ABSTRACT: The impetus for this research was the enigma regarding the origin of the extensive silicic volcanic units in the Paraná-Etendeka Igneous Province. Are they rheoignimbrites, lava flows, or a combination of both? The São Joaquim silicic eruptive sequence is comprised of Palmas-Type aphyric dacites and is located in the northern segment of Torres Trough, a dissected mountainous region in southern Brazil. These dacite outcrops form plateaus and remnant hills that are scattered above a basaltic andesitic landscape. The original morphology of the silicic volcanic flows was constrained based on the internal architecture, geometry, and cyclicity of the lithofacies, in conjunction with petrographic and geochemical information. The data suggests the presence of ancient large tabular and lobate silicic lava flows (~100 m thick and ~10-40 km extents). No pyroclastic features were found. Eight interdigitating units were mapped. These large aspect ratios are similar to ones for basaltic flows and those of Snake River-type rhyolites, and they suggest high effusion rates and high temperatures as well as the presence of well-insulated cooled crusts. Lavas would have overflowed from long fissure vents that had created an extensive volcanic silicic surface body composed of conjugated and interdigitated flows above the north segment of the Torres Trough region.

ABSTRACT: The Paraná Continental Flood Basalt (PCFB) province is one of the largest igneous provinces on Earth, but little is known about the architecture and geochemical characteristics of the lava flows. In this context, a set of borehole samples arranged in an NEE-SWW profile - more than 600 km long - located in the northern PCFB was used to define its chemostratigraphy. In this region, the outcrops are rare, and the volcanic piles are covered by hundreds of meters of sedimentary rocks from the Bauru Sub-Basin. This study aimed to explore the chemostratigraphic column from northern PCFB to improve the understanding of its internal architecture and to characterize the flood basalt sequences by using the petrography and elemental geochemistry of surface and borehole samples. The results showed the presence of the main four magma types, which were essentially distinguished by their P 2 O 5 , TiO 2 and Sr concentrations, as well as Zr/Y ratios, with excellent chemostratigraphy relations.

ResumoEsse estudo consiste na aquisição de dados magnetométricos na região de São Sepé (RS), sul do Brasil, com o propósito de avaliar o potencial de mineralização aurífera de um gabro, a partir de indícios obtidos em prospecção geoquímica de sedimentos de corrente prévios. O gabro denominado Santa Catarina está intrudido no Complexo Metamórfico Vacacaí, uma sequência metassedimentar ,onde foram reconhecidas diversas jazidas de ouro. Nesse estudo, foram realizadas nove linhas de magnetometria terrestre, com 1.200 m de comprimento cada, num total de 10.800 m e 720 estações de leitura. Os resultados sugerem a presença de mineralizações filonianas em fraturas, algumas das quais sujeitas à ação intempérica, com liberação e transporte de ouro por ação fluvial e deposição junto a sedimentos de corrente a jusante do gabro. O mapa de sinal analítico descreve as áreas de alta susceptibilidade magnética dentro de seus limites, coincidentes com lineamentos estruturais nas direções NE e E-W, associadas a à última das quatro fases de deformação regional, possivelmente responsável, tanto por fraturamento e abertura de espaços no gabro, quanto pela remobilização de sulfeto e ouro reconhecidamente descritos em diversas jazidas de ouro no âmbito do Complexo Metamórfico Vacacaí.

AbstractThis study involved magnetometric exploration in the region of São Sepé (RS), southern Brazil, to evaluate the potential for gold mineralization in a gabbro using evidence previously obtained from the geochemical exploration of stream sediments. The gabbro named Santa Catarina is intruded in the Metamorphic Complex Vacacaí, a metassedimentary sequence where several gold deposits were recognized. In this study, nine lines of land magnetometry were performed with 1.200m in length each, a total of 10.800m and 720 reading stations. The results suggest the presence of filonian mineralization in fractures, some of which are subject to weathering action, releasing and transporting gold by water action and depositing it along the stream sediments downstream from the gabbro. The analytical signal map outlines areas of high magnetic susceptibility within its boundaries, coinciding with structural lineaments in the NS and EW directions, associated with the last of the four stages of regional deformation, possibly responsible for both fracturing and opening of spaces in the gabbro, as by remobilization of sulfide and gold recognizably described in several gold deposits within the Vacacaí Metamorphic Complex.

Resumo: Rochas metamáficas, ortoderivadas, caracterizadas por assinaturas do tipo MORB e idades paleoproterozoicas ocorrem na região de Cajamar, centro leste do Estado de São Paulo, associadas a filitos, metarenitos e metacalcários da sequência metavulcano-sedimentar do Grupo São Roque, Faixa Ribeira Meridional. Para essas rochas, a associação de pico metamórfico é representada por actinolita-albita-epidotoclorita, típica de metamorfismo regional da fácies dos xistos verdes. Para melhor caracterizar os parâmetros de pressão e temperatura a que foram submetidas essas rochas, os principais minerais foram analisados por microssonda eletrônica. O metamorfismo não destruiu completamente a mineralogia primária dos basaltos. Microgabros originais e cristais de augita são registrados em muitos lugares, preservados junto à associação metamórfica. Eles permitem estimar as possíveis temperaturas ígneas entre 900 e 1.100oC. O equilíbrio mineral para as paragêneses de pico metamórfico é observado apenas em algumas amostras, dificultando a obtenção desses valores. Além disso, um evento retrometamórfico, resultante de processo de cisalhamentos superimpostos, pode ter afetado este equilíbrio. Onde o equilíbrio é observado, principalmente nas ocorrências da porção sul da área, determinações de temperatura e pressão indicam valores de 487 ± 42oC e 8,2 ± 2,0 kbar, respectivamente, para o pico metamórfico. Outros valores de T e P, menores que os de pico, foram também registrados nas porções central e norte da área e são: 450 ± 68oC e 4,7 kbar e 315 ± 26oC e 3,0 kbar e podem representar pontos onde o equilíbrio mineral não foi atingido ou corresponder a valores referentes ao evento de retrometamorfismo. Considerando esses resultados e, a trajetória metamórfica que eles representam, pode-se supor que o processo de exumação dessa área ocorreu por descompressão quase isotérmica.

Abstract: Metamafic orthoderivate rocks, characterized by MORB signatures and Paleoproterozoic ages occur in Cajamar region central east part of São Paulo State, Brazil. They are associated to philites, metasandstones and metalimestones of São Roque Group volcanosedimentary sequence, Southeastern portion of the Ribeira Belt. For the metamafic rocks the assemblage of metamorphic peak is represented by actinolitealbite-epidote-chlorite typical of regional metamorphism, greenschist facies. For a good characterization of P and T parameters of metamorphism microprobe analyses of main minerals were performed. Metamorphism did not destroy completely the primary mineralogy of original basalt and microgabbro, and augite crystals are registered in many samples, preserved together with metamorphic assemblage, and possibly indicate that the igneous temperatures of crystallization are between 900 and 1.100oC. Textures of mineral equilibrium for the peak paragenesis are not always present in the studied samples. It may be partially caused by the superposition of retrometamorphic events that is a consequence of regional shearing process. Where the equilibrium is reached, chiefly in the occurrences in the south of the area temperature and pressure determinations indicate values close to 487 ± 42oC and 8,2 ± 2.0 kbar, respectively. Other values for P and T lower than that of the peak were also obtained in central and north portion of the area and they are: 450 ± 68oC for T e 4.7 kbar for P and 315 ± 26oC for T and 3.0 kbar for P and may indicate that the mineral equilibrium was not reached in these samples or that they represent the retrometamorphic event. As a concluding remark considering the metamorphic P-T path now obtained one can say that the exhumation process in this area occurred by isothermal decompression.