Petrology of low-grade high-pressure metapelites from the External Hellenides (Crete, Peloponnese); a case study with attention to sodic minerals

In the Phyllite-Quartzite Unit of the External Hellenides regional differences in the mineralogical compositions of equivalent rocks indicate a gradation in P-T conditions of metamorphism from E-Crete via W-Crete to the Peloponnese. In metasediments of Eastern and Central Crete, sodic minerals appear in rather monotonous assemblages, i.e. albite -I- chlorite and paragonite + chlorite. For this region, the estimated P and T are about 300-350 degrees C, 8-10 kbar, based on parageneses with carpholite, chloritoid or sudoite. Passing to W-Crete (400 + or - 50 degrees C, 10 + or - 3 kbar, using chloritoid-carpholite assemblages), paragonite + chlorite is still present in widespread chloritoid schists, and blue amphiboles (as pseudomorphs) have been observed in paragonite-bearing metasediments. A greater variety of mineral parageneses has been observed in the Peloponnese, where chlorite + paragonite in chloritoid schists and blue amphibole + paragonite in glaucophane schists are widespread. Paragonite-bearing chloritoid-glaucophane schists also occur, which in some cases contain additional sodic pyroxene, or locally garnet. For the Peloponnese, application of thermodynamic data of Holland & Powell (1990) to constrain P and T of metamorphism yields 450 + or - 30 degrees C and 17 + or - 4 kbar. Whole-rock and mineral analyses demonstrate the importance of compositional parameters, besides P and T, on the development of a specific mineral paragenesis. Carpholite is restricted to magnesian rocks ; for intermediate Mg/Fe (super 2+) ratios, the occurrence of assemblages with either chloritoid or glaucophane, with chloritoid and glaucophane, or with Na-pyroxene, is determined by increasing Fe (super 3+) /Al ratio, at given P and T. Garnet occurs in rocks with higher Mn/Fe (super 2+) ratio. Reactions responsible for the prograde changes are identified, and the metamorphic evolution may be connected towards higher grades with the eclogite facies.