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

Within the Isortoq Complex of central West Greenland, doleritedykes have been metamorphosed in upper amphibolite to garnetgranulite facies. Detailed sampling and petrographic study ofthese dykes demonstrate that the entire sequence of assemblagesmay occur in individual dykes, with the amphibolite facies assemblagesnormally restricted to dyke margins, garnet granulite faciesrocks occurring in the dyke cores, and pyroxene granulite faciesrocks intervening between these two. Electron microprobe analysisof all coexisting phases demonstrates that the progression fromamphibolite facies to granulite facies assemblages results inthe extraction of edenite and tschermakite components from theamphibole in approximately a 2: 1 ratio. As amphibole breakdownproceeds, systematic changes in element abundance within amphiboleoccur when reactions are crossed, due to the development ofnew phases. The algebraic removal of small but systematic compositionalvariations within the dykes allows derivation of six equilibria,which provide a general model for metamorphism of basaltic rocksunder high grade metamorphic conditions. This model takes theform of the idealized assemblage sequences which will developalong (regional) gradients in X_H2O, temperature and/or solidpressure. Analysis of these equilibria demonstrates that variablesilica activity and Fe²⁺–Mg²⁺ exchange will modify reactionstoichiometry, will affect the ratio of edenite to tschermakitein amphibole breakdown reactions, and will modify the compositionof the product plagioclase. These variations together with variationsin water fugacity will locally modify the ideal assemblage sequences,and will explain the complex assemblage relationships observedin metabasic rocks of natural high grade terranes. ^*Present address: Instituttet for Teknisk Geologi, Danish Technical University, Building 204, DK–2800 Lyngby, Denmark