The mafic dike swarm of Uauá (Brazil): geological and petrographical aspects

Abstract

The Uaua mafie dlke swarm stands out as one of the most extensive swarms in the Sao Francisco eraton. These dikes erop out around the town of Uaua (State of Bahia) anel to the east near the Rio Capim greenstone belt (Flg. 1). The dlkes Intrude the Arehean basement whleh is composed of mediumto high-grade gneisses and migmatltes. Table 1 shows the geochronological results for basement rocks, whleh have yielded ages between 2,730-3,016 m.y. ( Rb-Sr,whole roek isochrons) indicatlng a Late Arehean evolution for these rocks. On the other hand, available 4OAr_39Ar (blotlte) and K-Ar (amphibole) determlnations for basement rocks yield Earty Proterozoie ages, the result of overprlnting of the Transamazonieo orogenie eycle. On the basis of field evldenee (degree of deformatlon) and on petrographie grounds (metamorphle faeles and mlneralogy), two generatlons of mafle dlkes have been reeognized in the reglon. The first generation oceurs mostly close to the Rio Capim sequenee and presents a paragenesis eompatlble wlth medium to high amphibollte metamorphie faeles. Penetrative foliation and folding are well~eveloped. These dlkes are eut by the second generatlon and have been Interpreted by WINGE (1984) as possibly representing volcanie eondults for the basie volcanism of the Rio Capim greenstone belt. The second generation is more widespread than the first one. The dikes are usually associated with extensional or shear fraetures and in general show straight or eurved shapes. Most strtke N-S or NE-SW anel less aften NW-SE. Their thieknesses range from a few eentimeters to 30 meters anel thelr lengths up to 8-10 kilometers.