Origine métamorphique des grenats des volcanites acides d'âge viséen supérieur dans le nord-est du Massif central français

Garnets of acid volcanics (called « tufs anthracifères ») from the upper Visean, North-East of the French Central Massif, which occur either in metamorphic enclaves or in free xenocrysts, are of metamorphic origin. This result is based upon a comparison of these garnets with those of the neighbouring metamorphic basement of the Forez area. The chemical composition of garnets shows that they first belonged to sillimanite-cordierite bearing rocks. Original metamorphic zoning is preserved in most of the mineral. At the rim, the evolution of the almandine/pyrope ratio in garnets from volcanics is the opposite of that in garnets from the metamorphic basement. This is a consequence of the ultimate magmatic stage of these minerals, during which a plagioclase-biotite reaction rim appears around free garnets. These results suggest the need for reexamining the magmatic origin proposed by several authors for garnets of intermediate and acid volcanics occurring in different areas, these garnets having zoning and chemical characteristics similar to those of the « tufs anthracifères » garnets. Finally, this study of garnets supports the assumption of the origin of acid magmatism from the upper Visean by anatectic melting of continental crust.