High-grade Archean rocks are exposed in the central Superior Province of the Canadian Shield in the Kapuskasing Structural Zone, a partly fault-bounded region up to 50 x 500 km. In migmatitic mafic gneiss, paragneiss, dioritic and tonalitic rocks, a lower-grade garnet-clinopyroxene-plagioclase (Gt-Cpx-Pl) zone and patchy, higher-grade orthopyroxene zones are distinguished. Grade decreases abruptly to greenschist in the Abitibi subprovince to the east across the Ivanhoe Lake cataclastic zone. Grade decreases gradually through amphibolite to greenschist facies in the Wawa subprovince to the west. Based on mineral-melt equilibria, minimum conditions for the Gt-Cpx-Pl zone are 750°C, 6 kbar, aH2O = 0.5-0.7 and for the orthopyroxene zone, 770°C, 6 kbar, aH2O < 0.5.

Various garnet-biotite and gamet-pyroxene geothermometers and geobarometers yield apparent temperatures ranging from < 600°C in the west to locally > 800°C. Apparent pressure values derived from the pyrope-grossular-anorthite-diopside-quartz equilibrium are 5.4-8.4 kbar (average 6.3) and define a NNE-trending area of relatively high P in the eastern and central Kapuskasing Zone, supporting the interpretation of a tilted crustal section.

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