Petrogenesis and tectonic implication of the Neoproterozoic Shuimo alkaline-carbonatite complex from Northwestern margin of the Yangtze Block (China)

Abstract

To investigate the petrogenesis of the alkaline-carbonatite complex and the Neoproterozoic tectonic evolution for the northwestern margin of the Yangtze Block, we conducted a combined study of petrology, LA-ICP-MS zircon U-Pb geochronology, mineral chemistry, whole-rock geochemistry and Sr-Nd-C-O isotopes on the Shuimo alkaline-carbonatite complex in the area. The complex consists of ultrabasic alkaline rocks (jacupirangite and ijolite), syenites (aegirine-augite syenite and biotite syenite), and igneous carbonatites. Zircon U-Pb weighted mean age of the ijolite is 855 ± 4 Ma, consistent with previous studies that report ages of 860 ± 5 and 869 ± 4 Ma for syenites, indicating that the Shuimo alkaline magmatic activity occurred at approximately 860 Ma. The Shuimo alkaline rocks show relative enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) while being depleted in high field strength elements (HFSEs). The alkaline rocks and carbonatites have similar Sr-Nd isotopes (I_Sr = 0.7045–0.7056 and εNd(t) = 0.28–4.97, I_Sr = 0.7042–0.7044 and εNd(t) = 1.07–2.55, respectively), indicating a common enriched lithospheric mantle source. Geochemical characteristics of minerals and whole rocks suggest that the Shuimo alkaline rocks originated from a nephelinitic alkaline magma formed by low-degree partial melting of garnet-bearing spinel lherzolite in the lithospheric mantle. The primary magma underwent fractionation of clinopyroxene during its early stage, resulting in gradual formation of jacupirangite and ijolite. In contrast, syenite formation involves crustal assimilation and fractional crystallization of the parent magma. Carbonatites are genetically associated with ultrabasic alkaline rocks and underwent fractional crystallization and liquid immiscibility during their formation. Tectonically, the Shuimo alkaline-carbonatite complex was formed in a continental back-arc extensional setting, where the mantle was metasomatized by slab-derived fluids, leading to carbonation and triggering partial melting. The results support the plate-subduction model for the northwestern margin of the Yangtze Block during the Neoproterozoic period and suggest that early back-arc extension occurred during the subduction. We propose a model for the formation of alkaline-carbonatite complexes in subduction-related extensional settings, which involves enriched mantle metasomatized by slab-derived fluids, fractional crystallization and crustal contamination of alkaline rocks, immiscible separation of carbonatites from evolved silica-undersaturated alkaline rocks, and late-stage fluid metasomatism that modifies the carbonatites.