1. Late‐Triassic high‐Mg andesite from the Zhangguangcai Range (NE China): Hydrous melting of depleted mantle wedge in active continental margin.
- Author
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Zheng, Guoshun, Qin, Jiangfeng, Lai, Shaocong, Long, Xiaoping, Ju, Yinjuan, Zhang, Zezhong, and Zhang, Hongbing
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CONTINENTAL margins ,ANDESITE ,IGNEOUS rocks ,HYDROUS ,MELTING - Abstract
The Zhangguangcai Range is located in the junction zone of the Palaeo‐Asian and Palaeo‐Pacific tectonic regimes; Mesozoic igneous rocks in this area are considered to have been formed in an active continental margin, which is related to the western subduction of the Palaeo‐Pacific Plate beneath the Eurasian continent. However, the nature of the mantle wedge and related melting mechanism remains unclear. This paper reports zircon U–Pb data, whole‐rock geochemistry, and zircon Lu–Hf isotope for a Late‐Triassic high‐Mg andesite in the southern part of the Zhangguangcai Range. Zircons from the andesite yield a concordant 206Pb/238U age of 218.6 ± 3.3 Ma, which is contemporary with the diorite in the study area. The Lengshan high‐Mg andesite is characterized by moderate SiO2 (57.30–60.15 wt.%), high Mg# (61–67), Al2O3 (15.05–17.35 wt.%), (La/Yb)N (16.9–22.4), and variable Na2O/K2O (1.06–1.59). Its insignificant Eu negative anomalies (Eu/Eu* = 0.88–0.99) and high Sr/Y (68.4–96.4) may prove the existence of garnet or amphibole in the residual melt, where feldspar is more or less absent. The andesite is enriched in Rb, Ba and depleted in Nb, Ta, Ti, and other high‐field‐strength elements, with Nb/Ta ratios of 14.6–17.2, U/Th ratios of 0.28–0.31. In combination with its depleted zircon Lu–Hf isotopic compositions (e.g., εHf(t) = −0.48 to +13.1), it is proposed that the high‐Mg andesite formed by hydrous melting of the mantle wedge, which was metasomatized by slab‐derived adakitic melts. The subsequent assimilation of ancient crust results in the formation of some zircons with negative Eu*/Eu anomalies (0.05–0.1) and εHf(t) values (−13.8 to −18.5), which is approved by the relative evolved whole‐rock Sr–Nd isotopic compositions. [ABSTRACT FROM AUTHOR]
- Published
- 2022
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