Fertile mantle domains with different metasomatic features beneath the eastern North China Craton: Transformation from variably auto-metasomatized asthenospheric upwelling

Abstract

The eastern North China Craton is known for replacement of refractory lithospheric mantle by fertile one relevant to the (Paleo-) Pacific subduction. However, the formation mechanism of the fertile mantle domain remains enigmatic, thus hindering profound understanding of the associated lithosphere-asthenosphere interaction. To explore the deep dynamic processes, we present petrology, in-situ element and Sr isotopic data on spinel peridotite xenoliths entrained by the Cenozoic basalt in the Qingyuan county, Liaoning Provence.The Qingyuan mantle xenoliths can be categorized into two groups based on textural characteristics and mineral compositions. Group A peridotites consist of lherzolites containing patch of fine-grained aggregates and have low olivine Fo (100×Mg/(Mg + Fe) ≤ 89, atomic ratios) and mineral major-element composition trends deviating from residual mantle. The coarse-grained clinopyroxenes outside the patch and the patch-hosted fine-grained clinopyroxenes and amphiboles display similar light rare earth element (LREE)-enriched patterns and comparable 87Sr/86Sr ratios (0.7035–0.7039, 0.7040–0.7062 and 0.7029–0.7033, respectively). These characteristics suggest that Group A peridotites may represent fragments of lithospheric mantle strongly modified by melt impregnation. In contrast, Group B xenoliths, typically lherzolites, have protogranular texture without the patches of fine-grained aggregates. The olivines have moderate Fo (ca. 90–91) and mineral major-element compositions highly resembling melting residua. The melting is estimated to occur at about 1370–1590 °C and 2–4 GPa, corresponding to mantle potential temperatures of around 1450–1550 °C. The clinopyroxenes show variably enriched REE patterns and depleted 87Sr/86Sr ratios (0.7020–0.7044). These peridotites are thus interpreted as fragments of residual lithospheric mantle transformed from small-scale asthenospheric upwelling. Diffusion modeling of trace elements in clinopyroxenes suggests ubiquitous Mesozoic-Cenozoic metasomatism in the studied peridotites.Combining with other Cenozoic basalt-borne peridotite xenoliths from the eastern NCC and associated tectonic setting, we conclude that the fertile lithospheric mantle domains with different metasomatic features may be transformed from variably auto-metasomatized asthenospheric upwelling. That is, the subduction-related small-scale upwelling not only leaves behind residual domains with weak metasomatism but also produces mantle regions strongly infiltrated by the decompressional melts. This model links the mantle domains of different formation mechanisms to coherent geodynamic processes and emphasizes the complex mantle processes associated with the fluctuated lithosphere-asthenosphere boundary.