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Oxygen fugacity variations and mineral reactions in sapphirine-bearing paragneisses, E. Grenville province, Canada

Published online by Cambridge University Press:  05 July 2018

R. K. Herd ,
D. Ackermand ,
A. Thomas  and
B. F. Windley
R. K. Herd
Affiliation:
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada, K1 OE8
D. Ackermand
Affiliation:
Mineralogisches Institut der Universität, D-2300 Kiel, Federal Republic of Germany
A. Thomas
Affiliation:
Department of Mines and Energy, Govt. of Newfoundland and Labrador, P.O. Box 4750, St Johns, Newfoundland, Canada
B. F. Windley
Affiliation:
Department of Geology, The University, Leicester, LE1 7RH, U.K.
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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

sapphirineparagneissoxygen fugacityGrenville provinceCanada
Type
Petrology
Information
Mineralogical Magazine , Volume 51 , Issue 360 , June 1987 , pp. 203 - 206
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1987

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