Abstract

Progressive mineralogical and mineral–chemical changes are described for metapelitic rocks from an Abukuma-type metamorphic series ranging from greenschist to upper amphibolite – granulite facies in the Bear Structural Province, Northwest Territories, Canada.The first appearance of the following minerals defines six isograds: biotite; andalusite; cordierite (muscovite + chlorite out); sillimanite (andalusite out); sillimanite + K-feldspar (muscovite + quartz out); and almandine + K-feldspar ± cordierite (biotite + sillimanite + quartz out).Electron microprobe analyses of the Fe–Mg silicates, biotite, cordierite, and garnet, display two distinct trends of mineral chemistry with increasing metamorphic grade. In the almandine + K-feldspar ± cordierite zone, where garnet is present, Fe/(Fe + Mg) decreases in all of the Fe–Mg silicates observed. However, in the cordierite zone and in the higher grade rocks where garnet is absent, Fe/(Fe + Mg) increases in both biotite and cordierite. Ilmenite and rutile are involved in all continuous reactions and lead to increasing Fe/Mg with grade unless garnet is a product of reaction. There is also a displacement towards lower Fe content at the sillimanite + K-feldspar isograd.The scale of equilibration decreases to 1–2 mm in the almandine + K-feldspar ± cordierite zone, which is most probably a function of the decrease of graphic and therefore graphicin the metamorphic fluid with increasing metamorphic grade.The physical conditions of metamorphism in the Arseno Lake area range from graphic at 2–2.5 kbar withgraphic in the chlorite zone to ≥650 °C at 3.5–4.0 kbar where graphic in the almandine + K-feldspar ± cordierite zone.

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