Estratigrafia dos derrames da Província Vulcânica Paraná na região oeste do Rio Grande do Sul, Brasil, com base em sondagem, perfilagem gamaespectrométrica e geologia de campo

The Serra Geral Formation has a wide distribution in the western region of Rio Grande do Sul (200 x 200 km) and consists of six tholeiitic lava flows which constitute the Alegrete facies. This number of lava flows was evidenced through geological mapping integrated with lithological and scintillometric description of 19 boreholes drilled for groundwater by the Geological Survey of Brazil (CPRM) in the years 1983-1986. The general thickness of the Alegrete facies is <100 m, locally reaching 300 m in the borehole UR-13. The Botucatu desert dunes were partially buried by the Mata Olho (basalt) flow, followed by the Catalán (andesite), Cordillera, Muralha, UR-13 and Coxilha (basaltic andesites) flows. The scintillometric measurements are typical for each one of lava flows, varying between 45-120 cps and act as a stratigraphic guide, specially when integrated with field work and lithochemistry. This study enables the systematic geological mapping of each lava flow on a regional scale, with great significance for the comprehension of the geology of the Serra Geral Formation.A Formação Serra Geral ocupa uma área de cerca de 200 x 200 km na região oeste do Rio Grande do Sul e consiste de seis derrames de lavas toleíticas, que constituem a fácies Alegrete. Esse número de derrames foi constatado em mapeamento geológico, integrado com a descrição geológica e com a perfilagem cintilométrica de 19 furos de sonda efetuados para água subterrânea pelo Serviço Geológico do Brasil (CPRM) nos anos 1983-1986. A espessura média da fácies Alegrete é inferior a 100 m, atingindo até 300 m no furo UR-13. As dunas da Formação Botucatu foram parcialmente soterradas pelo derrame Mata Olho (basalto), seguido pelos derrames Catalán (andesito), Cordillera, Muralha, UR- 13 e Coxilha (andesitos basálticos). As medidas cintilométricas são típicas de cada derrame,variando entre 45-120 cps, e servem como guia estratigráfico, especialmente com o uso integrado com as observações de campo e litoquímica. Este estudo apresenta uma forma de abordagem integrada entre diferentes ferramentas geológicas aplicadas à estratigrafia de basaltos, possibilitando o mapeamento geológico sistemático de cada derrame em escala regional, com grande significado para o entendimento da geologia da Formação Serra Geral

Coeficientes de distribuição de elementos-traço em clinopiroxênio e plagioclásio de sills basálticos da Formação Serra Geral, Brasil

The use of an electron microprobe (EPMA) integrated with inductively coupled plasma mass spectrometer laser ablation (LA-ICP-MS) for the determination of major and trace elements in igneous minerals advances the knowledge of the distribution coefficients “D”in cores, intermediate portions and rims of crystals. Crystals of clinopyroxene and plagioclase of tholeiitic sills from the Serra Geral Formation, Paraná Basin, Brazil, display positive correlations between many major and trace elements. Plagioclase has core concentrations of SiO2 and Na2O wt.% which increase to the rim and Al2O3 and CaO wt.% decreasing from the core to the rim, corroborated by similar variations of distribution coefficients of Sr and Eu as a result of substitution of these compatible elements for Na in the structure of plagioclase. Clinopyroxene has the decreases in MgO, Al2O3 and CaO wt.% core to the rim and decreased D of Ni, Zn and V, and to increased concentration of FeOtotal wt.% core to the rim related increase in D of Sc, Y andHREE. This investigation is innovative because it presents the study of distribution coefficients oftrace elements with spatial control of the analyses, taking into account the effect of inclusions, fractures and compositional zoning.A utilização de técnicas de microanálise (microssonda eletrônica - EPMA e ablação a LASER em espectrômetro de massa com plasma indutivamente acoplado - LA-ICP-MS) de elementos maiores, traços e terras raras em minerais ígneos amplia o conhecimento do coeficiente de distribuição “D” em porções de núcleo, intermédio e borda do cristal. Cristais de clinopiroxênio e plagioclásio de sills toleíticos da Formação Serra Geral, Bacia do Paraná, Brasil, possuem correlações positivas entre muitos elementos maiores e traços. O plagioclásio possui em seu núcleo concentrações de SiO e Na O (peso %) 2 2 que aumentam para a borda e concentrações de Al O e CaO (peso %) que diminuem do núcleo para a 2 3 borda do cristal. Isto é corroborado pelas mesmas variações dos coeficientes de distribuição de Sr e Eu em decorrência de substituições destes elementos, nas posições estruturais do Na no plagioclásio. O clinopiroxênio mostra decréscimo nos conteúdos de MgO, Al O e CaO (peso %) do núcleo para a borda, 2 3 diminuição dos D de Ni, Zn e V e aumento das concentrações de FeO (peso %) do núcleo para a borda total relacionados com o aumento do D de Sc, Y e ETR pesados. Este trabalho é inovador, pois apresenta o estudo de coeficientes de distribuição de elementos traços com controle espacial das análises, controlando- se desta forma, a influência de inclusões, fraturas e zoneamentos composicionais

Tectonic evolution of the southern margin of the Amazonian craton in the late Mesoproterozoic based on field relationships and zircon U-Pb geochronology

New U-Pb zircon geochronological data integrated with field relationships and an airborne geophysical survey suggest that the Nova Brasilândia and Aguapeí belts are part of the same monocyclic, metaigneous and metasedimentary belt formed in the late Mesoproterozoic (1150 Ma-1110 Ma). This geological history is very similar to the within-plate origin of the Sunsás belt, in eastern Bolivia. Thus, we propose that the Nova Brasilândia, Aguapeí and Sunsás belts represent a unique geotectonic unit (here termed the Western Amazon belt) that became amalgamated at the end of the Mesoproterozoic and originated through the reactivation of a paleo-suture (Guaporé suture zone) in an intracontinental rift environment. Therefore, its geological history involves a short, complete Wilson cycle of ca. 40 Ma. Globally, this tectonic evolution may be related with the final breakup of the supercontinent Columbia. Mafic rocks and trondhjemites in the northernmost portion of the belt yielded U-Pb zircon ages ca. 1110 Ma, which dates the high-grade metamorphism and the closure of the rift. This indicates that the breakup of supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia at ca. 1.1-1.0 Ga and that the Western Amazon belt was formed during the accretion of the Arequipa-Antofalla basement to the Amazonian craton

Mid amphibolite facies metamorphism of harzburgites in the Neoproterozoic Cerro Mantiqueiras Ophiolite, southernmost Brazil

Valuable information is retrieved from the integrated investigation of the field relationships, microstructure and mineral compositions of harzburgites from the Neoproterozoic Cerro Mantiqueiras Ophiolite. This important tectonic marker of the geological evolution of southernmost Brazilian Shield was thoroughly serpentinized during progressive metamorphism, because the oldest mineral assemblage is: olivine + orthopyroxene + tremolite + chlorite + chromite. This M1 was stabilized in mid amphibolite facies - 550-600ºC as calculated from mineral equilibria. No microstructural (e.g. ductile deformation of olivine or chromite) or compositional (e.g. mantle spinel) remnant of mantle history was identified. A metamorphic event M2 occurred in the low amphibolite facies along 100 m-wide shear zones, followed by intense serpentinization (M3) and narrow 1-3 m-wide shear zones (M4) containing asbestos

Stratigraphy of amethyst geode-bearing lavas and fault-block structures of the Entre Rios mining district, Paraná volcanic province, southern Brazil

The Entre Rios mining district produces a large volume of amethyst geodes in underground mines and is part of the world class deposits in the Paran&#225; volcanic province of South America. Two producing basalt flows are numbered 4 and 5 in the lava stratigraphy. A total of seven basalt flows and one rhyodacite flow are present in the district. At the base of the stratigraphy, beginning at the Chapec&#243; river bed, two basalt flows are Esmeralda, low-Ti type. The third flow in the sequence is a rhyodacite, Chapec&#243; type, Guarapuava subtype. Above the rhyodacite flow, four basalt flows are Pitanga, high-Ti type including the two mineralized flows; only the topmost basalt in the stratigraphy is a Paranapanema, intermediate-Ti type. Each individual flow is uniquely identified from its geochemical and gamma-spectrometric properties. The study of several sections in the district allowed for the identification of a fault-block structure. Blocks are elongated NW and the block on the west side of the fault was downthrown. This important structural characterization of the mining district will have significant consequences in the search for new amethyst geode deposits and in the understanding of the evolution of the Paran&#225; volcanic province

Paralavas in the Cretaceous Paraná volcanic province, Brazil - A genetic interpretation of the volcanic rocks containing phenocrysts and glass

ABSTRACT The occurrences of glassy rocks containing long and curved phenocrysts in the Paraná volcanic province, South America, are here interpreted as paralavas. The large number of thin (0.1-0.5 m) dikes and sills of glassy volcanic rocks with hopper, hollow or curved, large crystals of clinopyroxene (up to 10 cm), plagioclase (up to 1 cm), magnetite and apatite are contained in the core of thick (>70 m) pahoehoe flows. They are strongly concentrated in the state of Paraná, coincident with the presence of the large number of dikes in the Ponta Grossa arch. These rocks were previously defined as pegmatites, although other names have also been used. A paralava is here interpreted as the product of melting of basaltic rocks following varied, successive processes of sill emplacement in high-kerogen bituminous shale and ascent of the resultant methane. As the gas reached the lower portion of the most recent lava flow of the volcanic pile, the methane reacted with the silicate and oxide minerals of the host volcanic rock (1,000 ºC) and thus elevated the local temperature to 1,600 ºC. The affected area of host rock remelted (possibly 75 wt.%) and injected buoyantly the central and upper portion of the core. This methane-related mechanism explains the evidence found in the paralavas from this volcanic province, one of the largest in the continents