PHASE-RELATIONS IN THE GREENSCHIST-BLUESCHIST-AMPHIBOLITE-ECLOGITE FACIES IN THE SYSTEM NA2O-CAO-FEO-MGO-AL2O3-SIO2-H2O (INCFMASH), WITH APPLICATION TO METAMORPHIC ROCKS FROM SAMOS, GREECE

Calculated phase equilibria among the minerals sodic amphibole, calcic amphibole, garnet, chloritoid, talc, chlorite, paragonite, margarite, omphacite, plagioclase, carpholite, zoisite/clinozoisite, lawsonite, pyrophyllite, kyanite, sillimanite, quartz and H2O are presented for t he model system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH), which is re levant for many greenschist, blueschist, amphibolite and eclogite faci es rocks. Using the activity-composition relationships for multicompon ent amphiboles constrained by Will and Powell (1992), equilibria conta ining coexisting calcic and sodic amphiboles could be determined. The blueschist-greenschist transition reaction in the NCFMASH system, for example, is defined by the univariant reaction sodic amphibole + zoisi te = calcic amphibole + chlorite + paragonite + plagioclase (+ quartz + H2O) occurring between approximately 420 and 450 degrees C at 9.5 to 10 kbar. The calculated petrogenetic grid is a valuable tool for reco nstructing the PT-evolution of metabasic rocks. This is shown for rock s from the island of Samos, Greece. On the basis of mineral and whole rock analyses, PT-pseudosections were calculated and, together with th e observed mineral assemblages and reaction textures, are used to reco nstruct PT-paths. For rocks from northern Samos, pseudomorphs after la wsonite preserved in garnet, the assemblage sodic mphibole-garnet-para gonite-chlorite-zoisite-quartz and the retrograde appearance of albiti c plagioclase and the formation of calcic amphibole around sodic amphi bole constrain a clockwise PT-path that reaches its thermal maximum at some 520 degrees C and 19 kbar. The derived PT-trajectory indicates c ooling during exhumation of the rocks and is similar to paths for rock s from the western part of the Attic-Cycladic crystalline complex. Roc ks from eastern Samos indicate lower pressures and are probably relate d to high-pressure rocks from the Menderes Massif in western Turkey.