Navegando por Autor "LEMOS, Ronaldo Lima"

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Petrografia, geocronologia e significado tectônico do Nefelina Sienito Brejinho: extremo noroeste da Província Borborema
(2011-09) GORAYEB, Paulo Sérgio de Sousa; BARBOSA, Rita de Cássia de Oliveira; MOURA, Candido Augusto Veloso; LEMOS, Ronaldo Lima
In the extreme northwest of the Borborema Province was identified an alkaline massif named Brejinho Nepheline Syenite, emplaced in Paleoproterozoic gneisses of the Granja Complex. The investigation involved the mapping, detailed petrographic analyses and Rb-Sr geochronology, allowing to the reconstruction of the evolutionary history of this pluton in the tectonic context of the Borborema Province. Five petrographic facies have been mapped and the mineralogical and textural/structural analyses performance suggest the processes of fractional crystallization, with strong control of gravitational action and liquid immiscibility in the history of magmatic crystallization of the massif. The age of 554 ± 11 Ma was obtained by Rb-Sr wholerock geochronology and was interpreted as the minimum age for crystallization and emplacement of Brejinho Nepheline Syenite in the late Neoproterozoic. The tectonic context of the alkaline magmatism may be related to the extensional event responsible for the formation of Jaibaras Graben in western Ceará, as well as the emplacement of a number of granitic plutons whose ages range from 530 to 590 Ma. Similar situation was identified in the north of the Paleozoic Amazon Basin involved the Maicuru Alkaline-Ultramafic-Carbonatite Complex (589 Ma) emplaced in the Paleoproterozoic gneissic basement of the Amazoniam Craton. The geological framework places the Brejinho Nepheline Syenite after the strike-slip tectonics represented by Santa Rosa Shear Zone (Transbrasiliano Lineament), and before the formation of the Parnaíba Basin. Therefore, this alkaline magmatism may be interpreted as an important record of the rift phase that heralded the installation of the basin in the early Paleozoic. The identification of this alkaline massif, so far without any similar occurrence in the Borborema Province, opens new perspectives for research in basement of Parnaiba Basin, considering the tectonic and metallogenic importance that represents this type of magmatism.
Acesso aberto (Open Access)
Petrologia do maciço alcalino de Peixe-Goiás
(Universidade Federal do Pará, 1983-12-21) LEMOS, Ronaldo Lima; ALBUQUERQUE, Carlos Alberto Ribeiro de
The Alkaline Fish Massif consists of a nepheline syenite gneiss body, leucocratic, foliated and banded. Foliation is accentuated at the edges and incipient or absent at the center, while banding is restricted to the edges. The Massif is surrounded by a narrow band of quartz syenite and granite gneisses, in contact with pre-Serra da Mesa Group metasediments, predominantly pelitic (graphite-sillimanite micaschist and sillimanite-quartz micaschist). The nepheline syenite gneiss consists essentially of nepheline, mesopertite, peristerite and microcline, with biotite being the mafic mineral and, more rarely, magnetite. Its agpaitic coefficient is typical of miaskitic rocks, and its mineralogy is also of this type. The different textural types of nepheline show variation in the Si/Al ratio, while the Na/K ratio is approximately constant. Peristerite has composition An4.0-5.1 -An0.5-2.8 and, in a narrow band of the edge, it is An7.0-10.7 -An0.4-4.0. The , microcline and potassium phase of mesopertite (microcline) have composition Or8 3.0-9 4.4, while the sodium phase of mesopertite is predominantly peristerite (An4.0-5.1 -An0.7-2.8) and sometimes it is albite (An0.4-2.6). Biotite is of the lepidomelane-siderophyllite type. The composition of nepheline syenite gneiss, similar to the minimum point in the Ne-Ks-Qz diagram, the variation in the ratio, Si/Al. of nepheline, typical of nepheline crystallized from a nepheline-like syenite magma under plutonic conditions, as well as the trends of the elements Si, Al, Na and K and the K/K + Na ratio in differentiation diagrams, which are coincident with the variation observed in series of alkaline igneous rocks, indicate that the nepheline syenite gneiss resulted from a nepheline syenitic magma, generated in the lower crust or upper mantle, in a tectonically stable environment. During the ascent of this magma, the increase in the geothermal gradient caused fusions in rocks of the crust, generating magmas of quartz syenitic and granitic composition. The nepheline syenite body, the quartz syenite and granite belt and the pre-Serra da Mesa Group metasediments were later affected, together with the sediments of the Uruaçú Development Belt (Serra da Mesa Group), by a regional medium pressure type metamorphism. in the amphibolite facies, with a temperature of 650 ± 30°C and a pressure of 6 ± 0.5 kb. The metamorphism took place in the presence of a fluid phase rich in H2O and CO2, in addition to S03, F and P originated by the decomposition of late-magmatic minerals from nepheline syenite, rich in volatile components, which had great importance during the metamorphism in the formation and recrystallization of minerals and, in local processes, of autometasomatism. This Massif presents striking analogies with the alkaline complexes of Blue Mountain, Bigwood and Darkainle.
Acesso aberto (Open Access)
Vivianite and siderite in lateritic iron crust: an example of bioreduction
(2007-02) LEMOS, Vanda Porpino; COSTA, Marcondes Lima da; LEMOS, Ronaldo Lima; FARIA, Mario Sergio Gomes de
Lateritic iron crust (LIC) samples from Padauari (AM) were analysed by XRD, optical microscopy and SEM-EDS. The equilibrium of iron minerals (IM) was studied using Eh-pH diagram. It was shown that the minerals of the LIC are goethite (a-FeOOH), vivianite [Fe3(PO4)2.8H2O] and siderite (FeCO3). Carbonate grains are a solid solution of FeCO3-MnCO3. The LIC presents textures and structures of dissolution of IM. The siderite and vivianite are stable from Eh =-0.3 to 0.0 V and pH=5.0-7.5. These results indicate that vivianite and siderite are products of bioreduction through biogenic dissolution of IM, the new conditions of ecosystems of the Amazon region.