Stratigraphy of the greenstone belts, Crixás region, Goiás, central Brazil

Abstract

Archean rocks in the Crixás region, central Brazil, comprise polydeformed low-grade metamorphic greenstone belt volcano-sedimentary sequences of 2.8 Ga arranged in three subparallel belts separated by domal granitoid gneiss blocks. From west to east the supracrustal belts are named Crixás, Guarinos and Pilar de Goiás. A revised stratigraphy is proposed for each belt. The Crixás Group is composed of: (a) the Córrego Alagadinho Formation, consisting of an approximately 500 meter thick sequence of komatiites interlayered with banded iron formations; (b) the Rio Vermelho Formation (350 m), locally pillowed metabasalts with intercalations of oxide and/or silicate facies banded iron formations; and (c) the Riberrão das Antas Formation (700 m), with a lower sub-unit of carbonaceous phyllite-manganiferous metachert assemblage, a middle carbonaceous phyllite-marble assemblage, and an upper metapelite sub-unit. The Guarinos Group is subdivided into: (a) the Serra do Cotovelo Formation (500 m), made up of komatiites with intercalations of carbonaceous phyllite; (b) the Serra Azul Formation (300 m), consisting of metabasalts with intercalations of carbonaceous phyllite; (c) the São Patricinho Formation (300 m), metagraywacke interlayered with chlorite phyllites and locally banded iron formations and orthoquartzite; (d) the Aimbé Formation (50 m), a stratigraphic marker made up to diverse banded iron formations; and (e) the Cabaçal Formation (400 m), consisting of carbonaceous phyllite. The Pilar de Goiás Group contains four sub-units: (a) the Córrego Fundo Formation (900 m), komatiites interlayered with thin banded iron formations; (b) the Cedrolina Formation (500 m), consisting of metabasalts with thin iron and/or manganese formation layers; (c) the Boqueirão Formation (200 m), made up of 50 meters of lower metachert grading up into calc-silicate rocks with dolomitic marble lenses close to the top; and (d) the Serra do Moinho Formation (400 m), composed of phyllite with variable proportions of carbonaceous matter. Each of the greenstone belts has a gross stratigraphic framework made up of a lower volcanic and an upper sedimentary sequence. Major stratigraphic differences occur among the upper sedimentary sections of the three belts; these are interpreted as resulting from different paleographic settings. Vertical facies change from deep to shallow water facies in each belt. Lateral facies change from an outer shelf euxinic environment in the Crixás greenstone belt, eastward into slope graywackes in the Guarinos greenstone belt, and deep water cherts and calc-silicate rocks in the Pilar de Goiás greenstone belt. A carbonaceous sequence on top of both the Guarinos and Pilar de Goiás Groups may represent either a blanket of euxinic shelf prograding onto deep water facies or allochtonous slices of outer shelf type metasediments transported during late Archean thrusting.