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Silicocarbonatites are carbonatite rocks containing > 20% silicate minerals. Their formation is not well understood due to low silica solubility in carbonatite melts and negligible amounts of silicate minerals on carbonatite melt cotectics at upper crustal conditions. We explore whether silicocarbonatites can be thought of as antiskarns: rocks formed by leaching of SiO2 from siliceous wall rocks by carbonatite melts, and its deposition as solid silicate minerals by reaction with chemical components already present in the carbonatite melt. Solid state thermodynamic modelling at 1–5 kbar and 500–800 °C predicts that calcite–dolomite–magnetite assemblages will transform to dolomite-free silicocarbonatites with an increase in silica contents. In sodic systems, the formation of aegirine and alkali amphiboles suppresses silica activity despite elevated silica contents. Therefore, dolomite remains stable, but Fe3+ is consumed, firstly from magnetite breakdown, and secondly by coupled Fe oxidation and reduction of CO2 to CO, CH4, and graphite, particularly at higher pressures. Despite a net increase in Fe3+/Fe2+, the system evolves to increasingly lower oxygen fugacity. In aluminous systems, nepheline indicates high temperatures whereas alkali feldspars form at lower temperatures. Modelling of potassic systems demonstrates stability of mostly phlogopite-rich biotites, leading to Fe2+ increase in all other carbonate and silicate phases. We find that perthites are expected in high pressures whereas two feldspars are more likely in lower pressures. Aspects of the clinopyroxene natural compositional trend (diopside to hedenbergite to aegirine) of carbonatite systems can be explained by silica contamination. Ferrous clinopyroxenes typically require low alumina and are predicted in potassic or low temperature sodic systems, primarily at mid to high pressures. Silica contamination permits the formation of silicocarbonatite-like assemblages in a way that is not limited by SiO2 solubility in carbonatite melts. Glimmerites and clinopyroxene-rich rocks (such as the ijolite series) that often occur around carbonatite rocks at the contact with silica-oversaturated wall rocks can be explained as the extreme end of silica contamination of carbonatite melts. Therefore, these clinopyroxenites and glimmerites can form solely via metasomatic processes without the presence of a silicate melt. [ABSTRACT FROM AUTHOR]

New 40Sr/39Ar dating revealed that the magnetite phlogopitite at the center of the Araxá complex reached the surface at 98 Ma, and generated volcanoes with mineralized volcanic tuffs and crater lakes. At the bottom of the crater lakes, micro-graywacke deposited. Based on the mineral composition, hydrothermal structures, and δ18O and δD values, a hydrothermal episode occurred between ∼98 and 86 Ma Ma. The δ18O of phlogopites of phlogopities vary between 4.8‰ and 5.7‰, and those of magnetitites are 4.3‰ and 6.8‰. The δ18O of the water in equilibrium with phlogopites of the phlogopitites vary between 2.3‰ and 3.2‰, and those with magnetitite are 4.3‰ and 1.8‰. The δD of phlogopites of phlogopitites vary between −86.5‰ and −95.2‰, and those of magnetitite are −84.9‰, and −85.8‰. The δD of the water in equilibrium with phlogopites of the phlogopitites vary between −26.8‰ and −35.5‰, and with magnetitite are −25.6‰, and −26.26‰. During the hydrothermal event, magnetite phlogopitites were hydrothermally altered into phlogopite magnetitite, magnetite apatitite (epifoskorites), and mineralized with pyrochlore. At 84 Ma, the phlogopitic core was permeated by veins and micro-veins of Sr-rich norsethite carbonatites. At 83.5 Ma, calcitic carbonatites and micro-phlogopitites originating from the same mantle source as the magnetite phlogopitite intruded, which resulted in the formation of a carbonatite ring around the core and the assimilation of part of the phlogopitic core. This magmatism generated a new hydrothermal event of limited coverage that mineralized phlogopitites and carbonatites with monazite and massive pyrite ± chalcopyrite. The magmatism ended at 77 Ma, when phosphorous-saturated calcitic carbonatite surrounded the core. The saturation of phosphorous favored the crystallization of apatite, which generated phosphate deposits. • About 70% of the world's production of niobium is from the Araxá complex (Brazil). • The Araxá's rocks contain magmatic and hydrothermal pyrochlore. • The complex' nucleus is composed mainly by pyrochore-rich phlogopitites with magnetite and apatite. • A P 2 O 5 -rich and Nb-poor carbonatite ring surrounds the complex' nucleus. • Niobium and phosphate ores are regoliths formed by weathering of phlogopitites and carbonatites, respectively. [ABSTRACT FROM AUTHOR]