1. Geochronology and geochemistry of volcanic rocks of the Bima Formation, southern Lhasa subterrane, Tibet: Implications for early Neo-Tethyan subduction.
- Author
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Lang, Xinghai, Deng, Yulin, Wang, Xuhui, Tang, Juxing, Yin, Qing, Xie, Fuwei, Yang, Zongyao, Li, Zhuang, He, Qing, Li, Liang, Zhang, Zhong, and Jiang, Kai
- Abstract
Bima Formation volcanic rocks, which record the history of Neo-Tethyan subduction, are found within the central and eastern segments of the southern Lhasa subterrane, Tibetan Plateau. Zircon U Pb dating, whole-rock major and trace element analysis, and Sr–Nd–Pb–Hf isotopic compositions of Bima Formation volcanic rocks from the central segment of the southern Lhasa subterrane were used to constrain the magmatic and tectonic evolution of the Lhasa terrane during the early Mesozoic. Zircon U Pb dating of five samples yielded consistent ages of 184.3 ± 2.4 to 176.8 ± 3.5 Ma. The dominant volcanic rock types within the Bima Formation are basalts, basaltic andesites, andesites, and dacites, which are enriched in the large-ion lithophile elements (e.g., Rb, Sr, and Ba) and depleted in high-field-strength elements (e.g., Nb, Ta, and Ti). (
87 Sr/86 Sr) t ratios are low (0.702900–0.704146), ε Nd (t) and ε Hf (t) values are high and positive (+4.4 to +6.9 and + 9.6 to +15.7, respectively), and Pb isotope ratios are homogeneous (initial206 Pb/204 Pb = 18.28–18.40;207 Pb/204 Pb = 15.53–15.56;208 Pb/204 Pb = 38.21–38.38). Combining the new data with those from a previous study of Bima Formation volcanic rocks from the eastern segment of the southern Lhasa subterrane indicates that the Bima Formation formed between the Middle Triassic and Early Jurassic. It suggests that more widespread early Mesozoic volcanic rocks in the southern margin of the Lhasa terrane. The basaltic rocks of the Bima Formation were generated by partial melting of a depleted mantle wedge metasomatized by slab-derived fluids, and subsequently experienced fractional crystallization without significant crustal contamination. The andesitic and dacitic rocks were formed by fractional crystallization of the basaltic magma. Our study indicates that the Bima Formation volcanic rocks were generated within a continental island arc setting related to northward subduction of the Neo-Tethyan oceanic slab during the early Mesozoic. Unlabelled Image • Bima Formation in the central segment of the southern Lhasa subterrane formed in Early Jurassic. • It suggests that more widespread early Mesozoic volcanic rocks in the southern margin of the Lhasa terrane. • Bima Formation volcanic rocks were generated within a continental island arc setting. [ABSTRACT FROM AUTHOR]- Published
- 2020
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