DUCTILITY OF GARNET AS AN INDICATOR OF EXTREMELY HIGH-TEMPERATURE DEFORMATION

Discoidal shaped garnets (Alm63 Prp33 Grs3 Sps1) occur in the foliation plane of lineated quartzo-feldspathic mylonites of the Morin shear zone located along the eastern boundary of the Morin anorthosite complex (Grenville Province, Quebec). The flattened shape of garnets records a component of coaxial strain (X = Y much greater than Z), different from the dominant rotational shear strain (X > Y > Z) in this mylonite. TEM observations indicate dislocation slip and recovery to be the main mechanism for the deformation of garnet. Extrapolation of available experimentally-determined flow laws to natural strain-rates shows an inversion of flow strength between quartz +/- feldspar and garnet: garnet, much stronger than quartz and feldspar at temperatures below 700-degrees-C, becomes weaker than quartz and feldspar above 900-degrees-C. The coaxial ductile strain recorded by the garnets could be produced by a local, uncharacterized deformation, under extremely high temperature conditions at the roof of the rising anorthositic diapir (approximately 1155 Ma). Subsequent, lower temperature rotational, deformation is related to strike-slip movement along the Morin shear zone (approximately 1020 Ma).