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Estudo petrológico e geoquímico do magmatismo transicional na Bacia Lusitânica
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Resumo(s)
Apresentam-se e discutem-se as características petrológicas e geoquímicas do magmatismo ocorrido há cerca de 145 Ma na Bacia Lusitânica, de que resultaram dois alinhamentos sub-meridianos de rochas intrusivas entre as latitudes de Rio Maior e Soure. Este ciclo magmático, intermédio dos três que afectaram as margens Sul e Oeste da Ibéria no Mesozóico, associados à abertura do Oceano Atlântico, teve características transicionais, distinguindo-se do primeiro, sub-alcalino, toleítico, ocorrido há cerca de 200Ma, e do terceiro, alcalino, datando do Cretácico Superior. Apresentam-se, para rochas deste ciclo transicional, três novas idades 40Ar/39Ar que, juntamente com idades U-Pb recentemente publicadas, sugerem uma duração de cerca de 6 Ma para este ciclo magmático, muito inferior à anteriormente considerada (~32 Ma). Os novos dados atribuem também, contrariamente ao que se supunha, idade semelhante às rochas de ambos os alinhamentos. A variabilidade química destas rochas, demonstrada por análises de rocha total de um conjunto representativo de amostras, com determinação de elementos maiores e traço, é atribuída a variações nas percentagens de fusão parcial do manto e aos efeitos de processos de cristalização fraccionada e de alteração hidrotermal. Análises isotópicas de Sr e Nd sugerem fortemente que algumas destas rochas terão sofrido contaminação, possivelmente por via hidrotermal, de um agente com 87Sr/86Sr_0.7075. Um bom candidato a agente contaminante seriam fluidos hidrotermais isotopicamente equilibrados com os evaporitos e dolomitos da Formação de Dagorda, do Hetangiano.
Petrological and geochemical characteristics of magmatism occurred about 145 My ago in the Lusitanian Basin, Portugal, are presented and discussed. The intrusive rocks studied here belong to two sub-meridian alignments located between the latitudes of Rio Maior and Soure. This magmatic cycle, the intermediate one of the three that affected South and West Iberian margins during the Mesozoic, associated with the opening of Atlantic Ocean, had transitional character, distinctive from the first, sub-alkaline, tholeiitic, occurred about 200 My ago, and from the third, alkaline, of Upper Cretaceous age. Three 40Ar/39Ar new ages for rocks of this transitional cycle are presented here, witch together with recently published U-Pb ages, suggest a total duration of about 6 My for this magmatic cycle, much shorter then the previously accepted (~32 My). According with the new data the rocks of both alignments are contemporaneous, in opposition to previous ideas. The chemical variability of this rocks, inferred from chemical analysis of major and trace elements from a representative set of samples, is considered to reflect variation in the weight fraction of mantle partial melting and processes of crystal fractionation and hydrothermal alteration. Sr and Nd isotopic analysis suggest that some of this rocks has been contaminated, possibly by hydrothermal circulation of an agent with 87Sr/86Sr_0.7075. A good candidate for contamination agent are fluids isotopically equilibrated with the evaporates and dolomites of Dagorda Formation, of Hetangian age.
Petrological and geochemical characteristics of magmatism occurred about 145 My ago in the Lusitanian Basin, Portugal, are presented and discussed. The intrusive rocks studied here belong to two sub-meridian alignments located between the latitudes of Rio Maior and Soure. This magmatic cycle, the intermediate one of the three that affected South and West Iberian margins during the Mesozoic, associated with the opening of Atlantic Ocean, had transitional character, distinctive from the first, sub-alkaline, tholeiitic, occurred about 200 My ago, and from the third, alkaline, of Upper Cretaceous age. Three 40Ar/39Ar new ages for rocks of this transitional cycle are presented here, witch together with recently published U-Pb ages, suggest a total duration of about 6 My for this magmatic cycle, much shorter then the previously accepted (~32 My). According with the new data the rocks of both alignments are contemporaneous, in opposition to previous ideas. The chemical variability of this rocks, inferred from chemical analysis of major and trace elements from a representative set of samples, is considered to reflect variation in the weight fraction of mantle partial melting and processes of crystal fractionation and hydrothermal alteration. Sr and Nd isotopic analysis suggest that some of this rocks has been contaminated, possibly by hydrothermal circulation of an agent with 87Sr/86Sr_0.7075. A good candidate for contamination agent are fluids isotopically equilibrated with the evaporates and dolomites of Dagorda Formation, of Hetangian age.
Descrição
Tese de mestrado em Geologia (Geoquímica, Mineralogia e Petrologia), apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2010
Palavras-chave
Bacia Lusitânica Magmatismo mesozóico Magmatismo transicional Alteração hidrotermal Teses de mestrado - 2010