Petrography and mineral chemistry of carbonatites and mica-rich rocks fromthe Araxa complex (Alto Paranaiba Province, Brazil)

The Araxa complex (16 km(2)) comprises carbonatites forming a central core and a complex network of concentric and radial dykes as well as small veins ; additionally, it includes mica-rich rocks, phoscorites and lamprophyres. Fenites also occur and are represented by Proterozoic quartzites and schist s of the Araxa Group. The petrographic study of 130 burehole samples indicates that the complex i s basically made up by two rock-types, carbonatites and mica-rich rocks, an d subordinately by a third unit of hybrid composition. Carbonatites: range chemically in composition, the most abundant type being magnesiocarbonatite s. Dolomite and calcite correspond to the chief constituents, but other car bonate phases, including the Ce-group RE minerals, are also recognized. Pho sphates and oxides are widespread accessories whereas silicate minerals con sist of olivine, clinopyroxene, mica and amphibole. Mica-rich rocks are represented by abundant glimmeritic rocks and scarce cu mulitic phlogopite-, olivine- and diopside-bearing pyroxenites. Hybrid rock s mainly contain phlogopite and tetraferriphlogopite as cumulus and intercu mulus phases, respectively; carbonate minerals may also be found. Chemical data indicate that the carbonatites are strongly enriched in REE a nd have lower contents of Nb, Zr, V, Cr, Ni and Rb compared to the mica-ric h rocks. The higher K, Nb and Zr contents of the latter rocks are believed to be related to metasomatic processes (glimmeritization) of the pyroxenite s. Similar REE patterns for carbonatites and mica-rich rooks seem to suggest t hat they are related to a single parental magma, possibly of ijolitic compo sition. Steep LREE/HREE fractionation and high Sigma REE content of some ca rbonatite samples would be explained by hydrothermal and supergenic process es.