Mineral and rock compositions of mafic enclaves in the Morton Gneiss

Two major mineralogical types of mafic enclaves (amphibolite and pyroxene amphibolite) have been characterized in the Morton quartzofeldspathic gneiss. Some amphibolite contains plagioclase megacrysts with An (sub 66) cores and An (sub 43) rims. One indication of compositional equilibration of the amphibolite with host gneiss is the presence of biotite-rich rims on many enclaves. Textural and compositional reaction relationships in the enclaves are interpreted as relict igneous features. Bulk compositional data from three representative enclaves at Morton suggest that the pyroxene amphibolite could be derived from amphibolite by crystal fractionation and that both enclave types could have crystallized from a tholeiitic magma.