Oxygen Fugacity Variations and Mineral Reactions in Sapphirine-Bearing Paragneisses, E. Grenville Province, Canada

R. K. Herd, D. Ackermand, A. Thomas and B. F. Windley
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada, K1 OE8
Mineralogisches Institut der Universität, D-2300 Kiel, Federal Republic of Germany
Department of Mines and Energy, Govt. of Newfoundland and Labrador, P.O. Box 4750, St Johns, Newfoundland, Canada
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 °C to 6 kbar at 700 °C which changed from early isobaric to late isothermal. Based on successive generations of sapphirine and orthopyroxene with constant XMg and decreasing Fe3+/Fe2+ 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

Mineralogical Magazine; June 1987 v. 51; no. 360; p. 203-206; DOI: 10.1180/minmag.1987.051.360.02
© 1987, The Mineralogical Society
Mineralogical Society (www.minersoc.org)