A crystal size-distribution study of the Kiglapait layered mafic intrusion, Labrador, Canada: evidence for textural coarsening

Abstract

Crystal size distributions (CSD) of plagioclase, olivine and clinopyroxene from the Kiglapait intrusion, Labrador were measured for a series of 13 samples spanning most of the exposed intrusion. All CSDs lack small crystals and are slightly curved to the right, concave up. All CSDs of a mineral sweep out a fan, as expected where there is relatively little variation in phase proportion. CSD dispersion can be replicated using the communicating neighbours model of textural coarsening (Ostwald ripening). Most of the textural variation in the intrusion is the result of variable degrees of coarsening. It is proposed that coarsening occurred in the outer part of the crystallisation boundary layer where release of latent heat and changes in the intercumulus liquid composition resulted in a reduction in undercooling, suppression of nucleation and coarsening. Particularly coarse-grained areas reflect focusing of circulating magmatic fluids during coarsening. The effects of crystal settling were not observed in the CSDs, even within single well-developed layers. They were probably concealed by the pervasive textural coarsening. CSDs do not suggest that mechanical compaction has played an important role in the formation of texture, although it is very likely to have occurred. However, samples from the lower part of the intrusion show textural evidence of pressure-solution compaction.