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Referência Bibliográfica


ZANON, V., PIMENTEL, A. (2008) - Evolution of OIB magmas towards peralkaline silicic compositions: a comparison between Terceira and Pantelleria islands (Poster). 84º Congresso Nazionale - Societá Geologica Italiana, Sassari (Sardenha), Itália, 15 - 17 Setembro.

Resumo


The islands of Terceira (Azores, Portugal) and Pantelleria (Italy) are two world-class examples of peralkaline volcanism. These islands are located in geodynamic settings dominated by active rift zones. These two islands show large similarities in terms of chemical composition. The samples from Terceira constitute a trend from transitional basalts to comendites and pantellerites, showing the existence of a minor compositional gap in the intermediate fields. In the case of Pantelleria, the dataset shows a marked bimodal trend with a large compositional gap between mafic and silicic rocks, towards pantellerites. Terceira basalts are more mafic (higher Mg, Ni, Cr) and less aluminiferous than the equivalent from Pantelleria. Among the evolved compositions the presence of two different evolutive trends (comendites and pantellerites) suggests a different role played by Fe, Ti and Al, during evolutive processes. Pantelleria evolved lavas are more enriched in REE and other trace element with the exception of U, Ta, Pb and Pr. The mineral assemblage present in the two islands is also very similar, consisting of olivine + clinopyroxene ± plagioclase + oxides in the mafic compositions and alkali feldspar (anortoclase/sanidine) ± plagioclase + clinopyroxene + olivine (fayalite) + oxides with rarer aenigmatite and arfvedsonite in the evolved rocks.

 

The origin of these peralkaline magmas can be modelled starting from poorly evolved basalt with different contemporaneous processes occurring within the same magma reservoir located at depth. Polybaric fractional crystallization starting from 5 kbars seems to be responsible for the evolution towards peralkaline compositions, while isobaric in situ crystallization and mixing of parental alkali/transitional basalts can account for the evolution towards intermediate compositions and then changing crystallization fractionation conditions towards trachytes. During these processes it is remarkable the change in the behaviour of Sr and Rb which behave as a perfect compatible/incompatible pair in the during the polybaric basalt-rhyolite fractionation process, inverting their behaviour in the isobaric basalt-trachyte in situ crystallization process, due to the abrupt stop in feldspar and apatite crystallization and the biotite formation. Relatively low QFM buffering conditions (QFM-2) plays a main role in determining the liquid line of descent and to allow the formation of high-silica compositions.

Observações


Anexos