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Contrasting Fractionation Trends in Coexisting Continental Alkaline Magma Series; Cantal, Massif Central, France
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1995
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Abstract Two coexisting series of strongly alkaline (basanite-tephritephonolite) and weakly alkaline (alkali basalt-trachyandesite-trachyte-rhyolite) lavas occur in the Cantal volcano (French Massif Central). The parental magmas appear to be derived by variable degrees of partial melting of a common asthenospheric mantle source. Derivative trachyandesites and feldspathoidbearing 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 titaniferous magnetite±plagioclase. In the most extreme differentiates (trachytes, rhyolites and phonolites) fractionation of zircon, sphene and alkali feldspar exerts a major control on the trace element characteristics of the magmas. Sr-Nd-Pb isotopic data for the two magma series suggest the importance of combined assimilation-fractional crystallization processes (AFC) within the lower crust in their evolution. Modelling demonstrates that the AFC process amplifies the original compositional differences between the parent magmas. After ∼55% crystallization in the strongly alkaline series and 65% in the weakly alkaline series crustal contamination ceases, although fractional crystallization continues beyond this point to produce the most evolved differentiates (phonolites and rhyolites). This may reflect progressive sealing of the lowercrustal magma reservoirs. The tendency of the magmas to follow over- or under-saturated evolutionary trends, producing rhyolitic and phonolitic residua respectively, appears to be established at the early stages of magmatic differentiation, reflecting inherent differences in the compositions of the parental magmas.