Petrology of low-grade high-pressure metapelites from the External Hellenides (Crete, Peloponnese) A case study with attention to sodic minerals
Theye, Thomas; Seidel, Eberhard
European Journal of Mineralogy Volume 3 Number 2 (1991), p. 343 - 366
published: Apr 18, 1991
manuscript accepted: Dec 4, 1990
manuscript received: Jan 26, 1990
ArtNo. ESP147050302008, Price: 29.00 €
Abstract 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°C, 8-10 kbar, based on parageneses with carpholite, chloritoid or sudoite. Passing to W-Crete (400 ± 50°C, 10 ± 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 ± 30°C and 17 ± 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/Fe2+ ratios, the occurrence of assemblages with either chloritoid or glaucophane, with chloritoid and glaucophane, or with Na-pyroxene, is determined by increasing Fe3+/Al ratio, at given P and T. Garnet occurs in rocks with higher Mn/Fe2+ ratio. Reactions responsible for the prograde changes are identified, and the metamorphic evolution may be connected towards higher grades with the eclogite facies.