Constant Ps-T Amphibolite to Granulite Facies Transition in Agto (West Greenland) Metadolerites: Implications and Applications

• Published in: Journal of Petrology
• Main Authors: GLASSLEY, WILLIAM E., SØRENSEN, KAI
• Format: Text
• Language: English
• Published: Oxford University Press 1980
• Subjects: Articles; Greenland; Isortoq
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/21/1/69; https://doi.org/10.1093/petrology/21.1.69

Within the Isortoq Complex of central West Greenland, dolerite dykes have been metamorphosed in upper amphibolite to garnet granulite facies. Detailed sampling and petrographic study of these dykes demonstrate that the entire sequence of assemblages may occur in individual dykes, with the amphibolite facies assemblages normally restricted to dyke margins, garnet granulite facies rocks occurring in the dyke cores, and pyroxene granulite facies rocks intervening between these two. Electron microprobe analysis of all coexisting phases demonstrates that the progression from amphibolite facies to granulite facies assemblages results in the extraction of edenite and tschermakite components from the amphibole in approximately a 2: 1 ratio. As amphibole breakdown proceeds, systematic changes in element abundance within amphibole occur when reactions are crossed, due to the development of new phases. The algebraic removal of small but systematic compositional variations within the dykes allows derivation of six equilibria, which provide a general model for metamorphism of basaltic rocks under high grade metamorphic conditions. This model takes the form of the idealized assemblage sequences which will develop along (regional) gradients in X H 2 O , temperature and/or solid pressure. Analysis of these equilibria demonstrates that variable silica activity and Fe2+−Mg2+ exchange will modify reaction stoichiometry, will affect the ratio of edenite to tschermakite in amphibole breakdown reactions, and will modify the composition of the product plagioclase. These variations together with variations in water fugacity will locally modify the ideal assemblage sequences, and will explain the complex assemblage relationships observed in metabasic rocks of natural high grade terranes.