O SUPERGRUPO ESPINHAÇO DA SERRA DO ESPINHAÇO MERIDIONAL (MINAS GERAIS): O RIFTE, A BACIA E O ORÓGENO

Abstract

The Serra do Espinhaço is a mountain chain built up mainly of quartzitic rocks of the EspinhaçoSupergroup. It extends from the central part of Minas Gerais to the north of Bahia, a length of 1200 km in a NSdirection. The southern part of the Serra do Espinhaço (SdEM), which is 300 km long, extends fromQuadrilátero Ferrífero to Olhos d’Água (Fig. 1), and during its initial rifting phase of development wasseparated from the northern part by a NE-trending sinistral transfer fault system (Fig. 2).The Guinda Group contains the synrift sedimentary and volcanic rocks of the SdEM (Tab. 1 and 2). Therifting processes were thermally controlled and started at 1750 Ma. The rifting evolved in 3 distinct phases:the first phase was marked by sedimentation of the Bandeirinha Formation (fluvio-eolian environments) withstrong extensional activity that caused block tilting of the units of this formation and the exhumation of a corecomplex in the median-central part of the SdEM; during the second rifting phase the fluvial sediments of theSão João da Chapada Formation were deposited, following minor thermal subsidence of the rift; anasthenospheric plume upwelling below the continental crust along the central part of the rift (Fig. 3) causedthe third phase of rifting by intensive vertical block movements, generating half-grabens that propagated tothe east. The fluvial and lacustrine sediments of the Sopa-Brumadinho Formation (Fig 4) mark this third phaseof rifting. During the eastward propagation of the extensional tectonics (Fig. 5), the half-grabens wereprogressively filled up by eolian sediments of the Galho do Miguel Formation.The crustal extension culminated in mantle-uncovering at the axial zone of the rift (Fig. 3), generally withserpentinization, and a marine basin was formed. Thick piles of clastic sediments and banded iron formationswere deposited in the neritic and bathyal/abyssal zones, and at the same time the alluvial sediments (fandeltas) of the Sopa-Brumadinho Formation prograded out into the coastal parts of the basin (Fig. 6).Because of the strong thermal subsidence of the basin, the locus of deposition of the marine coastalsediments was at least 80 km westward (Fig. 7), this being indicated by the occurrence of the ConselheiroMata Group (Tab. 1) in the western part of the SdEM. The thermal subsidence of the basin marks the start ofthe Espinhaço passive continental margin and the end of rift-forming processes.Tectonic inversion started at about 1.5 Ga with eastward subduction of oceanic crust, and this precededthe continental collision that built up the SdEM. The units of the Espinhaço Supergroup in the western andcentral parts of the orogen were folded in N-S trending open folds during the initial phase of the tectonicinversion. The deformation related to the subsequent collision was strongly partitioned, consequent both onthe plate indentation and on transcurrent/strike-slip fault movements (Fig. 8, 9 and 10).In the proximal collision zone (eastern edge of the SdEM), a thrust belt developed, and in the southcentralpart of the SdEM and the eastern region of the Quadrilátero Ferrífero the thrust sheets propagatedonto the western domain of the orogen, destroying either partially or totally the previous folding system (Fig.9 and 11).The wandering of the São Francisco craton through high latitudes at about 1.05 Ga caused a mountainglaciation in its higher parts, as indicated by the meta-tillites of the Macaúbas Group and equivalent units.Around 2.5% of the SdEM surface is formed of basaltic rocks derived from basic magmatism ( 906 Ma)that preceded the fragmentation of the continental mass that had been amalgamated by the end ofMesoproterozoic time. This continental fragmentation formed the Neoproterozoic basins of the westernAfrican continent and caused extensive subsidence of São Francisco craton. A large epicontinental basinwas developed there, now indicated by units of Bambuí Group (Fig. 1).The orogenesis of the West Congolian belt ( 750-500 Ma) caused a strong reactivation of the EspinhaçoSystem, with the units of the Macaúbas and Bambuí groups being overthrusted by sequences of the EspinhaçoSupergroup (Fig. 9 and 11). The westward subduction of the oceanic plate of the West Congolian basin underthe southeastern edge of the São Francisco craton (Fig. 12) is indicated by the size and frequency of graniticplutons in the eastern parts of the Araçuaí belt and SdEM, as well as by thermal event of Brasiliano age thatis recorded in metamorphic and granitic rocks over the whole eastern part of this craton.