An examination of modal variation mechanisms in the contact sublayer of the Sudbury Igneous Complex, Canada

The origin of ultramafic inclusions in the Ni-Cu-PGE sulphide-mineralized c ontact sublayer of the Sudbury Igneous Complex (SIC) is critical to our und erstanding of the evolution of the complex and its mineral deposits, to our understanding of impact processes and to the ongoing debates between endog enic and exogenic models for this and other similar structures. In this stu dy crystallization modelling is conducted using various SIC starting compos itions as parent liquids to determine whether or not ultramafic material co uld be derived in situ. The software programs COMAGMAT (Ariskin et al., 199 3) and PELE (A. Boudreau) were used and gave broadly comparable results. Cr ystallization of a bulk SIC would not produce olivine-orthopyroxene assembl ages, and crystallization of any main mass composition produces plagioclase compositions consistent with those observed in the main mass but not suffi ciently calcic to account for sublayer inclusion feldspar compositions. Cry stallization of a liquid derived from a mixture of orthopyroxene-enriched n orite (such as the north range mafic norite) and tholeiitic basalt produces an appropriate crystallization sequence and assemblage. The localized addi tion of volatiles derived from granitic north range footwall in particular could account for the predominance of clinopyroxenitic (opx-poor, specifica lly) inclusions by crystallization in the presence of small amounts of wate r. 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 streng ths of siliceous, standing magma pools prohibits all but the largest crysta ls or agglomerates of dense crystals (such as olivine and orthopyroxene) to initiate settling. A model is proposed whereby the contact sublayer develo ps from the cooling and crystallization of a basaltic basal liquid componen t 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 significant mineral crystallization.