Cordierite Growth Textures and the Conditions of Genesis and Emplacement of Crustal Granitic Magmas: the Velay Granite Complex (Massif Central, France)

Abstract

Three morphological types of cordierite are distinguished in the Velay migmatites and granites: (1) anhedral cordierite mimetic on older biotite–sillimanite melanosomes; (2) prismatic cordierite; (3) cordierite–quartz aggregates (nodules, cockades and dendrites). Structural relationships show that all types of cordierite grew under conditions above the granite solidus. Prismatic cordierite, a first-formed phase in some granites, appears to result from equilibrium biotite dehydration melting under low-P conditions (peritectic cordierite, also possibly cotectic), and to reflect genesis of shallow crustal melts formed from the surrounding metamorphic series. Cordierite–quartz aggregates, overprinting the fabric of granites and migmatites, formed from earlier biotite-bearing assemblages. Phase assemblages and chemical data suggest two coupled reactions: (1) breakdown of biotite in the presence of melt producing cordierite; (2) dissolution of feldspars releasing Al and producing quartz. The data suggest that these aggregates result from diffusion-controlled growth at the melt interface in a highly crystallized mush, at αAl2SiO5 ≤ 1, as a result of decompression of biotite-bearing magmas equilibrated at depth, but which entered the cordierite phase volume during their final stage of emplacement. Cordierite formation in the Velay complex indicates the en masse ascent of the whole melt-bearing volume (migmatites and granites), which led to a diapir.