Isotopic studies bearing on the tectonics of the West Junggar Region, Xinjiang, China

Isotopic investigations from the Hercynian‐age fold belt between the Kazakhstan and Siberian cratons in the West Junggar region determine the timing of tectonic evolution and the petrogenesis of the granitic rocks of the region. Sphene from the leucogabbro phase of the Tangbale ophiolite melange, the oldest member of the ophiolite sequences in the fold belt, yields an isotopic Pb‐Pb age of 523.2±7.2 Ma. Zircon from a postcollision alkali granite yields slightly discordant isotopic U‐Pb ages that indicate magma crystallization at 321.4±6.7 Ma, dating it in the Lower Carboniferous period. Radiometric dating thus documents a time span of circa 200 Ma for igneous activity in the area. Petrogenetic studies were made to test whether Precambrian crustal rocks might underlie the Junggar sedimentary basin. Initial lead isotope ratios determined from potassium feldspars from five alkali granites show clear affinity with ratios from mid‐ocean ridge basalts in Pb isotope correlation diagrams. Sm‐Nd data from sphene and apatite from one of the granites yield an initial ε_Nd(T)=+6.1. The granite sources are depleted mantle rocks of oceanic affinity that show no involvement of recycled aged granitic crustal rocks.