Magmatismo basáltico miocênico de Pedro Avelino (RN): mecanismo de colocação e efeito termal na Bacia Potiguar
The cenozoic magmatism on the extreme northeastern of Brazil occurs as basic plugs, necks and flows with alkaline to sub-alkaline affinity; being referred in literature as Macau – Queimadas magmatism. One of these bodies, focus of this study, is located on the surroundings of Pedro Avelino, 150 km t...
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| Formato: | bachelorThesis |
| Idioma: | pt_BR |
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Universidade Federal do Rio Grande do Norte
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| Endereço do item: | https://repositorio.ufrn.br/handle/123456789/34397 |
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| Resumo: | The cenozoic magmatism on the extreme northeastern of Brazil occurs as basic plugs, necks and flows with alkaline to sub-alkaline affinity; being referred in literature as Macau – Queimadas magmatism. One of these bodies, focus of this study, is located on the surroundings of Pedro Avelino, 150 km to the west of Natal/RN. It is composed by basalts and diabases of the Serra Preta body, with 40Ar/39Ar whole rock ages of 14,1 ± 0,7 Ma, intrusive on Jandaíra (carbonate sequence with predominance of limestones) and Açu (siliciclastic sequence with arenites and pelitic levels) Formations of the Potiguar Basin. Previous publications interpret this occurrence as basaltic flows on the central portion and plugs on the extremities, controlled by a fault on its east border. The reported work characterizes the geometry, emplacement mechanism, modal composition and textures of these rocks and the thermal effect on its host rock. Remote sensing products, field observations and transmitted light microscopy were used. The basic rocks occur as in-situ outcrops and collapsed blocks which, in some areas, cover-up the contacts. They are olivine basalts and nepheline diabases with fine to medium and, commonly, microporphyritic textures. The matrix, crypto to microcrystalline, is composed by plagioclase and grains of augite, forsterite, opaque minerals and interstitial glass; being common amygdales filled by zeolites; There is also the occurrence of late veins interdigitated with the cryptocrystalline basaltic matrix, composed by nepheline, anorthoclase, red biotite, opaques, zeolites, clinopyroxene (Ti-augite) and apatite needles. On the host rock, the thermally affected limestones show carbonatic matrix partially or completely modified by static recrystallization, turning into fine to medium grained marbles. On the arenites and siltites, it is noticed the turbid aspect of the feldspars, neoformation of polygonal mosaics of quartz, fibroradial growth of chalcedony and interstitial glass; the group being denominated buchite. Cavities partially filled by calcite, quartz, chalcedony are observed, being product of dissolution and precipitation processes after the thermal event. These rocks are massive with extreme compaction, becoming hard to fragment. In geometric terms, it is remarkable the tectonic influence on the shape and alignment of the igneous bodies observed in orbital sensors imageries. For the Serra Preta, its geometry is similar to a inverted Y, interpreted as a dike system, where the main conduct would be the result of the filling of NNW dextral transcurrences and, the secondary conducts, NE second order fractures. This arrangement is consistent in time, kinematic and structures with the neotercicary-pleistocene deformation described in literature, with N-S horizontal compression (σ1) and E-W extension (σ3). Beyond that, the presence of conduct-fractures is corroborated by the orientation of phenocrysts of olivine and plagioclase microlites, indicating a NNE to NNW magmatic flow. |
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