Tectonic and chemical implications of cathodoluminescent microstructures in quartz, Parry Sound domain, Ontario, Canada

Abstract

Cathodoluminescence (CL) imaging of quartz in orthogneiss of the Parry Sound domain, Grenville Province, Ontario, Canada reveals four dark microstructures - medium-dark grains, mantles, sinuous lines, and straight lines. The Parry Sound domain experienced granulite-facies deformation and metamorphism followed by variable degrees of amphibolite-facies retrogression associated with the development of retrograde shear zones. We integrate our observations of CL-visible quartz microstructures with structures observable with optical and scanning transmission electron microscopy, analyses of water species concentration and trace element distribution in quartz, as well as with microstructures in plagioclase, to determine their characteristics and elucidate their timing and mechanisms of formation relative to the deformational history. Medium-dark grains are primarily located in and near kilometer-scale shear zones at the margins of the Parry Sound domain, have notably darker cores than most samples in the interior, and may have higher water contents than other samples in the interior. These grains formed during late shearing, at a lower temperature than CL-bright grains; they thus provide evidence that the large shear system narrowed with time. Dark mantles occur at grain boundaries and correlate with higher concentrations of Fe. They formed after the first shearing event, but while still at amphibolite facies, and they provide evidence for post-kinematic alteration at grain boundaries. Dark sinuous lines correspond to subgrain boundaries that formed after retrograde shearing, but while still at amphibolite facies. Dark straight lines correspond to fluid inclusion trails and most likely formed at low temperatures and pressures during exhumation.