Ductility of garnet as an indicator of extremely high temperature deformation

Abstract

Discoidal shaped garnets (Alm63 Prp33 Grs3 Sps1) occur in the foliation plane of lineated quartzofeldspathic 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 ⪢ 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°C, becomes weaker than quartz and feldspar above 900°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 (∼1155 Ma). Subsequent, lower temperature rotational deformation is related to strike-slip movement along the Morin shear zone (∼1020 Ma).