Petrologia dos diabásios da Região de Ubatuba, SP

• Ano de defesa: 2010
• Autor(a) principal: Silva, Fábio Peres da
• Orientador(a): Duarte, Beatriz Paschoal
• Banca de defesa: Almeida, Julio Cesar Horta de , Valente, Sérgio de Castro , Medeiros, Sílvia Regina de
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade do Estado do Rio de Janeiro
• Programa de Pós-Graduação: Programa de Pós-Graduação em Análise de Bacias e Faixas Móveis
• Departamento: Centro de Tecnologia e Ciências::Faculdade de Geologia
• País: BR
• Palavras-chave em Português: Diques toleíticos da Serra do Mar; Magmatismo mesozóico; Suites de alto-TiO2
• Palavras-chave em Inglês: Serra do Mar tholeiitic dikes; Mesozoic magmatism; High TiO2 suite
• Área do conhecimento CNPq: CNPQ::CIENCIAS EXATAS E DA TERRA::GEOCIENCIAS::GEOLOGIA::PETROLOGIA
• Link de acesso: http://www.bdtd.uerj.br/handle/1/7051

The diabase dykes of the study area (Ubatuba region) consist, predominantely, of NE-SW (N40-50E) subvertical bodies which follow the main trend of fractures in the area. This pattern is concordant with that of the high-TiO2 suite of the Serra do Mar Dykes Swarm in southeast coast of Brazil. Subordinately, these dikes follow a NW-SE or a NE-WSW orientation. The thickness of those bodies varies from 0,3 to 55 meters, although most of them have thickness between 1,5 and 8 meters. Regarding the forms of the fracture-conduit, the great majority of the dikes are emplaced within straight fractures. However, features like steps, bridges, stubs, bifurcations, stagger and dykes in zigzag are also observed. The studied diabases have black color, per times varying in red tones, as a reflection of the alteration degree. Most often, diabases are aphanitic or fine grained and, commonly, display porphyritic texture with plagioclase phenocrysts. In some cases, white carbonate-filled milimetric amigdals can be observed. The mineralogical composition of those diabases is little variable, consisting, essentially, of plagioclase, Ca-rich clinopyroxene (augite) and/or Ca-poor clinopyroxene (pigeonite). Acessory minerals are, commonly, olivine, opaque mineral(s) and apatite. Considering the lithogeochemical study, results indicate that, of the 27 samples selected for analysis of major and trace elements, 22 are tholeiitic diabases. The tholeiitic affinity of the great majority of the samples (22 in a population of 27) is corroborated by the presence of quartz and hypersthene (and absence of nepheline) in the CIPW norm, which lead to the classification of these diabases as quartz- tholeiites. In three of the studied samples, the presence of normative olivine and hypersthene (and absence of normative quartz) includes them in an olivine- tholeiite group. Of all the 25 samples of tholeiitic diabases, 24 were accounted to a high-TiO2 suite (TiO2> 2%peso) and only 1 was accounted to a low TiO2 one (TiO2 <2%peso). On the basis of La/Yb ratios and its relations to MgO, it was possible to discriminate four distinct high-TiO2 suites (suites 1, 2, 3 and 4). Of these, only suites 2 and 3 include a minimum amount of samples necessary for the evaluation of evolutionary process, and results indicate, for both suites, that fractional crystallization with change of fractional assembly as a more plausible evolutionary process. The discrimination of mantle sources was made exclusively on the basis of incompatible trace element ratios of samples considered as representative of parental liquids of the four discriminated suites. Results indicate that all four suites were generated by partial melting of an enriched source, which points to the consideration of lithospheric components contribution.