Grenats des andésites et des rhyolites de Slovaquie,origine des grenats dans les séries andésitiques

The investigated garnets from the andesitic series (andesites — rhyolites) occured as megacrysts. It was shown they were not formed in a late magmatic period and they are not xenocrysts coming from the metamorphic basement. Better, they indicate high pressure crystallisation phases in the eclogitic facies, during which the almandine content of the garnets increases with the rising degree of acidity of the basalt-eclogitic melt.The electron microprobe analysis showed that an isobaric evolution could manifest by a normal Mg-Fe zonation, whilst a polybaric crystallisation would result in an inverse Fe-Mg zonation. Two types of magmatic garnets are shown: one that has a low Ca content and is formed under medium pressure in the presence of H₂O, and the other with higher Ca content, formed under high pressure in the absence of H₂O.