinopyroxene,amphibole with minor quartz and plagioclase,and the latter contains mainly alkali feldspar,amphibole,biotite with minor quartz and plagioclase.Xiaohaizi syenites are ferroan,alkaline series and enriched in LREEs relative to HREEs with pronounced positive Eu anomalies,but no negative Nb and Ta anomalies.Geochemical and Sr-Nd isotopic compositions [(87Sr/86Sr)i=0.7033~0.7038,eNd(t)=+3.1~+3.8] indicate that they were derived from a depleted mantle source without crustal assimilation.Major and trace elements and Sr-Nd isotopic compositions suggest that the parental magma of Xiaohaizi syenites were residual melt left by olivine and clinopyroxene fractional crystallization of mantle-derived alkaline basaltic magma.They crystallized as fayalite syenite under relatively reduced condition,and as amphibole syenite under relatively oxidized environment accompanied by variable degree of plagioclase fractionation.