Navegando por Assunto "Basaltos"

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Alteração supergênica dos basaltos do Porto Franco-Grajaú-MA
(Universidade Federal do Pará, 1985-10-04) VAQUERA VARGAS, Antônio; OLIVEIRA, Nilson Pinto de
Acesso aberto (Open Access)
Basaltos almofadados da Suíte Ofiolítica Morro do Agostinho: registros de fundo oceânico na porção centro-oeste do Cinturão Araguaia
(2013-12) MIYAGAWA, Luciana de Jesus Penha Pamplona; GORAYEB, Paulo Sérgio de Sousa
Along the domain of low metamorphic grade (Midwestern portion) of the Araguaia Belt, outcrop dozens mafic and/or ultramafic bodies of ophiolitic suites. An example is the Morro do Agostinho Ophiolite Suite, in the surroundings of the city of Araguacema (Tocantins) that configures a small isolated body that sustains the Morro do Agostinho and is tectonically emplaced in meta-sandstones, slates and phyllites of Couto Magalhães Formation (Tocantins Group). The Morro do Agostinho Ophiolite Suite consists of serpentinized peridotites, basalts and ferriferous cherts. The association of basalts is characterized by a significant layer of submarine pillow lavas overlying serpentinized peridotites. The basalts were classified into massive and hialobasalts types. Massive basalts are homogenous, with intersertal texture that is essentially composed of thin laths and acicular crystals of plagioclase and clinopyroxene and present rare crystals of olivine, chalcocite and chalcopyrite. Hyalobasalt shows ultra-fast cooling features (quenching) as spherulites, radial, acicular and skeletal crystals of plagioclase and clinopyroxene with forms of swallowtail. Geochemical data, including (La/Yb)n and (La/Sm)n ratios less than 1, indicate a subalkaline-tholeiitic MORB nature of the basalts. These rocks show that during the geological evolution of Araguaia Belt, there was an oceanization stage of Araguaia Basin, with litospheric mantle's exposure, followed by lava's extravasation and sedimentation of cherts and banded iron formation in deep ocean environment. After sedimentary filling of the Couto Magalhães Formation (Tocantins Group), the tectonic inversion, which led to the exhumation of the ophiolitic bodies, especially along thrust surface, with fragmenting and tectonic mixing with supracrustal rocks, accompanied by regional metamorphism under conditions of low green schist facies. The Morro do Agostinho Ophiolite Suite therefore represents a small allochthonous fragment of a well-preserved ocean mantle/crust lithosphere, similar to the others examples found in the Araguaia Belt who marks the Araguaia Basin's moment of oceanization during the Neoproterozoic.
Acesso aberto (Open Access)
Magmatismo basáltico na sucessão sedimentar do Grupo Tucuruí - Cinturão Araguaia, Nordeste do Pará
(Universidade Federal do Pará, 2012-04-04) DUTRA, Alessandra de Cássia dos Santos; GORAYEB, Paulo Sérgio de Sousa; http://lattes.cnpq.br/4309934026092502
The main objective of this work was the study of magmatic and sedimentary environments associated to the rocks of the Neoproterozoic Tucuruí Group, and discusses their relation in the evolution of the Araguaia Belt. This unit is exposed in the region of Tucuruí, northeast state of Pará, northern Brazil, in transition zone between the Amazonian craton and Araguaia belt and is described as a volcano-sedimentary succession composed by basaltic volcanic deposits including occasional intrusions of diabase and basalt breccia intercalated with amalgamated arkosic arenite and siltstones interpreted as coastal deposits with layers oriented in the NNE-SSW dipping down to SE. The Tucuruí overthrusting fault affected this succession toward west, causing fracturing and fluid percolation. The siliciclastic deposits are organized and thickening sucession that include two facies associations: 1) Shoreface formed by arkosic with low angle cross-stratification; and 2) Storm influenced shoreface the comprise the arkosic sandstones with parallel lamination , siltstone with lamination parallel, laminated siltstone truncated by wave and siltstone with hummocky cross-stratification. These associations suggest processes for easy transport and sedimentation connected to a shallow marine environment, following the foreshore-shoreface zones influenced by storms. The petrographic analysis of arkosic arenite and siltstones reveals compositional and textural immaturity of these rocks, suggesting proximity of source area composed of igneous mafic to intermediate igneous or metamorphic rocks. The sedimentary sucession was submitted to eodiagenetic environment and mesodiagenetic conditions. The volcanic deposits are composed of mesocratics and hipocrystalline rocks, whose mineralogy consists of labradorite (An60), augite, Fe and Ti oxides and sulfides and apatite accessory. The basalt flows are aphanitic composed of amygdaloidal basalts, located on the bottom and top of them and massive basalts, located in the central portions of the basaltic flows. In type amygdaloidal, the amygdales are filled by chlorite, quartz, zeolite and green cryptocrystalline material and are embedded in the matrix or intergranular intersertal, the latter being the main plots of textural massive basalts. The diabases of the sills are phaneritic and have coarse grained, intergranular texture and micrographic intergrowth. The basaltic breccias, occur between basaltic flows and / or in contact with the sedimentary sequence and is composed of fragments of basalt showing flow structures involved in matrix grain size of sediment silt, locally intercalated with irregular concentrations of zeolites and epidote, indicating thus the concomitant processes of sedimentation and volcanism. The geochemical analysis carried out on 14 samples of volcanic rocks, using inductively coupled plasma spectrometry with mass spectrometry (ICP-MS) reveals concentrations of SiO2 ranging from 46-48% and alkalis (Na2O + K2O) between 2-4% predominantly. The concentrations of CaO, and TiO2 Fe2O3 are high and ranges from 8-128%, 12-16%, 2-3% and respectively, while the concentration of MgO is moderate and will vary between 5-7%. As for the trace elements have values of Cu, Cr and Co in moderate concentration between 102-216ppm, 160-560 ppm, 44-52 ppm, since the values of elements such as Ba, Rb and Sr vary between 115-350 ppm, 5 -34 ppm and 145-424 ppm, respectively, and ratios (La/Yb)n between 2-5. This framework is consistent with compositional affinity tholeiitic basalts.The behavior of trace elements is typical of continental basalt provinces, where the rare earth elements show weak fractionation. Anomalies are observed, which were negative in the case of Eu and Sr, indicating fractionation of plagioclase and positive in the case of Ti, indicating a high similarity with suites magmatic source, the reasons La/Yb and La/Nb, both greater than 1, allow us the derivation of enriched mantle sources, with a partial melting of subcontinental lithosphere mantle. Thus, can summarize that the Tucuruí group represents the portion preserved of coastal segment influenced by storm in a rift or foreland basin, with contribution for igneous or metamorphic rocks as source area marked for high topography and by physical weathering dominant that achieved during its formation by effusive volcanism with geochemical signatures of tholeiitic affinity and continental during the final stages of the geological evolution of the Araguaia Belt. Moreover, the fracturing and percolation of hydrothermal fluids from veins observed in clusters recorded rocky GT, coupled with the discrete mineralogical transformation that occurs in the basalts are understood to influence the later stages of deformation and regional metamorphism of the geological evolution of the Araguaia Belt.
Acesso aberto (Open Access)
Pesquisas mineralógicas e geoquímicas para a distinção de basaltos intrusivos e extrusivos no Vogelsberg Alemanha Federal
(Universidade Federal do Pará, 1977-07) OLIVEIRA, Elson Paiva de; SCHWAB, Roland Gottlieb
In a drill hole of 450m made in the central part of the volcanic complex Vogelsberg, West Germany many authors found an alternating sequence of alkali olivinbasalts and tholeiites, consisting of 7 series with a total of 293m. Radiometric age dating suggests that two of the alkali olivinbasalts are intrusive. One of them, serie 3, however shows traces of weathering on its surface. The present study includes chemical analyses of major and trace elements (Ni, Co, Cu, Zn, Li and Be) as well as mineralogical investigation by means of X-ray diffraction and optial methods. The objective of these analyses was the chemical and mineralogical characterization of the weathering horizons and of the possible effects of differentiation in the units considered intrusive. The commom association of haematite, montmorillonite and quartz fragments in the weathered horizons, as well as the increase of Ni, Co, Cu, H2O and Fe2O3 and the decrease of Li, Be, FeO, MgO and CaO from the bottom to the top of one of the "intrusive units" in serie 3, shows that this unit was exposed to the earth's surface and therefore cannot be considered intrusive. The other alkali olivinbasalt in serie 5 shows incipient gravitative differentiation effects but witho ut significant textural variations. On the other hand there is amigdaloidal texture on its top suggesting also an extrusive nature.