Early Carboniferous intra-oceanic arc and back-arc basin system in the West Junggar, NW China

摘要

A series of Lower Carboniferous volcanic rocks occur in the Hatu, Darbut, and Baogutu areas of Xinjiang Province. Secondary ion mass spectrometry (SIMS) zircon U-Pb isotopic data indicate that two samples of these rocks coevally erupted at 324.0 ± 2.8 Ma and 324.9 ±3.4 Ma. Three detailed profile measurements show that the volcanics include the Hatu basalt, the Baogutu andesite and dacite, and the Darbut andesite. Whole-rock compositions suggest that the Hatu volcanics are tholeiites and have a mid-ocean ridge basalt (MORB)-like signature with a small negative Nb anomaly, suggesting formation in a back-arc basin. Their isotopic compositions (eNd(t) = +2.2 to +4.0, (~(87)Sr/~(86)Sr)i = 0.70414 to 0.70517) suggest a mixing origin from depleted to enriched mantle sources. In contrast, the Baogutu and Darbut rocks are andesite and dacite possessing a transitional tholeiite to calc-alkaline character and have E-MORB-like and OIB signatures, with a marked neg ative Nb anomaly and Th/Yb-enrichment, indicating that they were generated in a subduction zone setting. Isotopically, they display consistently depleted Sr-Nd isotopic compositions [(~(87)Sr/~(86)Sr)i = 0.70377-0.70469, eNd(t) = 1.0-5.2], suggesting that they were derived from a depleted mantle, and that fluid and sediments were involved in their petrogenesis. These fea tures suggest that an early Carboniferous intra-oceanic arc and back-arc basin system generated the studied volcanic units in the West Junggar.