Dating low-grade metamorphism and deformation of the Espinhaco Supergroup in the Chapada Diamantina (Bahia, NE Brazil): a K/Ar fine-fraction study

Abstract

This study focuses on the northernmost part of the Mesoproterozoic Espinhaco Supergroup that crops out in the Chapada Diamantina. The fine-fraction K/Ar dating obtained on slightly metamorphosed sediments of the siliciclastic Espinhaco Supergroup shows a polyphase deformation history that corresponds to the Brasiliano (Pan-African) orogenic cycle. The isotopic results are interpreted to indicate three age domains coincident with three structurally different domains. Constrained by the Kubler Index ('illite crystallinity') and illite polytypism, the thermal conditions generated during the tectonic activity show a gradual trend from the craton margins to the interior from epizonal to diagenetic. The northern Chapada Diamantina is situated in the foreland of the Riacho do Pontal belt and comprises the sediments of the Espinhaco Supergroup northeast of the Irece basin. The K/Ar ages for < 2 mu m illite fractions range between 645 and 621 Ma [mean 637 +/- 9 Ma (2 sigma)] and for < 0.2 mu m fraction range between 625 and 603 Ma [mean 614 +/- 9 Ma (2 sigma)]. Samples from the central Chapada Diamantina east of the Irece basin are not affected by a Brasiliano deformation event and therefore, the N-S-trending structures are assumed to be older. The deformation of the southern Chapada Diamantina was established in conjunction with the formation of the Aracuai orogenesis and the inversion and reactivation of the Paramirim impactogen. The last stage of deformation in this area is recorded by the K/Ar fine-fraction dating between 470 and 460 Ma.