Thermobarometry of garnet–clinopyroxene–hornblende granulites from the Kapuskasing structural zone

Abstract

Amphibole mixing properties and standard state properties for the end members tremolite, tschermakite, pargasite, ferrotremolite, ferrotschermakite, and ferropargasite have been refined through analysis of relevant phase equilibrium experiments, coexisting cummingtonite–hornblende pairs, and natural mineral parageneses. When merged with the mineral properties given by R.G. Berman, these amphibole properties allow the calculation of numerous equilibria suitable for thermobarometry in a wide range of bulk compositions at amphibolite to granulite metamorphic grades. The method was applied to texturally equilibrated garnet–pyroxene–hornblende gneisses and compared both to results based on anhydrous assemblages and to those for garnet–biotite gneisses from the Kapuskasing structural zone. Thermometer resetting is apparent only in garnet–biotite gneisses, whereas touching garnet–pyroxene pairs in mafic gneiss show little evidence of retrograde Fe–Mg exchange. Computed metamorphic temperatures (garnet–clinopyroxene) for the 50 km wide garnet–clinopyroxene zone, in the range 750–790 °C, show no systematic variation with distance from the Ivanhoe Lake fault zone. Paleopressures computed from hydrous and anhydrous equilibria consistently indicate a north-trending high of 10–11 kbar (1 kbar = 100 MPa), decreasing to 3.3 kbar 60 km to the west in the Wawa gneiss domain. The pressure results define west-dipping Archean isobars with gentle (< 5°) attitudes through the Wawa gneiss domain and steeper dips (~ 17°) east of the amphibolite–granulite transition. The preserved record of high temperatures and pressures and moderate water activities (0.2–0.45) through a prolonged period of lower crustal residence and slow cooling indicated by zircon and titanite geochronology is difficult to reconcile at present.