Garnet-cordierite-anthophyllite rocks at Rat Lake, Manitoba

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Baldwin, David Arthur
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Anthophyllite - garnet - cordierite rocks occur in the PreCambrian of the Churchill province at Rat Lake, Manitoba. Texturally the rocks can be classified as granoblastites, porphyroblastites and schists. They occur in the area of deformed meta-sedimentary rocks that have been intruded by granodiorite, tonalite, granite and pegmatite. Three outcrops of the anthophyllite - garnet - cordierite rocks, covering an area of 200' x 90' were mapped on a scale of 1" = 10' and ninety-seven samples were collected. Chemical analyses were obtained for 10 whole rock samples, also for garnets separated from seven samples, and one sample of cordierite. Compositions of other cordierite and anthophyllite specimens were determined by measuring optical properties. Mineral relationships can be interpreted to have originated in two stages of metamorphism. An early high pressure, high temperature period of granulite metamorphism is evidenced by the relict mineral assemblage orthopyroxene - sillimanite - garnet - cordierite (?). A later retrograde garnet - cordierite grade of metamorphism of the uppermost amphibolite facies is indicated by the assemblage garnet - cordierite - anthophyllite - hercynite, (the garnet of this assemblage being a relict of the granulite grade metamorphism) plus textural evidence of reactions which are interpreted to be: Garnet + sillimanite---cordierite + hercynite (1) Orthopyroxene + quartz + water---anthophyllite (2) Anthophyllite + hercynite---cordierite (3) The presence of relict bedding, and unusual rock compositions support a theory of origin of these rocks as a weathering deposit from basic rocks.