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摘要
Intra-oceanic subduction is considered to be one of the most important contributors to the crustal growth of the immense Central Asian Orogenic Belt (CAOB), the study of which can deepen the understanding of plate tectonic processes and the history of continental growth. The Late Paleozoic tectono-magmatism in the central West Junggar has been widely considered to be related to the intra-oceanic subduction of the Junggar Ocean. However, robust petrologic and geochemical evidence for arc-related magmatism, which is critical to reconstruct the intra-oceanic subduction history, is still unavailable. A comprehensive study of geochronological, geochemical, and Sr-Nd-Hf isotopic compositions of the volcanic rocks from the Baogutu Formation in the Baikouquan region was carried out, aiming to provide constraints on the early Carboniferous tectonic evolution of the central West Junggar. LA-ICP-MS zircon U-Pb analysis of a representative basaltic andesite sample yielded an early Carboniferous age of 342 Ma. The volcanic rocks are calc-alkaline, with low SiO2 (48.38% to 54.45%) and high Al2O3 (19.19% to 19.78%), and are relatively enriched in light rare earth elements and large ion lithophile elements (e.g. Rb、Ba、Sr), but depleted in high field strength elements (e.g. Nb, Ta, Zr, Hf), these characteristics are similar to those of typical island arc volcanic rocks. The Sm/Yb-Sm and Sm/Yb-La/Sm covariation diagrams indicate that the Baikouquan volcanic rocks were derived from 10% to 20% partial melting of a garnet and spinel lherzolitic depleted mantle source that modified by fluids released from the subducted oceanic lithosphere. They have high positive εNd(t) values (+6.4 to +6.8), medium initial 87Sr/86Sr ratios with the range of 0.704593 to 0.704852 and variable εHf(t) values (+8.7 to +14.9), which are similar to those of the early Carboniferous island arc volcanic rocks and the Dalabute mafic rocks in West Junggar, indicating they were derived from depleted asthenosphere mantle in a normal subduction environment. Combined with the palaeogeography and previous research results, we suggest that the formation and tectonic setting of these volcanic rocks are related to the intra-oceanic subduction magmatism, which are resulted from the southeastward subduction of the late Paleozoic ocean basin in Western Junggar. ? 2023 Science Press.
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