Intrusão sienítica do Complexo Alcalino Floresta Azul, Bahia : mineralogia e geoquímica

Abstract

The “Alkaline Province of Southern Bahia State” – Província Alcalina do Sul da Bahia – consists of a NE-SW trend of SiO2 undersaturated syenitic massifs of Brasiliano age. One of these massifs, the Floresta Azul Alkaline Complex, is a batholith composed of two different intrusions, one of them granitic and the other one syenitic. The present study was aimed at identifying the accessory minerals of the syenitic rocks through SEM-EDS analysis besides studying the major and trace element geochemistry. The syenites are medium- to coarsegrained rocks with monotonous essential mineralogy including microcline, albite, annite, and nepheline. Secondary cancrinite and sodalite have been formed by low-T reaction between early formed nepheline and late-stage fluids; calcite is also a low-T secondary mineral. The identified accessory minerals were presentacilite, pyrochlore, monazite, baddeleyite, zirconolite, thorianite, strontianite and hydroxyfluorides besides magnetite, ilmenite, pyrite, sphalerite, apatite, ancyllite, zircon and monazite. The most common carbonate is calcite although strontianite has also been identified in places. Apatite is associated to ancylite and monazite. Zircon is always anhedral. In the Currie diagrams devised for the study of alkaline suites, it is noteworthy that most syenites are myaskitic although the more evolved terms show an agpaitic character. The syenites are metaluminous to peraluminous and their REE spectra show strong LREE enrichment and no Eu anomalies indicating oxidating conditions. The main contribution of this study was the identification of accessory minerals so far unknown in the Alkaline Province of Southern Bahia State and the geochemical characterization of the Floresta Azul Massif.