Geochemical and geochronological study of early Carboniferous volcanic rocks from the West Junggar : Petrogenesis and tectonic implications

A series of early Carboniferous volcanic rocks, i.e., basalt, andesite, and dacite, occur in the West Junggar. Zircon U-Pb isotopic data indicate that these rocks coevally erupted at 331-344Ma. These volcanic rocks have comparable trace element patterns, characterized by the enrichment of large ion lithophile elements (LILE) and the depletion of high field strength elements (HFSE). Isotopically, they display consistently depleted Sr-Nd isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr ratios=0.7034-0.7054, ε_Nd(t)=4.2-7.7). These rocks are interpreted to be derived from a slightly metasomatized depleted mantle in a normal subduction environment. In the context of a previously proposed ridge subduction model, this study constrains that ridge subduction, if it indeed occurred, commenced between 331 and 310Ma in the central West Junggar.