Dome emplacement and formation of kilometre-scale synclines in a granite–greenstone terrain (Quadrilátero Ferrífero, southeastern Brazil)

Abstract

The Quadrilátero Ferrífero is a portion of the Brazilian Precambrian shield containing a granite–greenstone terrain characterized by granite–gneiss domes encircled by kilometre-scale synclines. Integration of porphyroblast-matrix microtextures, c-axis fabrics analysis and strain analysis with outcrop-scale structural relationships indicates that the main phase of domes and synclines evolution occurred during the westerly directed 0.8–0.6 Ga Pan-African-Brasiliano regional shortening. This tectonic event has produced the single pervasive foliation observed in the folded supracrustal sequences of the Quadrilátero Ferrífero. 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 facilitated steeply directed downward flow of country rock material around the granite–gneiss domes. The marked density contrast between granite–gneiss material and the iron-rich supracrustals provided a gravitationally favourable environment for downward wallrock flow. Variations in shear zone kinematics away from the domes, combined with kilometer-scale viscous drag, contributed to produce kilometre-scale synclines in the supracrustal rocks constrained between upward moving domes.