Navegando por Autor "LOPES, Elem Cristina dos Santos"

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Acesso aberto (Open Access)
Estudos hidrogeoquímicos e geofísicos na região da Braquidobra de Monte Alegre-PA
(Universidade Federal do Pará, 2005-04-11) LOPES, Elem Cristina dos Santos; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983
The Monte Alegre brachyanticline is located in the central-western Pará and is one of the most striking structures of the Paleozoic Amazon sedimentary basin. The dome outcropping surface is elliptical with axes of 30 and 20 km trending NESW and NW-SE, respectively. At the present erosion level, rocks of the Ererê, Barreirinha, Curiri, Oriximiná, Faro, Monte Alegre, Itaituba and Alter do Chão formations, as well as the Penatecaua diabase are exposed. Thermal springs with temperatures from 29 to 37ºC issue out of the Ererê Formation. The present study focuses on the physico-chemical characterization of surface and groundwater that occur within the dome and on the interaction with their mineral environment. In addition, the thermal waters are also characterized isotopically and their subsurface temperatures estimated with basis on the silica and Na-Ca-K geothermometers. Gravimetric and resistivity methods were applied aiming at detecting potential sources that could account for the heating of the thermal waters. The Piper diagrams show a wide chemical variation for both the surface and groundwater which spread over the bicarbonate, calc-sulfate and sodic-chloride fields. The thermal waters are chemically more homogeneous and fall on the bicarbonate and sodic-chloride fields. This variability reflects the composition of the rocks through which the waters migrate, whose components result mainly from hydrolysis and redox reactions involving pyrite and sulfate minerals. Most water samples are in equilibrium with kaolinite as deduced from activity diagrams constructed at 25ºC, 1 atm and at quartz saturation. The thermal water samples cluster near the boundary kaolinite-sericite, whereas a few samples of surface waters that drain the Itaituba Formation plot near the line separating the kaolinite and leonhardite stability fields. The samples of the Menino Deus thermal spring collected in October/2002 reached equilibrium with sericite but failed to equilibrate with paragonite despite their Na/K ratios being higher than unity. Furthermore, most water samples are saturated with quartz. During the dry season, sample FT-27 and those from the Menino Deus thermal spring presented higher silica contents causing them to equilibrate with pyrophyllite. Isotopic data show that the Monte Alegre thermal waters have a meteoric origin with δ18O and δD values coincident with the global meteoric water line, although slightly enriched in deuterium. The excess of deuterium varies seasonally, being recorded values of 11,8-14,8 ‰ in the dry season and 4-9,5 ‰ in the wet season. Sub-surface temperatures for the thermal waters estimated with the silica geothermometer yield mean values of 71°C (wet season) and 83°C (dry season). Average water circulation depths ranging from 1560 m (wet season) to 1900 m (dry season) were calculated assuming a geothermal gradient of 30°/km and using the estimated sub-surface temperatures. After infiltrating into the ground, these meteoric waters are heated and rise back to the surface through a channel way network, particularly NE-SW and N-S-trending faults which truncate the brachyanticline flanks. Gravimetric surveys allowed to infer the presence of a lacolith-like body at a depth of about 1.3 km. This body is thicker close to the central portion of the brachyanticline. Contact relationships between sedimentary units and the laccolith could also be defined. Likewise, fractures and faults could be mapped at depth whose traces are visible on SRTM radar images. Resistivity profiles did not identify any thermal anomaly in the area, but they confirm the lithological diversity of the geological units as well as faults that may have served as conduits to the fluids.
Acesso aberto (Open Access)
U-Pb (LA-ICP-MS) of detrital zircon and whole rock Nd and geochemical constraints on the provenance, depositional age and tectonic setting of the metasedimentary Piriá Basin, northern Brazil: implications for the evolution of the Gurupi Belt
(Universidade Federal do Pará, 2016-06) LOPES, Elem Cristina dos Santos; KLEIN, Evandro Luiz; MOURA, Candido Augusto Veloso; LUCAS, Fernando Rodrigo dos Anjos; PINHEIRO, Bruno Luís Silva; RODRIGUES, Joseneusa Brilhante; SIMAS, Margarete Wagner
The Piriá Basin (Piriá Formation) is a hemi-graben shaped basin that developed over Precambrian rocks of the Gurupi Belt. The lithological content comprises four interfingered lithofacies: (1) arkose and greywacke with pelite layers, (2) laminated siltstones and pelites, (3) arkose with hummocky stratification, and (4) oligomictic conglomerate. This sequence was formed in alluvial fans (conglomerate), and fluvial systems (arkose, greywacke, siltstones and pelites) that were established and evolved during the migration of the subsidence. The sedimentary sequence underwent anquimetamorphism and very weak tectonic deformation. U-Pb analyses of detrital zircon set the maximum depositional age at 591 Ma and indicate several sediment sources, that range in age from the Neoproterozoic to the Archean. The main sources are from the Rhyacian, which is the main period of continental crust formation in the São Luís cratonic fragment and the basement of the Gurupi Belt. Neoproterozoic sources were important in the eastern segment of the basin. Ages from unknown sources in the region so far have also been recorded. Combined, the U-Pb zircon data, the whole-rock geochemical and Sm-Nd results, and the petrographic information revealed proximal felsic to intermediate provenance, in addition to recycled sedimentary sources. As a whole, our data suggest that the Piriá Formation was deposited in a post-orogenic basin related to the final stage of evolution of the Brasiliano cycle of orogenies, which built up the Gurupi Belt.