Evolução tectonoestratigráfica dos sistemas transcorrentes Carajás e cinzento, Cinturão Itacaiúnas, na borda leste do Craton Amazônico, Pará

Abstract

TECTONOSTRA TIGRAPHIC EVOL UTION O F THE CARAJÁS AND CINZENTO STRIKE SLIP SYSTEMS, ITA C AI UNAS BELT, EAST OF THE AMAZONIAN CRATON, PARÁ STATE The Carajás-Cinzento fault system occurs within the Itacaiúnas Belt, Amazonian Craton, Brazil. The regional tectonostratigraphy can be subdivided into: (1) the Basement Assemblage - orthogneisses, migmatites, and granulites (the Xingu and Pium Complexes; Plaque Suite) and a later volcano-sedimentary sequence of ironstones, quartzites, amphibolites and schists (Igarapé Salobo Group). These rocks underwent intense ductile shearing and high grade metamorphism along the Itacaiúnas Shear Zone (ISZ). (2) a Cover Assemblage represented by very low-grade volcanic and sedimentary rocks, rests unconformably on the basement rocks. Older clastic, volcanic and ironstone sequences (Igarapé Pojuca & Grão Pará Groups; ca.2.7 Ga) are overlain by a sequence of rnarine to fluvial deposits (Águas Claras Formation). Both Cover and Basement assemblages are intruded by ca.2.5 and ca. 1.8 Ga A-type granitic plutons and basic dykes. Ali units are unconformably overlam by a localised sequence of polymitic conglomerates (Gorotire Formation). A subvertical EW mylonitic fabric developed in the Basement Assemblage rocks where it preserves widespread sinistrally transpressional kinematic indicators (oz.2.8 Ga). At least three cycles of Archaean to Proterozoic brittle-ductile strike-slip reactivation at low metamorphic grades appear to postdate the development of the ISZ, leading to the formation of the Carajás and Cinzento fault systems. The Cover Assemblage sequences were faulted down into dilational jogs formed during dextral displacements. The effects of a later episode of brittle-ductile sinistrai transpression are preserved, localised in the region of the major fault strands. Circunstantial evidences indicate further minor fault reactivation during the Phanerozoic, and the region appears to be tectonically active in the present. There is widespread evidence that the mylonitic fabrics of the ISZ controlled the orientation of later structures. Long-term fault zone weakening is common in those regions where brittle processes caused increases in fault zone permeability allowing extensive syn-tectonic fluid circulation, some of which caused gold and copper mineralisation.