Petrography and mineral chemistry of serra do Barriga granitic stock (Sobral, CE, northeast of Brasil)

Abstract

The Serra do Barriga Granitic Stock (SGSB), located at NNW of the Ceará, northeast of Brazil, presents faciological variation that permits commercial exploitation in four types of ornamental rocks: Rosa Iracema (RI), Rosa Olinda (RO), Branco Savana (BS) and White Crystal Quartz (BCQ). The aim of this paper is to investigate mineral association, chemical composition and classification of the main minerals phases, as well as genetic relationships among the granitic facies. Petrographic analyzes were made using thin section and analyzes of mineral chemistry through electron microprobe. Petrographic and mineral chemistry analyzes show that SBGS lithotypes varies from inequigranular to porphyritic syenogranites and monzogranites and different facies do not show significant differences in mineral composition, except for small variations of Or amount in K-feldspar. Potassium feldspar is orthoclase and plagioclase ranges from albite to oligoclase (Ab-Or- to Ab-Or). Biotite corresponds to annita to siderophyllite, while amphibole corresponds to iron-edenite. Post-magmatic or hydrothermal mineral transformations such as ex-solution, potassification, albitization, chloritization, sericitization and neoformation of fluorite are common and control discoloration from the pink to the white facies (BS and BCQ). In the plagioclase of the white granites frequent sericitization occurs superimposed to the microclinization, with neoformation of fluorite. The whole data converge to the knowledge that all the facies were formed at the same magmatic event, being the syenogranites (RI, RO and BCQ) originated by an unique magma, followed by a new, more mafic magmatic pulse, represented by porphyritic monzogranite (RO).