Mantle Metasomatism and REE Enrichment in the Genetic Source of the Dalucao Carbonatite Complex (Sichuan, China): Insights from Elemental Geochemistry and In-Situ Sr Isotopes of Two Calcite Types

Abstract Carbonatites possess the highest rare earth element (REE) concentrations among all magmatic rocks, yet the mechanisms governing the enrichment of REEs in carbonatites remain enigmatic. Carbonatite-hosted calcites provide crucial mineralogical and geochemical insights into addressing this matter. In this study, we present a dataset comprising major and trace elemental geochemistry, as well as in-situ Sr isotopic data, for two types of calcites from the Dalucao carbonatite complex in Sichuan Province, China. Our primary objective is to shed light on the origin of fertile carbonatites. These calcites are believed to have originated at different stages of carbonatitic evolution. Type I calcites, characterized by a euhedral granular texture and a homogeneous surface, crystallized at the early stage of carbonatite magmatism. In contrast, type II calcites, closely associated with bastnäsite in a paragenesis, formed within a fluid setting of hydrothermal overprinting. Both calcite types exhibit highly radiogenic (87Sr/86Sr)i ratios, ranging from 0.7059 to 0.7060 for type I calcites and 0.7059 to 0.7068 for type II calcites. The high Sr concentrations (3646–7315 µg/g for type I calcites and 6566–13,427 µg/g for type II calcites) and uniform Pb isotopic compositions (206Pb/204Pb ratios of 18.23–18.27) in the Dalucao calcites refute the hypothesis of crustal assimilation as their genetic source. Instead, the radiogenic Sr isotopic compositions suggest that subducted marine sediments have contaminated the lithospheric mantle that generated the carbonatitic magma. Mineralogical evidence indicating the presence of abundant silicate minerals in the Dalucao carbonatites, combined with a compilation of published C-O isotopic data, suggests that these carbonatites may have resulted from low-degree partial melting (melting proportion <1%) of a metasomatized lithospheric mantle. Finally, we propose a comprehensive model for REE enrichment in the Dalucao carbonatite complex and explore the significance of tectonism in the genesis of fertile carbonatites.