Geochemical and Sr-Nd-Os Isotopic Characteristics of Middle Silurian Volcanic Rocks in Northwest of the Xiemisitai Mountains, West Junggar and Its Tectonic Implications

Volcanic rocks in the Xiemisitai Mountains, in Northwest Junggar were recently considered to form in the Silurian rather than the Devonian period. However, the characteristic and tectonic setting of Middle Silurian volcanic rocks in this area are poorly constrained. In this study, major and trace elements and Sr-Nd-Os isotopic composition analyses were synthetically conducted on the Middle Silurian basalt and basaltic andesite in northwest of the Xiemisitai Mountains, West Junggar. These volcanic samples are characterized by high MgO (5.15%-7.81%) with Mg of 56.0-60.7, low CaO (4.56%-5.09%) and FeO_T/MgO (1.15-1.40), similar to those of high-Mg volcanic rocks. Basalts have εNd(t) of 2.65, (⁸⁷Sr/⁸⁶Sr)_i of 0.70439, and (¹⁸⁷Os/¹⁸⁸Os)_i of 0.4901. The slightly enriched isotopic signatures, high Th/Yb (2.2-2.7) and low Ba/La (17.4-33.4) indicate that they originated from depleted mantle metasomatized by melts from subducted sediments and suffered from crustal contamination. The volcanic rocks are calc-alkaline and exhibit arc-like geochemistry with the enrichment of light REE and large ion lithophile elements. It is suggested that these rocks were probably formed in a back-arc setting due to northward subduction of the Junggar Oceanic slab.