ZONA DE CISALHAMENTO TRANSCORRENTE DORSAL DE CANGUÇU: CARACTERIZAÇÃO E IMPORTÂNCIA NA COMPARTIMENTAÇÃO TECTÔNICA DO CINTURÃO DOM FELICIANO

Abstract

Crustal-scale strike-slip shear zones of the inner part of Dom Feliciano Belt show evidence of a long history of left-lateral movement. They started as transtensional structures along which porphyritic granitoids with mantle-derived signature and a well-developed magmatic fabric were emplaced, followed by the intrusion of a suite of crustal-derived peraluminous leucogranites. Several stages of movement of these shear zones where recognized and characterized at different scales. The eldest is typified by a well- developed magmatic planar and linear fabric with NE-NS trends in syntectonic granites. Transitional magmatic to solid state deformational microstructures include undeformed quartz grains within feldspars showing high temperature deformation. Abundant myrmekites along contacts between feldspar porphyroclasts and matrix are suggestive of stress induced diffusion. Mechanical twining, kink bands and interpenetrating grain boundaríes in plagioclase crystals indicates that oriented growth was active. Other high-T structures include grain boundary migration in quartz and grain-size comminution of K-feldspars through ubiquitous development of myrmekites producing a granoblastic polygonal texture. Quartz occurs as mono or polycrystalline ribbons with large polygonal grains with lobate boundaries while biotite shows grain-size reduction keeping cleavage planes parallel to the main foliation. Lovv-T microstructures are abundant in thick sequences of phyllonites and quartz mylonites and nucleation of new grains was the main grain-size reduction process during this stage. Extensive fracturing of feldspar crystals and their replacement by white mica and biotite suggests abundance of fluids. Quartz shows basal slip during this stage. Mylonites of these lOs of km-thick shear zones are affected by several sets of syngenetic and late folds The former show variable geometry becoming cylindrical towards the high strain zones. These strike-slip shear zones which were formerly interpreted as boundaries between allochthonous terranes and collisional sutures are better interpreted as intracontinental structures responsible for the accomodation of late-orogenic belt-parallel movement.