The Guacuí and Batatal lineaments in the southern region of the state of Espírito Santo (southeastern Brazil) can be traced from satellite images for tens of kilometers. They are considered shear zones associated to the final tectonic stages of the Araçuaí-West Congo (A-WC) Orogen during the Ediacaran-Cambrian period. In order to understand their nature and tectonic implications for the development of the orogeny, we integrated geological and structural data based on fieldwork and detailed microstructural analysis. Data was collected along two profiles located at the central segments of both structures. The results demonstrated that the Guaçuí shear zone (GSZ) is a typical ductile transpressive shear zone with approximately 9 km width that shows numerous dextral kinematic indicators and records metamorphism of upper amphibolite facies. The GSZ shows different levels of intensity of dynamic deformation. The Batatal shear zone (BSZ) is a narrow brittle shear zone that shows essentially brittle microstructures (intercrystalline microfractures) revealing deformation at low-grade conditions (frictional regime). Based on their characteristics and the level of erosion of the rocks exposed in the region, we interpret that GSZ developed prior to BSZ.
Na região norte do Rio de Janeiro afloram rochas orto e paraderivadas neoproterozoicas de alto grau metamórfico, englobadas no Terreno Oriental da Faixa Ribeira. Dado a carência de estudos de detalhe na região, o presente trabalho contribui com novos aspectos de campo, petrografia, química mineral e geotermobarometria dos granitoides diatexíticos aflorantes na região entre São José de Ubá e São João do Paraíso, no noroeste fluminense. O litotipo é caracterizado por biotita-hornblenda gnaisses migmatíticos e protomiloníticos que são correlacionáveis a ortognaisses e granulitos do Complexo Serra da Bolívia (Domínio Cambuci da Faixa Ribeira). Dois litotipos neoformados são observados: um mesocrático, de caráter residual; e outro leucocrático, que correspondente ao leucossoma. O enriquecimento de K-feldspato no leucossoma em relação ao neossoma residual sugere baixos volumes de fusão por meio de reações de fusão envolvendo água. Aplicando-se diferentes geotermobarômetros, foram obtidas como condições de equilíbrio para o pico metamórfico 6,8 Kbar e 672–680°C (fácies anfibolito superior). Tais valores destoam dos encontrados na literatura para ouras rochas da região, reforçando a ideia de que os domínios do Terreno Oriental da Faixa Ribeira apresentam evoluções metamórficas distintas, em diferentes níveis crustais.
Optical physics applied to mineralogy becomes a complex concept, since it is based on concepts of refractoriness of isotropic and anisotropic materials, dependent on crystallographic parameters. The work aims to materialize these concepts in threedimensional schematic models in wood, such as optical indicatrix and figures of interferences, generated when the plane polarized or polarized light interacts with several transparent minerals. The pieces were applied in the classes of Mineralogy II of the Geology course of the Federal University of Espírito Santo in the years 2017 and 2018, presenting great receptivity by the dissenting body and ease in the profusion of the knowledge of the group of monitors and teachers. The geometric representations were also shown in workshops at the Natural History Museum of Southern Espírito Santo State with the objective of disseminating the geosciences. As a result, the classes obtained satisfactory results of approval and learning
The distribution of trace elements in pyrite has been documented for the first time in quartz veins hosted in the Mesoproterozoic metasedimentary sequence of the Tombador Formation, São Francisco Craton, northeast Brazil. In this study, Electron Microprobe Analyses (EPMA) were used to determine the trace-element compositions of pyrite in these hydrothermal quartz veins. Three pyrite types have been distinguished and interpreted from petrographical relationships and trace-element patterns. Pre-existing pyrite (Py1), derived from the host-rock quartzite, is Ni-poor with concentrations varying from 600 – 6,100 ppm. Elongated syn-tectonic pyrite (Py2) has similar trace-element composition to the Py1, with Ni amounts ranging between 830 and 7,870 ppm. In contrast, possibly post-tectonic, euhedral to subhedral hydrothermal pyrite (Py3), contains higher contents of Ni (7,970 – 26,120 ppm). Mafic and/or metasedimentary rocks from the Espinhaço Supergroup were probably the source of Ni for this fluid-flow event. Fluid generation is related to the devolatilization of the base of the thickened crust, with migration of fluids by preexisting structures. Several shear zones and large-scale NNW-trending folds were developed during the inversion of the Espinhaço basin, as a result of the ca. 0.6 Ga Brasiliano orogenic event. Regional fluid movement through the crust at this time is supported by several mineralized veins and hydrothermal deposits in the São Francisco Craton and adjacent Neoproterozoic belts.
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