9725158 Marsh It has long been a mystery why some large bodies of igneous rock are nearly uniform and other outwardly similar bodies are not. There are two groups, those initially containing large crystals (phenocrysts) and those initially free of phenocrysts. The former tend toward compositional and textural uniformity. Crystal nucleation and growth seem to be controlled by upper and lower inward propagating solidification fronts (SF) that stifle crystal fractionation. The Null Hypothesis says that sheet-like magmas initially free of phenocrysts undergo little differentiation. We have tested this hypothesis on a thick (330 m) diabase sill and find it holds. To be a general, fundamental concept it must be tested on a large body. The Sudbury Igneous Complex (SIC) is an ideal large body to both test the Null Hypothesis and also to gain insight into the solidification dynamics of phenocryst free systems. The history of solidification will be tracked by making a comprehensive study doing Crystal Size Distributions (CSDs) of the SIC using a series of drill cores through the entire body. For this purpose, Falconbridge Limited has allowed unlimited access to their vast holdings of core from the SIC. This work will contribute substantially to and complement other studies of Sudbury, and will contribute fundamentally to the understanding of large magmatic systems.
