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

Late Hercynian dyke magmatism of Sarrabus (SE Sardinia)

The late-Hercynian dyke magmatism of Sarrabus reflects the bimodal distribution observed in the other areas of Sardinia, the suite being represented by prevalent basaltic andesites and peraluminous rhyolites. Calcalkaline basalts and andesites are scarce, while dacites are extremely rare. The hypabyssal activity is closed by basaltic dykes with tholeiitic affinity. The chemical composition points out the 'medium-K' and 'high-K' character of the calcalkaline series. Fractionation links are lacking between calcalkaline and the majority of rhyolitic rocks. In comparison with dyke rocks of the other areas of Sardinia, many of the Sarrabus type show a more marked subalkaline character. The compositional variation of calcalkaline rocks can be partly explained by crystal-liquid fractionation processes. Among the rhyolitic rocks, two groups with different peraluminous characters have been recognized

Late to Post-Hercynian dyke activity in the Sardinia-Corsica domain a transition from orogenic calc-alkaline to anorogenic alkaline magmatism

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