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)
The important garnet resorption near the anatectic granitic veins implies that this process is favoured by a decrease in \(X_{{\text{H}}_{\text{2}} {\text{O}}} \), this factor being otherwise buffered by the reaction (1) assemblage.
The most probable P-T path, assuming these conditions, consistent with the AFM projection of the former (inferred) and present assemblages in the aggregates and in the matrix, implies a decrease in P coeval with a decrease in T (Fig. 4, path 2).
The most reliable P-T determination for the final stage of garnet breakdown through reaction (1), based on the coexistence of the seven phase assemblage garnet — cordierite — biotite — sillimanite — plagioclase — potash feldspar — quartz plus melt, gives 695° C, 4.3 kbar, \(X_{{\text{H}}_{\text{2}} {\text{O}}} \)= 0.5, The maximum pressure for this process, obtained from the garnet — plagioclase equilibrium, is 6.5±1 kbar at the same temperature.
The estimates of the T for the garnet core-garnet included biotite pairs are consistently lower, ca. 550° C, than those obtained for the garnet rim-biotite in aggregates, ca. 645° C, or garnet rim-adjacent cordierite pairs, ca. 695° C.
It may, therefore, be supposed that, during their evolution these rocks underwent first an increase in T and then, during the last stages, as garnet and biotite brokedown, a decrease in P and T. This represents an uplift of the core of El Tormes dome under high grade amphibolite to low pressure granulite facies conditions, accompanied by a process of partial melting with local decrase in \(X_{{\text{H}}_{\text{2}} {\text{O}}} \). It is suggested, from mineral growth-deformation relationships, that this process took place during the late hercynian deformation phases, P-3 or doming stage.
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Gil Ibarguchi, J.I., Martinez, F.J. Petrology of garnet — cordierite — sillimanite gneisses from the El Tormes thermal dome, Iberian Hercynian foldbelt (W Spain). Contr. Mineral. and Petrol. 80, 14–24 (1982). https://doi.org/10.1007/BF00376731
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DOI: https://doi.org/10.1007/BF00376731