The mégantic unstrusive complex, québec: A study of the derivation of silica-oversaturated anorogenic magmas of alkaline affinity

The Cretaceous Mégantic intrusive complex of southern Québec contains early noritic gabbrodiorites which represent cumulates from crustally contaminated hawaiite to syenite magmas. Whole rock and mineral chemistry, as well as textural evidence, indicate that post-cumulus recrystallization and reaction were important, and most of the amphibole and biotite are thought to have formed in this way. A younger plutonic quartz-syenite ringdyke may not be cogenetic with the gabbro-diorites sice it lacks orthopyroxene. It may, however, be cogenetic with basaltic to riebeckite granite dykes. Fractionation of olivine, plagioclase, aluminous clinopyroxene, and minor Ti-magnetite from critically undersaturated alkali basaltic magmas generated hawaiitic magmas. The development of quartzbearing mugearitic and syenitic residua from the hawaiites can best be modelled by fractionation of amphibole, plagioclase, olivine, oxides, and apatite. Attempts to model fractionation using observed phenocrysts (including clinopyroxene) were unsuccessful. Amphibole fractionation is interpreted to have taken place through a reaction with still-porous, higher-temperature cumulates on the walls of the magma chamber. The plutonic syenites probably represent alkali feldspar cumulates from the residual syenitic melts. Magnesian calc-alkaline lamprophyres exhibit olivine to phlogopite reaction textures, are enriched in Cr, Ni, K, Rb, Nb, Y, Zr, and Si relative to the basaltic dykes, yet have similar incompatible element ratios. Their relation to the basalts is problematical. The late biotite-granite core to the complex is identical to typical White Mountain granites and may have formed as an anatectic cap on rising, fractionating, mantlederived magmas.