Navegando por Assunto "Nefelina Sienito Brejinho"

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Geologia, petrografia e geocronologia do Plúton Alcalino Brejinho - nordeste do Piauí / noroeste do Ceará
(Universidade Federal do Pará, 2001-06-25) BARBOSA, RITA DE CÁSSIA DE OLIVEIRA; GORAYEB, Paulo Sérgio de Sousa; http://lattes.cnpq.br/4309934026092502; https://orcid.org/0000-0003-1877-9756
The present work reports geological, petrographic and geochronological data of the alkaline pluton Nefelina Sienito Brejinho. The Nefelina Sienito Brejinho is inserted in the northwest portion of the Borborema Province, geographically located on the border of the states of Piauí and Ceará, approximately 60 km southeast of the city of Parnaíba. The pluton has approximate dimensions of 16 km long and 8 km wide and has a roughly triangular shape, elongated in the NNE direction. It is a typically igneous body, where its intrusive character was evidenced in gneisses and amphibolites of the Granja Complex, with cooling edges, xenoliths and small zones of hornfels. In general, the rocks along the plutonic body present magmatic feeding structures defining a preferential orientation in the NNE-SSW direction marked by tubular alkaline feldspar crystals, prismatic pyroxenes, lamellar biotites and, in some cases, this foliation is also evidenced by amphiboles and titanite. The petrographic and field studies allowed identifying three petrographic facies internally in the pluton: Pyroxene Microclimate Syenite, Pyrogen Nepheline Syenite and Leucosienites. The first two are dominant, and the other has restricted occurrences. Mineralogically the pluton presents a variation, qualitatively and quantitatively. Rocks in general consist essentially of microclimate, nepheline and aegirine-augite as essential minerals. In smaller percentages occur biotite, amphibole (arfvedsonite and riebeckite) and titanite. Accessory to apatite, opaque and fluorite. With regard to textural aspects, trachytoid textures are predominant in the pluton, defined by crystals of alkaline feldspar, pyroxene, biotite, titanite and amphibole. Subordinately, simplectitic intergrowth textures between alkaline and nepheline feldspar are present. This textural aspect suggests subsolvus crystallization for Nefelina Sienito Brejinho. The geochronological studies, carried out using the Rb-Sr methodology in eight rock samples, provided the Nefelina Sienito Brejinho with an age of 545 ± 11 Ma, with an initial ratio of 0.7173 ± 0.0002 and MSWD of 1.32. Based on this age, interpreted as the minimum crystallization age, it is suggested that the syenitic pluton was placed in the Lower Cambrian, during the distensive tectonics that occurred in this portion of NE Brazil. This tectonic was marked by the implementation of the Ubajara-Jaibaras System and by plutonism and volcanism, with an alkaline tendency, such as the Meruoca, Mucambo, Pajé, Anil, Morrinhos and São Paulo granites, among others; and the alkaline volcanism of the Parapuí Suite.
Acesso aberto (Open Access)
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.