Abstract

Eighteen potash felspars from the potash felspar gneisses of the Connemara migmatite belt [western Ireland] have been partially analysed and the associated plagioclase optically determined. Comparisons have been made with seventeen similar felspar pairs from the Galway granite. Albite components are expressed graphically in an attempt to illustrate sodium distribution between associated felspar phases. On such a plot it is possible to contrast the 'magmatic' felspars of the Galway granite with those of the migmatitic gneisses; the separation of the two fields being a function of total rock chemistry. Crystallization temperatures have been determined by the two felspar method and the range 460 to 480 degrees C. is thought to represent retrograde amphibolite conditions in the migmatitic gneisses, while petrographic evidence supports 550 degrees C. as the temperature of amphibolite facies metamorphism in these rocks. Triclinicity measurements have also been made on the analysed potash felspars.

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