Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The dominant tectonic event in the Lake Superior area was the development of the Keweenawan rift zone at approximately 1110 Ma.;Lithological sequences present within the Nipigon plate and the ages of these rocks are: passive granite and associated sub-aerial rhyolite (1536.7 +10/{dollar}-2.3{dollar} Ma); alkali basalt and lamprophyre dikes (ca. 1500 Ma); epicontinental clastic sediments of the Sibley Group ({dollar}{dollar}1108 Ma); and extensive tholeiitic diabase sills, dikes and cone sheets (1108 +4/{dollar}-2{dollar} Ma). These sequences form a shallow basinal structure which overlies Archean crust and is connected to the Lake Superior basin in the south by the Black Sturgeon Graben. An increase in intensity of rifting toward Lake Superior is suggested by the form of the igneous intrusions feeding diabase sills in the Nipigon plate.;The diabase and picritic intrusions both crystallized from fractionated magmas. The picrites are cumulate rocks derived at shallow crustal depths from a magma controlled by olivine fractionation. Picrite chills are in equilibrium with olivine phenocrysts of composition Fo{dollar}\sb{lcub}80{rcub}{dollar} and are interpreted to be the least evolved liquids observed. The diabase sills crystallized from an evolved basaltic liquid controlled by cotectic crystallization of plagioclase and lesser olivine and pyroxene.;The diabase sills occur as two 150 to 200 m thick intrusions covering an area of 11,000 km{dollar}\sp2{dollar}. The sills were emplaced under conditions approaching hydrostatic equilibrium near the Archean/Proterozoic unconformity. At the time of emplacement, the lithostatic load was probably less than 0.4 kbars. Crustal loading of the sedimentary sequence with sills enabled later magmas to erupt as basalts. Chemical variation within the sills reflects crystallization from multiple pulses of magma, minor crystal accumulation and movement of volatiles to late crystallizing parts of the sills. Variation in pyroxene crystallization sequences and the persistence of olivine during fractionation are a result of variations in a(SiO{dollar}\sb2{dollar}) in the magma. This variation may reflect the degree of contamination with siliceous crustal material.;Pre-Keweenawan passive granites and rhyolites are associated with a zone of ring faulting in northern Lake Nipigon. Partial melting of the lower crust with a heat source provided by mafic magmatism is suggested as a mechanism of producing the granites. (Abstract shortened with permission of author.)



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