Petrografia e química mineral das rochas alcalinas do Batólito Sienítico Itabuna, província Alcalina do Sul do Estado da Bahia

Abstract

The only occurrence of SiO2 subsaturated alkaline rocks in the São Francisco Craton is bounded by the Southern Bahia Alkaline Province (SBAP). A province formed by a neoproterozoic anorogenic alkaline magmatism intrusive in granulitic and gneissic-migmatitic rocks from the São Francisco Craton. Among the various plutons that make up SBAP rocks of the largest body, the Itabuna Sienitic Batholith (IBS), were studied in this work. It outcrops in the northern portion close to the cities of Itabuna, Itajuipe and Uruçuca. The Itabuna Sienitic Batholith consists of syenites, monzonites, foid syenites and in smaller volumes diorites and gabbros. Representative rocks from IBS were investigated with classical and electronic petrography. Chemical minerals analysis was obtained with a Wavelength Dispersive Spectrometer (WDS) coupled to an Electron Probe Microanalyzer (EPMA) and Dispersive Energy Spectrometer (EDS) coupled to a Scanning Electron Microscopy. The rocks have fine to coarse granulation. Essential minerals are alkali feldspar, plagioclase and nepheline, which together with biotite, amphibole and clinopyroxene are the most frequent phenocrysts. The matrix is composed mainly of alkaline feldspars, plagioclase and nepheline. Plagioclase Anorthite contents vary from albite to labradorite. Chemically biotite corresponds to siderophyllite, lepidomelane and Fe-biotite. Clinopyroxene was classified as diopside, hedenbergite and augite. The amphibole compositions have great chemical variations that define them as belonging to the groups of the Calcium, Fe-Mg-Mn-Li and Na-Ca amphibole. The accessory minerals present are apatite, sodalite, cancrinite, ilmenite, magnetite, pyrite, chalcopyrite, zirconolite, olivine, allanite, baddeleyite, zircon, calcite, monazite, barite, bastnasite and thorite. Primary crystal compositions were identified in biotite, amphibole, allanite and zirconolite. The coupled substitution ETR3++ Si4+= Ca2++P5+ was identified in the apatite crystals and can assist in the formation of monazite crystals. In the studied rocks, small crystals indicating the action of magmatic fluids such as carbonates, monazite and thorite are present. Magmatic temperatures (<989 o C), mean pressures for placing magma (<5.2 kbar) and high oxygen fugacity were inferred on clinopyroxene crystals. Temperatures <790 o C and pressures <2.2 kbar were obtained in the primary biotite crystals. And post-magmatic parameters based on the pairs of ilmenite and magnetite exsolution (maximum temperatures of 765 o C). Fractional crystallization was inferred as the main process responsible for the evolution of the rocks of the Itabuna Sienitic Batholith, a hypothesis also adopted for other SBPA plutons. The compilation and interpretation of the generated data was synthesized in the form of a scientific article.