GEOLOGIC AND GEOCHEMICAL RELATIONSHIPS BETWEEN THE CONTACT SUBLAYER, INCLUSIONS, AND THE MAIN MASS OF THE SUDBURY IGNEOUS COMPLEX - A CASE-STUDY OF THE WHISTLE MINE EMBAYMENT

More than half of the giant Ni-Cu-platinum-group element (PGE) sulfide ores of the Sudbury Igneous Complex are associated with a discontinuo us unit at the base of the main mass known as the ''sublayer.'' The su blayer is comprised of two fragment-rich members: (1) a metamorphic-te xtured footwall breccia, and (2) an igneous-textured contact sublayer. The contact sublayer occurs as a thick unit in depressions at the bas e of tile Sudbury Igneous Complex termed ''embayments''; it is associa ted with disseminated to massive sulfides and contains a range of incl usion types such as diabase, melanorite, olivine melanorite, metamorph osed melanorite, wehrlite, and dunite. We present data for a case stud y of the Whistle mine embayment and show that the igneous-textured sub layer matrix is geochemically unlike the main mass norites, quartz gab bros, and granophyres. For example, the igneous-textured sublayer matr ix in the Whistle mine has La/Sm = 4.0, La/Nb = 5.1, and Th/Zr = 0.02, and the main mass norites, quartz gabbros, and granophyres have ratio s of 5.5 to 7, 2.8 to 4.2, and 0.04 to 0.05, respectively. This matrix contains partially digested hornfels diabase fragments and ghost text ures of magnetite remaining from the melting of diabase. The igneous-t extured sublayer matrix at the Whistle mine can be modeled with small amounts of assimilation of local country-rock granitoids (similar to 1 0%), large degrees of assimilation of diabase that are not derived fro m the immediate country rocks (similar to 70%), and small contribution s from the main mass magma type (similar to 20% mafic norite). The amo unt of diabase assimilation would be too large for traditional assimil ation models controlled by the available heat of the mafic magma; even with a superheated magma, 70 percent assimilation is too large. A mod el is proposed where the melting of the target rocks initially produce s a felsic melt sheet which is laden with mafic fragments. These fragm ents sink to the base of the melt sheet in the crater and are concentr ated and further melted to produce the mafic sublayer. There are signi ficant differences in the composition of the igneous-textured sublayer matrix between different embayments which may reflect differing degre es of digestion of compositionally different protolith fragments. The melanorite inclusions in the sublayer at the Whistle mine have ratios of the incompatible trace elements essentially similar to those of the inclusions in the igneous-textured sublayer matrix, but they have sim ilar high incompatible element concentrations (e.g., olivine melanorit es have 20-65 ppm Ce in rocks with 15-21 wt % MgO), 1 to 10 percent in terstitial sulfide, up to 0.5 percent apatite, 1 to 15 percent biotite , and 1.85 Ga age zircon and baddeleyite. The mafic-ultramafic inclusi ons are interpreted to be the broken-up remnants of an earlier cumulat e formed at depth from a main mass magma; this parental magma has comp ositional traits which suggest that crystal accumulation took place fr om a magma which contains a large contribution from the diabase. Olivi ne compositional data for the mafic inclusions (Fo(71-79); 500-3,800 p pm Ni), in the absence of reequilibration, indicate that olivine cryst allization both predated and postdated sulfur saturation of the magma. Cr-rich spinels from these rocks confirm that the parental magma was especially Cr rich, and based on the olivine compositional data, had a tholeiitic Mg/Fe ratio.