The geochemistry of the Barra do Itapirapua carbonatite (Ponta Grossa Arch, Brazil): a multiple stockwork

The Early Cretaceous Barra do Itapirapua carbonatite, in southern Brazil, belongs to the initial stages of the alkaline carbonatitic magmatism in the Ponta Grossa Arch Province, as related to the opening of the south Atlantic Ocean. The magmatic evolution of the complex comprises four phases of intrusive carbonatite that are composed of medium- to coarse-grained Mg and Fe carbonatites with small volumes of late fine- to very fine-grained Mg carbonatites and show evidence of pervasive hydrothermal events. Drill core samples reveal the existence of silicate rocks of syenitic composition of two different origins. Among the silicate rocks, there are syenites of a likely magmatic origin, as well as fenites derived from the granitic protolith, which constitutes the wall rock of the complex. Fenitization around the Barra do Itapirapua complex is mainly potassic, and fenites are Ca- and LOI-enriched in comparison with the magmatic syenites. Furthermore, the two types of silicate rocks differ in terms of REE and other trace elements. Associated syenites and carbonatites have similar Sr-Nd isotopic compositions, close to the Bulk Earth at 120 Ma. Mixing curves indicate that the carbonatite-syenite association may be accounted for by assimilation of up to 20% crustal material, whereas the late carbonatite composition is consistent with less than 10% assimilation of crustal rocks. An origin by liquid immiscibility for the carbonatite-syenite association can be considered, followed by magmatic differentiation through fractional crystallization of calcite and dolomite in the carbonatite magma. C-O stable isotopic data indicate further reequilibration under hydrothermal conditions at temperatures between 375 and 80 degreesC. (C) 2002 Elsevier Science Ltd. All rights reserved.