Abstract
Metadolerites occurring in the vicinity of Agto (within the Isortoq Complex of central West Greenland) have been metamorphosed in upper amphibolite to garnet granulite facies. Detailed sampling and petrographic study of these demonstrates that the entire sequence of assemblages may occur in individual dykes, with the amphibolite assemblages normally restricted to dyke margins, garnet granulite facies rocks occurring in the dyke cores, and pyroxene granulite facies rocks intervening between these two. Bulk rock chemical analyses demonstrate that the dykes are chernically homogeneous with respect to all oxides except Al2O3 and Na2O. Nevertheless, variation in the latter two oxides is small, and demonstrably is not a parameter which significantly controls assemblage development.
Highlights
Electron microprobe analysis of all coexisting phases demonstrates that the progression from amphibolite facies assemblages to pyroxene granulite facies assemblages results in the extraction of edenite and tschermakite components from the amphibole in approximately a 2:1 ratio, to generate clinopyroxene, orthopyroxene and plagioclase
The algebraic removal of small but systematic compositional variations within the dykes allows derivation of six equilibria (Table l), which provide a general model for metamorphism ofbasaltic rocks under high grade metamorphic conditions
Analysis of these equilibria demonstrates that variable silica activity and Fez+ - MgZ+ exchange will modify reaction stoichiometry, will affect the ratio of edenite to tschermakite in amphibole breakdown reactions, and will modify the composition of the produet plagioclase
Summary
Electron microprobe analysis of all coexisting phases demonstrates that the progression from amphibolite facies assemblages to pyroxene granulite facies assemblages results in the extraction of edenite and tschermakite components from the amphibole in approximately a 2:1 ratio, to generate clinopyroxene, orthopyroxene and plagioclase. Garnet granulite assemblages are generated by the further reaction of these amphibole components with orthopyroxene to generate clinopyroxene, garnet and plagioclase. The algebraic removal of small but systematic compositional variations within the dykes allows derivation of six equilibria (Table l), which provide a general model for metamorphism ofbasaltic rocks under high grade metamorphic conditions
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