MEASUREMENT AND INTERPRETATION OF CRYSTAL SIZE DISTRIBUTIONS IN THE KAMA POINT SILL, NIPIGON, ONTARIO

Sheetlike diabase intrusions, because of their typically simple geometry and uniform chemical composition, are ideal for examining relationships between thermal evolution of magma and igneous textures. The lowermost 35 meters of the 1100 Ma olivine diabase sill at Kama Point, approximately 25 km east of Nipigon, Ontario, has been sampled at ~3 meter intervals from the basal contact upwards. For each sample, modal mineralogy was determined through point counting and textures were quantified using crystal size distributions (CSDs). In this study, analysis of plagioclase CSD data is shown to provide a method for quantitatively evaluating competing hypotheses for magmatic processes as well as important constraints on crystallization kinetics.

In the Kama Point sill, the textures show a primary coarsening-inwards trend related to diffusion-controlled in-situ crystallization. On top of this trend, however, plagioclase CSDs show a clear coarsening excursion between 20 and 25 meters above the basal contact. The origin of this excursion is unclear. Two possible explanations include accumulation of pre-emplacement plagioclase phenocrysts or injection of a second pulse of magma into the chamber after an initial cooling period. The first explanation is preferred due to the presence of an olivine-rich layer at this same horizon. Additionally, the textures above the olivine-rich layer follow the same trends as textures below the olivine-rich layer, suggesting that they were controlled by a single thermal environment. For each sampling location, cooling durations have been calculated using various estimates for the physical geometry and initial conditions. These durations may be used together with CSD data to infer time-integrated crystallization kinetics. Inferred growth rates are on the order of 10^-9 – 10^-7 cm s^-1 and nucleation rates are on the order of 10^-4 – 10⁰ cm^-3 s^-1, which is consistent with crystallization kinetics in the Hawaiian lava lakes (Kirkpatrick, 1977) and many mafic intrusions.