Quartzito azul com dumortierita e fosfatos de alumínio do Espinhaço setentrional, Bahia: Mineralogia e petrogénese

Abstract

In the Northern physiographic domain of the Espinhaço Ridge, in Bahia State, Brazil, blue quartzite is mined as a valuable dimension stone The meter- to decameter-sized lenses are irregularly distributed within the whitish quartzites belonging to the Vereda Formation. Optical studies and microprobe analyses have identified dumortierite, kyanite, chromium-bearing muscovite, and lazulite as the main minerals responsible for the bluish and greenish colors. Phosphates, such as trolleite, augelite, svanbergite-goyazite and monazite, occur sporadically. Dumortierite and kyanite, the most abundant color-causing minerals, are mostly concentrated along millimeter-wide, plane-parallel to crossed sedimentary beds. The dumortierite is also found filling discordant fractures, evidencing the high mobility of boron during metamorphism. The estimated metamorphic conditions of P = 3.5 kbar and T = 475 to 560C are of higher greenschist to lower amphibolite facies. Rounded quartz grains inherited from the sedimentary protolith are conspicuous in the Northern part and absent in the Southern region of the studied area, which indicates increasing deformation southwards. In terms of the depositional milieu, the lithofacies of the Vereda Formation is interpreted as the initial filling product of a Mesoproterozoic rift associated with alluvial planes and lagoons or tidal planes in continental and coastal systems, where the boron that generated dumortierite could have been concentrated in sediments with contribution of evaporitic material.