The Volcanology And Trace Element Geochemistry Of Cyclical Volcanism In The Archean Confederation Lake Area, Northwestern Ontario

Archean volcanic rocks in the Confederation Lake area northwestern Ontario, are in three mafic to felsic cycles collectively 8500 to 11,240 m thick. Each cycle begins with pillowed basalt and andesite flows and is capped by andesitic and rhyolitic pyroclastic rocks and minor flows. Cycle I is believed to be entirely subaqueous as it is overlain by 90 m of limestone. Cycle II has pillowed basal basalt flows and andesitic to rhyolitic pyroclastic rocks locally overlain by 100 to 150 m of rhyolite tuff with intensely welded fragments. The environment of deposition of this unit appears to have been subaerial. Cycle II is overlain by stromatolitic limestone and pillowed basalt flows at the base of Cycle III. The remainder of Cycle III is dacitic pyroclastic rocks and rhyolitic flows, pyroclastic rocks and coeval hypabyssal intrusions.;Cycle I rhyolites are 2959 Ma old, Cycle II rhyolites are 2794 Ma and Cycle II rhyolites are 2739 Ma old by U-Pb age determination on zircons. These time periods are in accord with observed periodicity and volumes in young volcanic terrains.;In each cycle, tholeiitic basalts are overlain by calc-alkaline andesite to rhyolite. Fe enrichment in basalts is accompanied by depletion of Ti, Ca, Al, Cr, Ni, and Sr, and enrichment in P, the rare earth elements, and Nb, Y, and Zr. This is interpreted as open system fractionation of olivine, plagioclase, and minor clinopyroxene. Si enrichment in dacites and rhyolites is attributed to fractional crystallization of plagioclase, quartz, K-feldspar, and biotite. Tholeiitic basalt liquids are believed to be mantle derived. Intercalated andesites with flat rare earth patterns appear to be products of mixing of tholeiitic basalt and rhyolite liquids and andesites with fractionated rare earth patterns are probably produced by melting of amphibolite. Felsic magmas are partial melts of tholeiitic basalt, in some instances followed by magma mixing of tholeiites with trondhjemites.;Cycle I is interpreted as a platform upon which Cycle II was deposited in the early stages of caldera development. Cyle III is the central graben of the caldera and is the product of resurgent volcanism, hypabyssal intrusions, and late stage hydrothermal activity which also formed the Cu-Zn-Ag massive sulphide body at South Bay Mine. The tectonic scheme favored on the basis of structural, lithologic, and chemical parameters is a marginal basin behind a volcanic arc