Carpholite, sudoite, and chloritoid in low-grade high-pressure metapelites from Crete and the Peloponnese, Greece

Abstract

In the high-pressure metamorphic Phyllite-Quartzite Unit of Crete and the Peloponnese, aluminous metasediments contain assemblages with Fe-Mg-carpholite, sudoite, chloritoid, pyrophyllite, chlorite, garnet, white mica and quartz. Variations of parageneses and mineral reactions in response to varying pressure, temperature and activity of H 2 O have been studied in detail. In Eastern Crete (≃300 ° C, 8 kbar), metapelites with high Mg/Fe 2+ are characterized by sudoite-bearing assemblages with chlorite or pyrophyllite. In rocks with low Mg/Fe 2+ , chlorite+pyrophyllite occur instead, suggesting the (continuous) reaction sudoite+quartz=chlorite+pyrophyllite+H 2 O. Additional carpholite rarely occurs in chlorite-pyrophyllite schists, probably through the hydration reaction chlorite+pyrophyllite+H 2 O=carpholite+quartz. Progress of this reaction requires relatively high a H2O , otherwise chlorite+pyrophyllite persist. In Central Crete (≃350 ° C, 9 kbar), the paragenesis chlorite+pyrophylite is no longer stable and chloritoid appears as a widespread rock-forming mineral, occasionally associated with carpholite, through the reactions carpholite=chloritoid+quartz+H 2 O, chlorite+pyrophyllite+I2O=chloritoid+carpholite+quartz. As in Eastern Crete, reactions involving carpholite proceed only in the case of relatively high a H2O ; the estimated values range between 1.0 and 0.7. This variability of H2O is reflected in AFM diagrams by the compositional shift of the three-phase assemblages carpholite-chloritoid-pyrophyllite and carpholite-chloritoid-chlorite. At lower a H2O , probably the normal case inCrete, no carpholite appears and chloritoid directly forms after chlorite+pyrophyllite=chloritoid+quartz+H 2 O. In this case, no minerals or parageneses diagnostic for high-P/low-T metamorphism appear. The same parageneses as in Central Crete have been observed in Western Crete (400 ° C, 10 kbar) and in the Peloponnese (≃450 ° C, 17 kbar), but with carpholites richer in Mg. For these rocks, AFM three-phase parageneses show fixed mineral compositions, suggesting equilibrium with a hydrous fluid phase of constant composition. The highest grade of metamorphic evolution of metapelites from the Phyllite-Quartzite Unit is reflected by the appearance of almandine-rich garnet in the Peloponnese