Geochemical Characteristics of Granitoids in the Eastern Sector of the Northern Tianshan Mountains in Xinjiang

The granitioids in question are located in the geosynclinal fold belt of the Northern Tianshan Mountains. The magmas are intruded into a Carboniferous marine volcanic-sedimentary rock series, forming a contemporaneous, intermediate-basic to acid, composite intrusion. With a whole-rock Rb-Sr isochron age of 315.7 Ma, it is considered to be of middle Hercynian age. The granitoids have a SiO2 content of 48–78%, a calc-alkali index of 60 and silica-alkali indices ρ = 2.2–2.4 and σ= 1.5–2. From basic to acid compositions, the total REE content increases from 38 to 143 ppm. δEu ranges from 0.47 to 0.86 and the Eu anomalies are negative. The normalized curves of REE of various rock types are very similar. These coupled with the characteristics of trace element geochemistry show that the intrusion is an island arc-continental margin, calc-alkaline one. The δ18O values range from 5.8 to 8.9% and the 87Sr / 86Sr initial ratio is 0.705. The materials of the intrusion are composed of a mixture of the magmas at the top of the upper mantle and in the lower crust. This indicates that the rocks might be consanguineous, differentiated and evolved products and possess the characteristics of I -type granites.