A NEW PETROGENETIC GRID FOR LOW-GRADE METABASITES

We have used internally-consistent thermodynamic data to present calculated phase equilibria for the system Na2O-CaO-MgO-Al2O3-SiO2-H2O (NCMASH), in the range 0-500-degrees-C and 0.1-10 kbar, involving the phases anorthite, glaucophane, grossular, heulandite, jadeite, laumontite, lawsonite, paragonite, prehnite, pumpellyite, stilbite, tremolite, wairakite, zoisite with excess albite, clinochlore, quartz and pure water. Average activity terms derived from published mineral chemical data were included for clinochlore, glaucophane, prehnite, pumpellyite, tremolite, and zoisite. The new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite, prehnite-actinolite, prehnite-pumpellyite, pumpellyite-actinolite, blueschist and greenschist facies metabasites. The stability fields of mineral assemblages containing prehnite, pumpellyite, epidote, actinolite (+ albite + chlorite + quartz) were analysed in some detail, using activity data calculated from five specific samples. For example, the prehnite-actinolite facies covers a P-T field ranging from about 220 to 320-degrees-C at pressures below 4.5 kbar. The transition from the prehnite-actinolite and pumpellyite-actinolite to greenschist facies occurs at about 250-300-degrees-C at 1-3 kbar and at about 250-350-degrees-C at 3-8 kbar. P-T fields of individual facies overlap considerably due to variations in chemical composition.