Abstract
ABSTRACT: In the Paleoproterozoic Transamazonas Province, synkinematic granitogenesis has taken place synchronously with compressive tectonic stress. The synkinematic character of the granites is marked by their WNW elongate shape, and by the presence of pervasive and concordant synmagmatic foliation. Ductile shear zones are concordant to the previous regional WNW structures, and tend to be accommodated along contacts between Rhyacian synkinematic granitoids and both Archean orthogneisses and Siderian metabasites. Locally phyllonitic shear zones and brittle-ductile shear zones with cataclasites are oriented subparallel to the preexisting ductile foliation. Late orogenic brittle faults N30E-trending strike-slip faults are either sinistral or destral. 40Ar/39Ar dating of muscovite developed on fault planes gave ages of 1977 ± 8 Ma and 1968 ± 11 Ma. Structural and geochronological data from rocks of the Transamazonas Province permit to conclude that most mylonites and brittle structures were controlled by preexisting structures such as geological contacts and petrographic facies boundaries. Compressive tectonic stress would have initiated at ca. 2100 Ma, since the former magmatic arc (Bacajaí complex), still present at 2070 Ma when syntectonic granites were emplaced and remained until 1975 Ma after granite plutonism and regional cooling.
Highlights
METHODSSynkinematic granite bodies show elongated shapes concordant with the regional structural pattern and microstructural evidence of emplacement during tectonic stress field (e.g., Paterson et al 1991, Miller & Paterson 1994)
In the initial phases of pluton evolution, igneous layering was controlled by the dynamics of the magmatic chambers and by regional compressive forces, so that deformation would have occurred in not completely crystallized magma
The subsequent evolution from ductile, ductile-brittle, brittle-ductile shear zones, and eventually brittle faults testifies to deformation during a period of cooling
Summary
Synkinematic granite bodies show elongated shapes concordant with the regional structural pattern and microstructural evidence of emplacement during tectonic stress field (e.g., Paterson et al 1991, Miller & Paterson 1994). The magmatic rocks of the Bacajá Terrane have been studied by several authors (João et al 1987, Faraco et al 2005, Vasquez et al 2005, Barros et al 2007) who describe belts of metavolcanics and concordant elongate granitic bodies. Regional lithologic layering is marked by the presence of kilometer-long lenses of metabasites oriented along the WNW direction parallel to the elongate granite bodies composed of differently foliated rocks. Metabasites and amphibolites compose a N70W-striking belt having dozens of kilometers long and few hundred meters wide (Fig. 2) These rocks belong to the Itatá Formation of the Três Palmeiras Group (João et al 1987, Vasquez & Rosa-Costa 2008) and are limited to the north and south by granitic bodies. At least three phases of quartz-feldspathic veins have been identified, many of which have been affected by dextral faults running N80E and by en-echelon sinistral fractures striking N45E
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