Chemical variations of muscovites from the Sierra de Guadarrama metamorphic area, Sistema Central, Spain

Muscovites in metapelites belonging to the Chloritoid, Staurolite, Kyanite, Sillimanite-K feldspar and Sillimanite-K feldspar-Cordierite zones of the Sierra de Guadarrama metamorphic area, have a chemical composition close to the end member Ms (mean value around Ms 92); nevertheless, they show definite variations in Al2O3, K2O and Na2O contents and, to a lesser extent, in FeO and MgO. These chemical variations (i. e., a progressive decrease of celadonite content, a increase followed by a decrease in paragonite solid solution towards compositions close to the endmember muscovite) may be regarded as in a close relationship with metamorphic grade in the upper metamorphic zones (Chloritoid, Staurolite and Kyanite zones), but the corresponding variations in the remaining lower zones (that is, a slight increase in Fe and Mg coupled with a slight increase in AlVI and Ti, and more or less abrupt changes in Na and K contents) can only be explained considering the interference of anatectic phenomena, which modificate the normal trends due to changes in metamorphic grade. As a consequence of these observations, it has been concluded that muscovite composition in metapelites can only be used as an indicator of metamorphic grade in conditions where partial melting phenomena are absent.