Mineralogy and Petrology (2000) 68: 141±157 An examination of modal variation mechanisms in the contact sublayer of the Sudbury Igneous Complex, Canada S. A. Prevec Hugh Allsopp Lab, University of the Witwatersrand, Johannesburg, South Africa With 4 Figures Received November 17, 1998; revised version accepted September 15, 1999 Summary The origin of ultrama®c inclusions in the Ni-Cu-PGE sulphide-mineralized contact sublayer of the Sudbury Igneous Complex (SIC) is critical to our understanding of the evolution of the complex and its mineral deposits, to our understanding of impact processes and to the ongoing debates between endogenic and exogenic models for this and other similar structures. In this study crystallization modelling is conducted using various SIC starting compositions as parent liquids to determine whether or not ultrama®c material could be derived in situ. The software programs COMAGMAT (Ariskin et al., 1993) and PELE (A. Boudreau) were used and gave broadly comparable results. Crystallization of a bulk SIC would not produce olivine-orthopyroxene assemblages, and crystallization of any main mass composition produces plagioclase compositions consistent with those observed in the main mass but not suf®ciently calcic to account for sublayer inclusion feldspar compositions. Crystallization of a liquid derived from a mixture of orthopyroxene-enriched norite (such as the north range ma®c norite) and tholeiitic basalt produces an appropriate crystallization sequence and assemblage. The localized addition of volatiles derived from granitic north range footwall in particular could account for the predominance of clinopyroxenitic (opx-poor, speci®cally) inclusions by crystallization in the presence of small amounts of water. Crystal settling was unlikely to have been a major feature in the evolution of the SIC or in crustal melt sheets in general, as very high yield strengths of siliceous, standing magma pools prohibits all but the largest crystals or agglomerates of dense crystals (such as olivine and orthopyroxene) to initiate settling. A model is proposed whereby the contact sublayer develops from the cooling and crystallization of a basaltic basal liquid component to the SIC. This liquid is derived from melting of the basaltic basement following impact melting, and mixes with basal, orthopyroxenitic main mass above it prior to signi®cant mineral crystallization.