Estruturas sinmagmáticas e idade de zircão de granitos (evaporação de Pb) paleoproterozóicos da parte leste do domínio Bacajá, Província Maroni-Itacaiúnas

Abstract

Along Transamazônica road, between Xingu River and Novo Repartimento village, (eastern Bacajá domain), one has described predominant syenogranites, monzogranites and granodiorites, tonalites, syenogranites and quartz diorites. These rocks are rather homogeneously deformed in the regional scale and present foliation trajectories striking N60°W and WNW-ESE. In outcrop both subvertical and flat lying igneous layering can be found. These primary surfaces pass laterally to high temperature secondary foliation and also to conformable mylonite zones. The development of these structures was controlled by progressive deformation under decreasing temperature regime, so that one can ascertain the syntectonic emplacement of these granitoids during regional shortening stresses. Rhythmic primary layering with flat-lying and vertical dips would be created by laminar flow, probably during ascending and filling of magma chamber. Aplite veins translate shallow-level emplacement and would have been formed through stress-controlled segregation. Simplectites formed by corrosion of biotite and amphibole indicate that granites emplacement occurred under compressive stresses, with early coaxial flattening components. In strongly deformed rocks low-plunge lineations indicate late strike-slip components. Sigma-type porphyroclasts found in mylonites point to sinistral movement whereas the displacement of aplite veins indicates destral shear sense. Pb-evaporation dating of a granite sample yielded an age of 2076 ± 6 Ma that marks the end of the Transamazonian cycle in the southern part of the Maroni-Itacaiúnas Province. The structures and the widespread occurrence of the studied granites are similar to those observed in other Paleoproterozoic magmatic-arc environments developed during soft amalgamation of continental plates.