Carbonatite Metasomatism in a Subvolcanic Setting: Breccia at the Badou Carbonatite in the North China Craton and Implications for Magmatic Evolution and Eruptive Style

Abstract

The genesis and primary compositions of carbonatite melts are enigmatic owing to their reactivity and resulting contamination in the mantle and crust. This overprints earlier uncontaminated compositions and obscures their origins. The subvolcanic Badou carbonatite in the North China Craton is characterized by brecciated textures and comprises pristine clinopyroxene, mica, and apatite phenocrysts in a mixed, carbonate-silicate matrix. Here, we provide mineralogical, textural, and major and trace elemental results to reveal the mantle derivation of the Badou carbonatite melt, and its compositional modification during interaction with crustal materials. Clinopyroxene and apatite phenocrysts have initial Sr isotopes (0.70920-0.71037) similar to brecciated calcite (0.71028-0.71159) and the whole rock (0.70953-0.71061), indicating a common source. Rare Mg-rich clinopyroxene and mica contain high Cr and Ni contents, recording direct mantle derivation without immiscibility from silicate melts. Abundant feldspars and aegirine, occurring as both phenocrysts and fine-grained matrix, formed an antiskarn at relatively low temperatures indicating extensive silica contamination upon crustal emplacement. Silica contamination is further indicated by britholite-rich apatite rims and relict quartz cores in clinopyroxene. Zircon xenocrysts were partly assimilated, indicated by Zr and Hf enrichment in late-stage silicates, and partly underwent decomposition to baddeleyite in a decreasing silica activity environment. The assimilation process consumed the carbonate melt, forming refractory silicate minerals and CO2 vapor, leading to near-surface gas overpressure with a decrease in magma viscosity. This elevates the potential for explosive activity, which, in turn, provides a positive feedback mechanism for silica assimilation. The complex evolution of the carbonatite melt in the crust might be responsible for the compositional gap between the natural rocks and experimental outcomes, and results in different eruption styles.