Paleo-Asian Ocean subduction-related multiple mantle metasomatism: Constraints from in-situ SrMg isotopes of mantle xenoliths in the Xing-Meng orogenic belt

Abstract

Subduction-induced metasomatism profoundly modifies the chemical composition and physical state of lithospheric mantle. However, overprint of multiple mantle metasomatism impedes the identification of metasomatic agents and understanding of mantle dynamics. Here, in-situ major and trace elements, and SrMg isotopes of mantle xenoliths from the Abaga alkali basalts are conducted to decipher multiple mantle metasomatism and evaluate its impact on lithospheric mantle transformation. Based on mineral assemblages and chemistry, these mantle xenoliths classify into four groups. The clinopyroxenes in Group I peridotite exhibit light rare earth element (LREE) depletion, high Ti/Eu (3468–6386), low ⁸⁷Sr/⁸⁶Sr (0.7023–0.7029) and normal mantle-like δ²⁶Mg (−0.11 ‰ to −0.26 ‰), indicating a depleted mantle source. The clinopyroxenes in Group I pyroxenite also exhibit similar REE patterns, Ti/Eu (3792–5208) and δ²⁶Mg (−0.19 ‰ to −0.26 ‰) to those in Group I peridotite. In contrast, the clinopyroxenes in Group II peridotite exhibit LREE enrichment, high field strength elements depletion (HFSE, such as Nb, Ta, and Ti) and low Ti/Eu (2324–3301), signifying that the mantle underwent carbonate metasomatism. Most of them display high ⁸⁷Sr/⁸⁶Sr (0.7032–0.7036) and low δ²⁶Mg (−0.32 ‰ to −0.42 ‰), suggesting that the carbonate melts originated from the subducted carbonate sediments. While some clinopyroxenes in Group II peridotite show relatively higher U/Nb (0.12–0.39) and higher δ²⁶Mg (−0.09 ‰ to −0.27 ‰), reflecting an early-stage fluid metasomatism. Most clinopyroxenes in Group II pyroxenite present LREE enrichment, HFSE depletion, high Ti/Eu (4435–6177), and high Dy/Yb (2.04–3.70), signatures consistent with silicate melt metasomatism. Their Cr₂O₃ and TiO₂ contents further correspond to those generated by peridotite-silicate melt reactions, implying that those pyroxenites resulted from the reaction between eclogite-derived silicate melts and peridotite. Paradoxically, those clinopyroxenes exhibit high ⁸⁷Sr/⁸⁶Sr (0.7036–0.7045) and low δ²⁶Mg (−0.31 ‰ to −0.46 ‰), suggesting that the metasomatic agent retained subducted carbonate signatures. We suggest light Mg isotopes originate from “ghost carbonate”, where silicate melts preserve recycled carbonate’s isotopic fingerprint after carbonate component loss. Considering the tectonic setting and geophysical evidence, Paleo-Asian Ocean subduction-induced fluid/melt-peridotite reactions result in the transformation of the lithospheric mantle beneath the eastern Xing-Meng orogenic belt.