Structural sequence and petrogenesis of the polyphase deformed Precambrian terrane of the Vila Nova region, State of Rio Grande do Sul, Southern Brazil

Evidence is presented concerning the nature and development of the structural pattern existing in the region of Vila Nova, State of Rio Grande do Sul, Southern Brazil. A supracrustal rock assemblage made up predominantly of quartzofeldspathic gneisses, ranging in composition from dioritic to granodioritic; migmatites, and associated layers and lenses of amphibolite, have undergone an extensive polyphase deformational and metamorphic history, within which multiple igneous injections played a prominent role. This mixed rock assemblage which is here designated "Vila Nova Complex" forms part of the Precambrian rocks of the Sulriograndence Shield. Despite the high degree of structural complexity it has been possible to establish a deformational sequence mainly on the basis of refolding and cross-cutting relationships seen in the field. Correlation between different areas of outcrop has been carried out on the basis of eleven phases of deformation that have been recognized, and their relationships with various intrusive masses. A gneissose foliation, formed during the first deformational phase (D1), and folds of varying style and size and related igneous intrusions, developed during the third deformational phase (D3), are key elements in the elucidation of the geological development of the rock complex. The dominant foliation (S1) which generally parallels the original layering (S0) is commonly associated with and axial-planar to isoclinal, intrafolial, rootless folds (F1). The main phases of igneous activity took place during both late and with the emplacement of structurally-controlled tonalitic to trondhjemitic and granodioritic masses, respectively. The complex fold patterns displayed by various neosomes are the result of polyphase deformation rather than ptygmatic activity. On the basis of different structural features representing a long-lasting progressive deformation three markedly distinct general compressive stress-systems have been distinguished in the course of the orogeny which caused the formation of the Vila Nova Complex. This orogeny appears to have occurred along convergent plate boundaries involving a continent-continent collision model