Abstract

The within-plate volcanic island of Madeira is considered a hot-spot originating from a mantle plume. Lavas range from basanites to mugearites. The majority of primitive lavas are nepheline normative and characterized by strongly fractionated rare-earth elements, where (La/Yb)n = 12-25, and by pronounced normalized negative K but positive Nb-Ta anomalies, similar to high 238U/204Pb (HIMU) type basalts. Madeira basalts are highly depleted in terms of Sr- and Nd-isotope ratios, which are negatively correlated and define a trend steeper than the mantle array. Pb-isotope ratios are moderately high (206Pb/204Pb up to 19.582), with almost all samples plotting close to the Northern Hemisphere Reference Line. Radiogenic isotope systematics demonstrate that the mantle source of Madeira basalts is the result of a multicomponent mixture where HIMU and enriched mantle I (EM I) plume components are diluted by a depleted component (DMM) which represents more than 85% of the source volume. The presence of DMM is considered to result from entrainment of asthenospheric depleted material by the plume. There is trace-element evidence for liquids having equilibrated with garnet and amphibole, the presence of the latter mineral reflecting lithospheric involvement in the genesis of the magmas. Such was done via assimilation, by plume (plus entrained asthenosphere) magmas, of melts generated from oceanic lithosphere and leaving amphibole as residue.

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