Origin of Anorthosite by Textural Coarsening: Quantitative Measurements of a Natural Sequence of Textural Development

The textures of plagioclase crystals within large olivine oikocrysts preserve the sequence of the formation of anorthosite. Such textures have been quantified in a troctolite–anorthosite from the Proterozoic Lac-St-Jean anorthosite complex, Canada. Crystal size distributions (CSDs) indicate that initially plagioclase nucleated and grew in an environment of linearly increasing undercooling, producing a straight-line CSD. During this phase, latent heat of crystallization was largely removed by circulation of magma through the porous crystal mush. By about 25% solidification the crystallinity was such that it reduced, but did not eliminate, the circulation of magma, resulting in the retention of more latent heat within the crystal pile. The temperature rose until it was buffered by the solution of plagioclase close to the liquidus temperature of plagioclase. Nucleation of plagioclase was inhibited and conditions were suitable for textural coarsening of both plagioclase and olivine to occur (Ostwald ripening). In this process small crystals were resorbed, whereas larger crystals grew from both material recycled by the resorption of crystals smaller than the critical size and new material brought in by the circulating magmatic fluid. The results of this process resemble those of high-temperature metasomatism but there is no necessity for a magmatic fluid with a different composition. The shapes of the plagioclase CSDs fit better the communicating neighbours equation of textural coarsening, rather than the classic Lifshitz–Slyozov–Wagner equation, as do other examples drawn from the literature. If olivine started to nucleate at a higher temperature than plagioclase, then during the textural coarsening phase olivine would have been more undercooled than plagioclase, and would have had a higher maximum growth rate. In these conditions olivine would coarsen more rapidly than plagioclase and engulf it. Hence the order of crystallization determined from the textures would be the reverse of the order of first nucleation of the two phases, from equilibrium phase diagrams. Maintenance of the temperature near the plagioclase liquidus may also inhibit the nucleation and growth of other phases.