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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica


ALMEIDA M.H., GASPAR J.L., QUEIROZ G. (2004) - A fonte mantélica na região dos Açores: constrangimentos impostos pelas características geoquímicas de rochas vulcânicas da ilha Graciosa. 4ª Assembleia Luso Espanhola de Geodesia e Geofísica, Figueira da Foz, Fevereiro (Poster).

Resumo


The present work consists of the analysis of an eruptive sequence within a well-defined volcano-stratigraphic framework from Graciosa Island. Three geological units were considered: the Volcanic Complex of Serra das Fontes, the Volcanic Complex of Serra Branca and the Volcanic Complex of Vitória – Vulcão Central. The rocks show an alkalic affinity and range from basalts and basanites to trachytes in the Vulcão Central, with a more limited compositional spread in the other complexes. The petrographic and chemical variation suggest that the magmatic evolution of these rocks was strongly influenced by fractional crystallisation processes with separation of olivine, plagioclase, pyroxene and oxides in the more primitive members and of amphibole, biotite, alkaline feldspars and apatite in the intermediate and more evolved magmas. The fractionation of amphibole is found mainly to have occurred on Vulcão Central rocks. Generally, Graciosa's rocks show enrichment in incompatible elements, whose ratios indicate partial melting of a non primordial, fairly homogeneous, mantle region, with F values between 3 and 7%, in the presence of residual garnet. The isotopic signatures of Sr and Nd reflect a mantle source impoverished in Rb and Nd for a long period of its history. Additionally, the analysis of Pb isotopes suggests a rise of the U/Pb ratio in the sampled mantle in a recent period of its history. The enrichment pattern of incompatible elements normalised to primitive mantle show clear positive anomalies of Nb and Ta, which are exclusive to basalts with HIMU affinity. The analysis of the isotopic systems suggests that the source mantle region of these rocks contains the DMM component and the HIMU component in reduced proportions. The contribution of the enriched components, EM I or EM II, is not clear.

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