Petrology of garnet ? cordierite ? sillimanite gneisses from the El Tormes thermal dome, Iberian Hercynian foldbelt (W Spain)

Abstract

The core of the El Tormes thermal dome, situated in the central part of one of the main metamorphic belts of the Iberian Peninsula, is formed by garnet-cordierite-biotite-sillimanite pelitic gneisses. These rocks, that very often are cut by minor intrusions of Al-rich S-type granites, are metatexitic gneisses in which there exists garnet showing different stages of resorption and transformation into an aggregate of cordierite±plagioclase±biotite. The garnet, mantled and corroded mainly by cordierite, has never been found to occur in contact with the prismatic sillimanite of the matrix, thus indicating that the continuous reaction Gr+Sill+Q = Cd has taken place. The presence of corroded biotite inside the garnet-rimming cordierite of the aggregates as well as inside the cordierite of the matrix, which usually includes remmants of sillimanite, indicates that the continuous reaction Bi+Sill+Q = Cd+FK+H2O has occurred too. Therefore, a realistic net reaction for these aggretates should be represented by the univariant, at a given \(X_{{\text{H}}_{\text{2}} {\text{O}}} \), equilibrium: Biotite+Sillimanite+Garnet+Quartz = Cordierite+K-feldspar+H2O (1)