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

Abstract

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 INTRODUCTION environment of linearly increasing undercooling, producing a straight- The textures of plutonic rocks are one of their most line CSD. During this phase, latent heat of crystallization was striking aspects, but they have been little used to elucidate largely removed by circulation of magma through the porous crystal petrological processes as compared with chemical and mush. By about 25% solidification the crystallinity was such that isotopic methods. Where used, discussion has tended it reduced, but did not eliminate, the circulation of magma, resulting to be qualitative, in contrast to quantitative chemical in the retention of more latent heat within the crystal pile. The methods, hence reducing the weight of this approach. temperature rose until it was buVered by the solution of plagioclase Indeed, it is not possible to verify physical models if close to the liquidus temperature of plagioclase. Nucleation of quantitative data are not available. Quantitative textural plagioclase was inhibited and conditions were suitable for textural studies of volcanic rocks are becoming more common coarsening of both plagioclase and olivine to occur (Ostwald and these methods are applied here to the development ripening). In this process small crystals were resorbed, whereas of anorthosite. larger crystals grew from both material recycled by the resorption of Quantitative textural studies of plutonic rocks really crystals smaller than the critical size and new material brought in started with Jackson’s studies of the Stillwater complex by the circulating magmatic fluid. The results of this process resemble in 1961 ( Jackson, 1961). He measured crystal size disthose of high-temperature metasomatism but there is no necessity for tributions, but did not record the area that he measured, a magmatic fluid with a diVerent composition. The shapes of the hence his data cannot be completely compared with plagioclase CSDs fit better the communicating neighbours equation of modern data. His pioneering work was not really followed textural coarsening, rather than the classic Lifshitz‐Slyozov‐Wagner up, and since then most studies have looked only at equation, as do other examples drawn from the literature. If olivine ‘average’ or ‘typical’ crystal sizes. These data are much started to nucleate at a higher temperature than plagioclase, then less revealing of petrological processes than crystal size during the textural coarsening phase olivine would have been more distributions, partly because they only give one parameter undercooled than plagioclase, and would have had a higher maximum for each sample. The few existing studies of crystal size growth rate. In these conditions olivine would coarsen more rapidly distributions in plutonic rocks have not treated than plagioclase and engulf it. Hence the order of crystallization plagioclase. determined from the textures would be the reverse of the order of One problem especially pertinent to quantitative texfirst nucleation of the two phases, from equilibrium phase diagrams. tural studies is the way that many igneous petrologists Maintenance of the temperature near the plagioclase liquidus may look at rocks: they commonly try to reconstruct how a rock has formed by examining the final, fully crystallized also inhibit the nucleation and growth of other phases.