Dome emplacement and formation of kilometre-scale synclines in a granite-greenstone terrain (Quadrilatero Ferrifero, southeastern Brazil)

The Quadrilatero Ferrifero is a portion of the Brazilian Precambrian shield containing a granite-greenstone terrain characterized by granite-gneiss do mes encircled by kilometre-scale synclines. Integration of porphyroblast-ma trix microtextures, c-axis fabrics analysis and strain analysis with outcro p-scale structural relationships indicates that the main phase of domes and synclines evolution occurred during the westerly directed 0.8-0.6 Ga Pan-A frican-Brasiliano regional shortening. This tectonic event has produced the single pervasive foliation observed in the folded supracrustal sequences o f the Quadrilatero Ferrifero. Dip-slip ductile shear zones developed at the dome-supracrustal interface are synchronous with dome evolution during the Pan-African-Brasiliano deformation, as indicated by contemporaneous growth of aureole porphyroblasts in the shear planes. These shear zones facilitat ed steeply directed downward flow of country rock material around the grani te-gneiss domes. The marked density contrast between granite-gneiss materia l and the iron-rich supracrustals provided a gravitationally favourable env ironment for downward wallrock flow. Variations in shear zone kinematics aw ay from the domes, combined with kilometer-scale viscous drag, contributed to produce kilometre-scale synclines in the supracrustal rocks constrained between upward moving domes. (C) 2000 Elsevier Science B.V. All rights rese rved.