CONTRASTING FRACTIONATION TRENDS IN COEXISTING CONTINENTAL ALKALINE MAGMA SERIES - CANTAL, MASSIF-CENTRAL, FRANCE

Two coexisting series of strongly alkaline (basanite-tephrite-phonolit e) and weakly alkaline (alkali basalt-trachyandesite-trachyte-rhyolite ) lavas occur in the Cantal volcano (French Massif Central). The paren tal magmas appear to be derived by variable degrees of partial melting of a common asthenospheric mantle source. Derivative trachyandesites and feldspathoid-bearing tephrites show depletions and enrichments in trace elements which indicate that they have been generated by broadly similar fractionation processes, related to the removal of a mineral extract, from the parental alkali basalts and basanites respectively, dominated by, olivine, clinopyroxene, amphibole, apatite and titanifer ous magnetite +/- plagioclase. In the most extreme differentiates (tra chytes, rhyolites and phonolites) fractionation of zircon, sphene and alkali feldspar exerts a major control an the trace element characteri stics of the magmas. Sr-Nd-Pb isotopic data for the two magma series s uggest the importance of combined assimilation-fractional crystallizat ion processes (AFC) within the lower crust in their evolution. Modelli ng demonstrates that the AFC process amplifies the original compositio nal differences between the parent magmas. After similar to 55% crysta llization in the strongly alkaline series and 65% in the weakly alkali ne series crustal contamination ceases, although fractional crystallis ation continues beyond this point to produce the most evolved differen tiates (phonolites and rhyolites). This may reflect progressive sealin g of the lower-crustal magma reservoirs. The tendency of the magmas to follow over- or under-saturated evolutionary trends, producing rhyoli tic and phonolitic residua respectively, appears to be established at the early stages of magmatic differentiation, reflecting inherent diff erences in the compositions of the parental magmas.