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Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, U.K.
Department of Geology, Birkbeck College, 7-15 Gresse Street, London W1P 1PA, U.K.
Continental alkaline volcanics of the French Massif Central range in age from 65 Ma to 3450 years BP and can be divided by geography and age into 20 separate areas. There is no correlation between the age of an area and the geographic locality.
Two magma series are present: a silica-undersaturated basanite-tephrite-phonolite series and an alkali basalt-trachyandesite-trachyte-rhyolite series, the evolved members of which are silica-saturated. Rare nephelinites are also found. Primitive magmas of both main series may be derived from 2%–20% partial melting of a mantle enriched with light rare-earth elements with variable amounts of garnet in the residue; nephelinites were formed from a source which was isotopically and chemically distinct.
Fractional crystallization of observed phases accounts for the diversity of intermediate and evolved products. Amphibole fractionation in basalts at depth causes the trend towards silica saturation, while alkali feldspar fractionation dominates the final stages of crystallization. Removal of small quantities of sphene and zircon explains trace-element depletions in trachytes, phonolites and rhyolites.
Up to 30% crustal contamination has occurred in the evolved magmas but contamination is generally absent from their basaltic parents. Sr, Nd and Pb isotope data indicate that the most probable contaminant is undepleted meta-sedimentary granulite-facies lower crust. Variations in Sr and Nd isotope ratios also constrain petrogenetic hypotheses such as amphibole fractionation and magma mixing.
Mineralogical disequilibrium, compositional banding and emulsification textures indicate extensive magma mixing. Some flows are homogeneous hybrid lavas in which only the unusual composition and rare mineral disequilibria indicate the mode of formation.
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  M. Wilson and H. Downes Tertiary-Quaternary intra-plate magmatism in Europe and its relationship to mantle dynamics Geological Society, London, Memoirs, 2006; 32: 147 - 166. [Abstract] [PDF]
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