Impact Melt Emplacement, Cooling, and Crystallization in the Sudbury Igneous Complex

English Abstract:

Models for emplacement of mineralized quartz diorite dikes and cooling/crystallization of the Main Mass of the Sudbury Igneous Complex suggest that the dikes were emplaced during the crater modification stage, that the Main Mass cooled slowly from a superheated state, and that the mineralization exsolved from Main Mass during crystallization. Offset dikes have inclusion- and sulfide-poor QD margins and inclusion- and sulfide-rich IQD cores, implying emplacement of IQD into the semi-molten cores of QD dikes and therefore extremely rapid generation of IQD. During crater excavation impact melts are generated in a central melt pocket, injected into surrounding broken rocks, and driven upward and outward. Movement is reversed during formation of the central uplift, but returning melt is laden with fragments. Thus, it is more likely that QD was emplaced during melt excavation and that IQD formed by incorporation of fragments during formation of the central uplift. Incorporation of fragments into IQD and inclusion-rich Sublayer Norite would have rapidly cooled the melt to the liquidus. Contact ores are separated from the Main Mass by Sublayer Norite; although some sulfides may have percolated through partially-crystallized Sublayer, successively crystallizing norites would have isolated the ores from the melt, so observed depletion trends in upper norites may also reflect accumulation of sulfides in Sublayer and lower norites, not just the ore-forming process. Geochemical-petrographic data for traverses across the North and South Ranges show differences in composition across the melt sheet and that it was derived from at least two melts: 1) one that formed South Range Sublayer and Quartz-Rich Norite, and 2) one that formed overlying South Range Melanorite and Upper Norites, and North Range Mafic and Felsic Norites. Several models are being investigated: 1) inflow of melt from peripheral parts of the system (outer part of observed impact basin or a second impact basin), 2) collapse of the inner peak ring, and 3) collapse of crater walls.