A new petrogenetic grid for low-grade metabasites

Abstract 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° 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, prehniteactinolite, 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° C at pressures below 4.5 kbar. The transition from the prehnite-actinolite and pumpellyite-actinolite to greenschist facies occurs at about 250–300° C at 1–3 kbar and at about 250–350° C at 3–8 kbar. P-T fields of individual facies overlap considerably due to variations in chemical composition.     

The Occurrence of Garnet in the Granulite-Facies Terrane of the Adirondack Highlands

Based on the presence or absence of almandine in charnockiticand metabasic rocks, Buddington (1963) recognized two garnetisograds in the northwestern Adirondacks which were inferredto represent a general increase in grade of regional metamorphismfrom northwest to southeast and east. ACF plots of analyzed Adirondack rocks demonstrate that thepresence or absence of garnet is primarily controlled by thebulk chemistry of the rocks. In metabasites, garnet forms partof the type assemblage of the almandine-amphibolite facies inwhich it developed. The assemblage becomes unstable in the granulitefacies where quartz is present. Garnet does not occur in themore calcic metabasites. In charnockites, garnet develops concomitantlywith orthopyroxene in the aluminous varieties of this rock typeand not in the calcic varieties. The two may occur intimatelyinterlayered. The present definition of the granulite facies, however, doesnot cover clinopyroxene-almandine-bearing assemblages reportedin rocks of diverse composition from the proximity of anorthositemassifs. Reaction rims prove that within the granulite faciesa reaction occurs in rocks with excess silica by which the typomorphicorthopyroxene-plagioclase association is replaced by the higherdensity clinopyroxene-almandine-quartz association. To accommodate this assemblage a fourfold subdvision of thegranulite facies is proposed. Four subfacies are obtained bytwo intersecting boundaries: (1) PH₂O-T controlled dehydrationreactions separating pyroxene-granulite from hornblende-granulitesubfacies, and (2) the anhydrous, P_load-T controlled reactionwhich separates subfacies with orthopyroxene-plagioclase assemblagesfrom those with clinopyroxene-almandine assemblages.