The Graphical Analysis of Greenschist to Amphibolite Facies Mineral Assemblages in Metabasites

Abstract

The mineral assemblages of greenschist to amphibolite facies metabasites may usually be represented in a system of principal components: CaO–Al2O3–(Fe2O3)–FeO–MgO–Na2O–SiO2–CO2–H2O Assemblages co-existing with quartz, ‘albite’, ‘epidote’ and a fluid of restricted composition, may be shown by projection in a CAFM subsystem from ‘epidote’ onto an extended AFM plane. This projection is analogous to the Thompson projection for pelites and is particularly useful in displaying the effects of Fe/Mg and Al substitution in the silicates as well as incorporating CaO; it is illustrated by plotting assemblages from the Southern Alps of New Zealand and the Scottish Highlands and demonstrates changes occurring with grade in the assemblages. Some common isograds and facies boundaries are seen to be strongly dependent on bulk rock composition. In some cases MnO must be considered as an additional component. A model of Psolids=Pfluid, where the fluid is composed of CO2+H2O is consistent with many greenschist to amphibolite facies metabasic assemblages. Natural assemblages indicate this fluid phase to have restricted mobility. Theoretical consideration of mineral reactions resulting from increasing Xco2, in conjunction with data from natural mineral assemblages, leads to the distinction of five principal types of assemblage which may be expected as a function of varying XCo2. Recognition of these assemblage types provides a useful guide to relative XCo2 during metamorphism.