The crystal nucleation and growth in metamorphic processes based on the Crystal Size Distributions (CSD) of mineral phases. The Grenville province, Ontario, Canada
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A systematic evaluation of Crystal Size Distribution (CSD) in metamorphic phases was conducted. Biotite, amphibole, chlorite, quartz and plagioclase mineral phases were selected, in order to evaluate how different metamorphic events are recorded in their CSDs, when they are over-imposed. The Grenville terrain was then selected as natural laboratory because it has a well-documented evolutionary history of several orogenic cycles, punctuated with intrusive events. Digital imaging using a polarized microscope equipped with a camera was performed and the subsequent images were then measured using ImageJ Open Source Java code. Many length and width measurements in each phase were taken to calculate the CSD of each mineral phase. The measurements were then processed using a script written in GNU Octave 3.2.2, based directly on the area measured, and CSDCorrections (Higgins, 2003), based on an estimation of the volume of the crystal. We have found that the samples that were located near younger intrusions revealed two CSD population densities, whereas, the samples that were not located near intrusions revealed only a single population density. The existences of crystal populations with a single sample are related to the first regional metamorphic activity associated an accretionary event during the Grenville orogeny. The second population of crystals relates to the relaxation which occurred post regional activity and intrusive behavior began in select locations. The samples also revealed prograde growth and retrograde annealing associated with the various cooling and thermal events, which, in general is in agreement with typical CSD’s of metamorphic terrains. Another key conclusion is that both processing methodologies produce similar results and are consistent with each other. This would suggest that although a volumetric estimation might reflect the true crystal habit, a direct measurement of the area in a thin section could also be used to calculate the CSD of a mineral phase.
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