The Graphical Analysis of Greenschist to Amphibolite Facies Mineral Assemblages in Metabasites |
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| Authors: | HARTE B; GRAHAM C M |
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| Institution: | Grant Institute of Geology, Edinburgh University West Mains Road, Edinburgh EH9 3JW, Scotland |
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| Abstract: | The mineral assemblages of greenschist to amphibolite faciesmetabasites may usually be represented in a system of principalcomponents: CaO-Al2O3-(Fe2O3)-FeO-MgO-Na2O-SiO2-C02-H2O Assemblages co-existing with quartz, ‘albite’, ‘epidote’and a fluid of restricted composition, may be shown by projectionin a CAFM subsystem from ‘epidote’ onto an extendedAFM plane. This projection is analogous to the Thompson projectionfor pelites and is particularly useful in displaying the effectsof Fe/Mg and Al substitution in the silicates as well as incorporatingCaO; it is illustrated by plotting assemblages from the SouthernAlps of New Zealand and the Scottish Highlands and demonstrateschanges occurring with grade in the assemblages. Some commonisograds and facies boundaries are seen to be strongly dependenton bulk rock composition. In some cases MnO must be consideredas an additional component. A model of Psolid=Pfluid, where the fluid is composed of CO2+H2Ois consistent with many greenschist to amphibolite facies metabasicassemblages. Natural assemblages indicate this fluid phase tohave restricted mobility. Theoretical consideration of mineralreactions resulting from increasing XCO2 in conjunction withdata from natural mineral assemblages, leads to the distinctionof five principal types of assemblage which may be expectedas a function of varying XCO2. Recognition of these assemblagetypes provides a useful guide to relative XCO2 during metamorphism. |
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