Evidence of a Temperature-dependent 'Blueschist' to 'Eclogite' Transformation in High-pressure Metamorphism of Metabasic Rocks

Textures in the blueschist metabasic rocks of the island ofSyros, Greece, indicate a reaction forming omphacite and garnetfrom earlier glaucophane and epidote. A balanced ‘wholerock’ reaction can be written using the phase compositionsand modal mineralogy of the products observed. This reactionis continuous in P-T space if Mg and Fe²⁺ are independent components.The available thermodynamic data suggest that the ‘eclogite’(garnet plus omphacite) assemblage is favoured by higher temperatures. It cannot, however, be conclusively shown that the rocks wentthrough an up-temperature interval of metamorphism. The reactionseen could be promoted by reduced silica, or water activity.The details of the reaction textures and mineral zonation patternsshow furthermore that chemical equilibrium was not maintainedthroughout the rock development. The reaction textures differin detail in different samples. In those samples in which thetextures are most consistent with maintained chemical equilibriumhowever, the mineral zonation patterns do imply up-temperaturemetamorphism.