Gneisses mapped in the Thompson nickel belt are the product of a Hudsonian metamorphic (M3) overprinting of an Archean granulite terrain. Pikwitonei region granulites may be partially preserved in amphibolite facies migmatite gneisses of the Thompson nickel belt. Temperatures attending the earlier Archean (M2) metamorphic event have been estimated to be 910 ± 70 °C. More detailed work on the younger overprinting suggests that the metamorphic conditions were relatively constant during the M3 metamorphism. Estimated temperature and pressure during the Hudsonian metamorphism are 650 ± 50 °C and 4–5 kbar (4–5 × 105 kPa), respectively.Although total pressure may have been constant, calculated values for graphic during the retrograde overprinting indicate a variability due to lithologic control. This variability in graphic is shown to be a controlling factor in the development of hydrous retrograde mineral assemblages. Metapelitic gneisses are characterized by graphic values of 0.05Ptotal, which may explain the absence of muscovite as a stable M3 mineral phase. Enderbitic gneisses exhibit higher values of graphic (0.25Ptotal) due to the emplacement of syn-M3 pegmatite dykes producing a more hydrous stable M3 mineral assemblage.

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