Cordierite–gedrite rocks and associated gneisses of Fishtail Lake, Harcourt Township, Ontario

Abstract

The cordierite–gedrite rocks and associated gneisses on the north side of Fishtail Lake, Harcourt Township, Ontario, occur within the Grenville gneiss complex of the Haliburton Highlands. The investigation comprises a petrographic study, based on geologic mapping, and supplemented by new chemical analyses of gedrites, cordierites, garnets, biotites, and typical rocks. Comparison with compositional data from other metamorphic environments shows that the compositions of associated cordierite, garnet, and anthophyllite (gedrite) and garnet–cordierite–biotite have lower FeO/(MgO + FeO) ratios in high-grade regional metamorphic environments such as exemplified at Fishtail Lake than in contact metamorphic associations. The chemical composition of the rocks is characterized by high MgO and FeO and low lime and alkalies, compared with argillaceous rocks and metamorphic rocks derived from them. It is suggested that this unusual composition results from the removal of an anatectic granite fluid from the parent rock during partial melting associated with high-grade metamorphism. Pegmatites and aplites associated with the gneisses may represent a part of this granite fluid. This hypothesis is shown to be consistent with published experimental data, field observations, and the composition of the cordierite–gedrite rocks compared with hypothetical argillaceous parents.The rocks of the area were metamorphosed initially to the staurolite–almandine subfacies, as indicated by the occurrence of inclusions of staurolite in garnet. With increasing intensity of metamorphism, in the sillimanite–almandine–orthoclase subfacies, the staurolite became unstable, and apart from relicts, is now represented by garnet and sillimanite. Partial anatexis and removal of a melted fraction of granite composition took place, leading to the crystallization of cordierite–gedrite assemblages. Subsequent retrograde metamorphism altered some of the cordierite to kyanite–andalusite–chlorite and pinite. This secondary generation of kyanite and andalusite has resulted, in two samples studied, in the association of kyanite, andalusite, and sillimanite.