Aluminium in coexisting pyroxenes as a sensitive indicator of changes in metamorphic grade within the Mafic Granulite Terrane of the Fraser Range, Western Australia

Abstract

Three pairs of coexisting pyroxenes of mafic granulites from each of two locations 100 km apart show large chemical differences, especially in Al, Fe, Mn, Ti and Na. Al content of the pyroxenes at the higher pressure locality is more nearly independent of Al of the host rock than are the pyroxenes from the lower pressure locality. All the data confirm that although no significant difference in temperature has emerged, there was a large difference in pressure between the two localities. Al is found to be a more effective discriminant of metamorphic conditions than {Mathematical expression}. As the three pairs of pyroxenes cover a wide range of Fe at each locality, the close relationship of Al and Na (and of Ca-tschermak and jadeite) to Fe becomes evident. This shows that a normalizing procedure should be adopted before comparing localities with different Al, Mn, Ti, Na and other elements or derived components such as jadeite and Ca-tschermak.