Oxygen fugacity variations and mineral reactions in sapphirine-bearing paragneisses, E. Grenville province, Canada R. K. HERD Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada, KI OE8 D. ACKERMAND Mineralogisches Institut der Universit/it, D-2300 Kiel, Federal Republic of Germany A. THOMAS Department of Mines and Energy, Govt. of Newfoundland and Labrador, P.O. Box 4750, St Johns, Newfoundland, Canada AND B. F. WINDLEY Department of Geology, The University, Leicester, LE1 7RH, U.K. Abstract Sapphirine-bearing assemblages occur in paragneisses in a 200 km long block in the Grenville province in Labrador-Quebec. The occurrence of some of these rocks was previously known, but their considerable extent is now recognised from regional mapping. The mineral assemblages, reactions, and compositions and the tectonic structure in the paragneisses of this block are surprisingly uniform. Within feldspar- quartz layers we recognise assemblages with sapphirine, quartz, iron titanium oxides, spinel, corundum, diaspore, orthopyroxene, sillimanite, cordierite, garnet, and biotite in metre to millimetre-thick layers. These minerals reacted with their matrix, especially quartz, during cooling and uplift. At least 11 retrograde reactions gave rise to spectacular corona textures and define a P-T-time trajectory from c. 8 kbar at 900 ~ to 6 kbar at 700 ~ which changed from early isobaric to late isothermal, Based on successive generations of sapphirine and orthopyroxene with constant XMg and decreasing Fe 3 +/Fe 2+ ratio in recalculated formulae, we deduce an accompanying change from high to low oxygen fugacity along this trajectory. The isothermal section of the trajectory is consistent with predicted rapid uplift and with field evidence for thrust tectonics and mylonitisation. KEYWORDS: sapphirine, paragneiss, oxygen fugacity, Grenville province, Canada. SAPPHIRINE-BEARING assemblages occur through- out a 200 km long block within the late Proterozoic eastern Grenville province extending from Ptar- migan Lake (Emslie et al., 1978) via Wilson Lake (Morse and Talley, 1971; Leong and Moore, 1972; Gittins and Currie, 1979; Nielsen and Gittins, 1977; Jackson and Finn, 1982) to Lac Ghyvelde and Lac Long (Thomas and Wood, 1983; Thomas et al., 1984) (Fig. 1). Within this granulite fades terrain the major lithological units are in general paragneiss (containing the sapphirine rocks), gabbronorite, Mineralogical Magazine, June 1987, Vol. 51, pp. 203-6 9 Copyright the Mineralogical Society gabbro, hypersthene granite and charnockitic gneiss, as well as late microcline granite (Fig. 1). The sapphirine-bearing assemblages are often associated with oxide minerals in millimetre-scale layers which have a strong aeromagnetic signature (Thomas and Wood, 1983). At Wilson Lake, the oxide minerals form lenses which reach widths as great as 7 m (Leong and Moore, 1972). The rocks strike SW-NE, dip strongly to the SE or NW, and have a well developed lineation which plunges 40 ~ S in the Ptarmigan Complex (Emslie et al., 1978).