Petrography and mineral chemistry of carbonatites and mica-rich rocks from the Araxá complex (Alto Paranaíba Province, Brazil)

The Araxá complex (16 km²) 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 schists of the Araxá Group. The petrographic study of 130 borehole samples indicates that the complex is basically made up by two rock-types, carbonatites and mica-rich rocks, and subordinately by a third unit of hybrid composition. Carbonatites range chemically in composition, the most abundant type being magnesiocarbonatites. Dolomite and calcite correspond to the chief constituents, but other carbonate phases, including the Ce-group RE minerals, are also recognized. Phosphates and oxides are widespread accessories whereas silicate minerals consist of olivine, clinopyroxene, mica and amphibole. Mica-rich rocks are represented by abundant glimmeritic rocks and scarce cumulitic phlogopite-, olivine- and diopside-bearing pyroxenites. Hybrid rocks mainly contain phlogopite and tetraferriphlogopite as cumulus and intercumulus phases, respectively; carbonate minerals may also be found. Chemical data indicate that the carbonatites are strongly enriched in REE and have lower contents of Nb, Zr, V, Cr, Ni and Rb compared to the mica-rich rocks. The higher K, Nb and Zr contents of the latter rocks are believed to be related to metasomatic processes (glimmeritization) of the pyroxenites. Similar REE patterns for carbonatites and mica-rich rocks seem to suggest that they are related to a single parental magma, possibly of ijolitic composition. Steep LREE/HREE fractionation and high sigmaREE content of some carbonatite samples would be explained by hydrothermal and supergenic processes

ITALICA (SEVILLE). USE OF LOCAL MARBLE IN AUGUSTAN AGE

ATTI DEL CONGRESSO ASMOSIA VIII- GIUGNO 200

Petrological, geochemical and Sr-Nd isotopic features of alkaline rocks from the Arraial do Cabo Frio peninsula (southeastern Brazil)

The alkaline activity in the Cabo Frio region is made up mainly of intermediate and felsic differentiate rocks emplaced about 55 Ma ago into the crystalline basement as sills, plugs and dykes. Two magma suites can be distinguished: (a) a strongly silica-undersaturated tephriphonolite to phonolite serie; (b) a weakly silica-undersaturated to satured trachyandesite-trachyte series. Petrography, mineral chemistry, whole-rock chemistry trends and isotopic data strongly support a genetic link among the lithotypes of each suites consistent with fractional crystallization processes dominated by the observed phases. The same data, however appear to exclude a link between the two groups of rocks, supporting a petrogenesis by prolonged differentiation processes starting from two distinct parental magma with a slight difference in the SiO2 saturation. The scarce basanites and trachybasalts reported in the same area could represent the most probable compositions for the parental magmas of the two suites. The initial 87Sr/86Sr (0.70401-0.70458) and 143Nd/144Nd (0.51239-0.51247) isotopic ratios and the significant potassic component indicate derivation of these rocks from an enriched lithospheric mantle source

Petrography and mineral chemistry of carbonatites and mica-rich rocks from the Araxá complex (Alto Paranaíba Province, Brazil)

The Araxá complex (16 km²) 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 schists of the Araxá Group. The petrographic study of 130 borehole samples indicates that the complex is basically made up by two rock-types, carbonatites and mica-rich rocks, and subordinately by a third unit of hybrid composition. Carbonatites range chemically in composition, the most abundant type being magnesiocarbonatites. Dolomite and calcite correspond to the chief constituents, but other carbonate phases, including the Ce-group RE minerals, are also recognized. Phosphates and oxides are widespread accessories whereas silicate minerals consist of olivine, clinopyroxene, mica and amphibole. Mica-rich rocks are represented by abundant glimmeritic rocks and scarce cumulitic phlogopite-, olivine- and diopside-bearing pyroxenites. Hybrid rocks mainly contain phlogopite and tetraferriphlogopite as cumulus and intercumulus phases, respectively; carbonate minerals may also be found. Chemical data indicate that the carbonatites are strongly enriched in REE and have lower contents of Nb, Zr, V, Cr, Ni and Rb compared to the mica-rich rocks. The higher K, Nb and Zr contents of the latter rocks are believed to be related to metasomatic processes (glimmeritization) of the pyroxenites. Similar REE patterns for carbonatites and mica-rich rocks seem to suggest that they are related to a single parental magma, possibly of ijolitic composition. Steep LREE/HREE fractionation and high sigmaREE content of some carbonatite samples would be explained by hydrothermal and supergenic processes

FEIçOES QUIMICAS DE PEROVSKITAS EM KIMBERLITOS E KAMAFUGITOS DA PROVINCIA ALCALINA DO ALTO PARANAIBA (MG-GO)

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