Northwestern Junggar Basin, Xiemisitai Mountains, China: A geochemical and geochronological approach

Abstract

We report SIMS U–Pb zircon, elemental and Nd–Sr–Pb isotopic data for the volcanic rocks and their subvolcanic units from the Xiemisitai Mountains in the West Junggar Region (Xinjiang, China), aiming to determine their ages, source regions and tectonic setting. Zircon U–Pb dating results of the volcanic rocks (422.5±1.9Ma–411.2±2.9Ma) indicate that the volcanic strata occurred in the Xiemisitai Mountains at Late Silurian–Early Devonian time rather than Middle Devonian as suggested previously. They are probably the eastern extension of the early Paleozoic (Cambrian–Silurian) Boshchekul–Chingiz volcanic arc of East Kazakhstan in China.The Xiemisitai volcanic rocks include andesite, rhyolite, and their pyroclastic equivalents. The subvolcanic units are felsite and granite porphyry. Andesite is moderately LREE-enriched, with a marked negative Nb anomaly and Th/Yb-enrichment. Rhyolite, felsite and granite porphyry are enriched in LREE and Th and depleted Nb. The initial Sr isotope values of the Xiemisitai volcanic rocks and their subvolcanic units range from 0.702294 to 0.704456, εNd(420Ma) from +0.19 to +1.88 with Nd model ages ranging from 600 to 803Ma. Initial Pb isotope ratios (206Pb/204Pb) are 17.77–17.96, (207Pb/204Pb) are 15.49–15.54 and (208Pb/204Pb) are 37.56–37.80. These features suggest that the Xiemisitai volcanic rocks and their subvolcanic units derived from a mantle wedge significant mixed by subducted material (EMI) in a subduction zone. They also indicate significant involvement of Neoproterozoic primitive crust in the formation of the Xiemisitai magmatic arc. These rocks are associated with a Late Silurian–Early Devonian southward subduction of the oceanic lithosphere in the northern West Junggar Region. This gives rise to an EW-trending Boshchekul–Chingiz volcanic arc and associated metallogenic belt in the northern West Junggar Region and adjacent Kazakhstan.