DEGEO - Departamento de Geologia
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8
Navegar
3 resultados
Resultados da Pesquisa
Item Strain localization and fluid-assisted deformation in apatite and its influence on trace elements and U–Pb systematics.(2020) Ribeiro, Bruno Vieira; Lagoeiro, Leonardo Evangelista; Faleiros, Frederico Meira; Hunter, Nicholas J. R.; Queiroga, Gláucia Nascimento; Raveggi, M.; Cawood, Peter Anthony; Finch, M.; Campanha, Ginaldo Ademar da CruzThis paper presents electron backscatter diffraction (EBSD), trace element and U–Pb data of apatite grains from a granitic mylonite from the Taxaquara Shear Zone (SE Brazil). The mylonite recrystallized under upper-greenschist facies and presents two types of apatite with distinct microstructures. Type1 apatite appears in quartz-rich layers and does not exhibit any microstructural, crystallographic, or chemical evidence of deformation/recrystallization, and resembles the original igneous apatite. Type2 apatite appears in mica-rich layers and exhibits core-and-mantle microstructures, and intragranular subgrain development, suggesting that they have undergone dynamic recrystallization. Recrystallized tails of type-2 apatite grains exhibit a strong c-axis crystallographic preferred orientation parallel to the X-direction (stretching lineation), and lack evidence of dislocation density. This evidence from type-2 apatite grains, combined with REE depletion, high La and a negative Ce anomaly compared to type-1 grains, suggests that type-2 apatite tails underwent recrystallization via dissolution-precipitation creep, whereas parental grains underwent crystal-plastic deformation and subgrain formation through dynamic recrystallization. Phase-equilibrium modelling and quartz CPO opening-angle thermometry are consistent with recrystallization at ∼480 – 530◦C and 2.2 – 5.0 kbar. We were not able to determine precise deformation ages from type-2 apatite because fluid-assisted recrystallization appears to have substantially decreased the U/Pb ratio. We find that preferential fluid flow along high-strain, biotite-rich layers in the mylonite caused type-2 apatite to recrystallise, whereas type-1 apatite in low strain layers was unaffected and retained the characteristics of the protolith.Item Deformation partitioning during folding and transposition of quartz layers.(2003) Lagoeiro, Leonardo Evangelista; Hippertt, João; Lana, Cristiano de CarvalhoBanded iron formation (BIF) from the Quadrilátero Ferrífero (southeastern Brazil) shows a compositional layering with alternating iron-rich and quartz-rich layers. This layering was intensively folded and transposed at a centimeter/millimeter scale through a component of bedding-parallel shear related to flexural slip at middle to high greenschist facies conditions (400–450 jC). The microstructure and c-axis fabrics of normal limbs, inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy (SEM) and digital image analysis. In the normal limbs, recrystallized quartz grains show undulose extinction, relatively dry grain boundaries, c-axes at high angle to foliation and a pervasive grain shape fabric (GSF) indicating operation of crystal-plastic processes. In the inverted limb, quartz grains show more serrated and porous (‘‘wet’’) grain boundaries; the GSF is similar to that of the normal limb, but c-axes are oriented at 90j to those of the normal limb. We interpreted these characteristics as reflecting operation of solution-precipitation deformation in inverted limbs, as a consequence of grains having been rotated to an orientation that was hard to basal hai glide, but easy to dissolution-precipitation creep. This deformation partitioning between crystal-plasticity and solution-transfer during folding/ transposition of quartz may explain the common occurrence of layered quartz rocks, where individual layers show alternating caxis fabrics with opposite asymmetries but a consistent GSF orientation. Such characteristics may reflect an earlier event of pervasive folding/transposition of a preexisting layering.Item Magnetic fabric of Araguainha complex impact structure (Central Brazil) : implications for deformation mechanisms and central uplift formation.(2012) Yokoyama, Elder; Trindade, Ricardo Ivan Ferreira da; Lana, Cristiano de Carvalho; Souza Filho, Carlos Roberto de; Baratoux, D.; Marangoni, Yára Regina; Tohver, EricThe weakening mechanisms involved in the collapse of compleximpact craters are controversial. The Araguainhaimpact crater, in Brazil, exposes a complexstructure of 40 km in diameter, and is an excellent object to address this issue. Its core is dominated by granite. In addition to microstructural observations, magnetic studies reveal its internal fabric acquired during the collapse phase. All granite samples exhibit impact-related planar deformation features (PDFs) and planar fractures (PFs), which were overprinted by cataclasis. Cataclastic deformation has evolved from incipient brittle fracturing to the development of discrete shear bands in the center of the structure. Fracture planes are systematically decorated by tiny grains (< 10 μm) of magnetite and hematite, and the orientation of magnetic lineation and magnetic foliation obtained by the anisotropies of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) are perfectly coaxial in all studied sites. Therefore, we could track the orientation of deformation features which are decorated by iron oxides using the AMS and AAR. The magneticfabrics show a regular pattern at the borders of the central peak, with orientations consistent with the fabric of sediments at the crater's inner collar and complex in the center of the structure. Both the cataclastic flow revealed from microstructural observations and the structural pattern of the magnetic anisotropy match the predictions from numerical models of compleximpactstructures. The widespread occurrence of cataclasis in the central peak, and its orientations revealed by magnetic studies indicate that acoustic fluidization likely operates at all scales, including the mineral scales. The cataclastic flow made possible by acoustic fluidization results in an apparent plastic deformation at the macroscopic scale in the core.