Browsing by Subject "North Shore Volcanic Group"
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Item The effects of contact metamorphism by the Duluth complex on proterozoic footwall rocks in Northeastern Minnesota.(2010-06) Frost, Shelby JeanneThe Duluth Complex is composed of numerous mafic intrusions that were emplaced in northeastern Minnesota during formation of the Midcontinent Rift approximately 1.1 Ga (Miller et. al., 2002). When it intruded, the heat of this igneous body significantly affected the wall rocks around it and created a distinctive contact metamorphic aureole. Footwall rocks directly west of the Duluth Complex include the Animikie Group and the North Shore Volcanic Group. The purpose of this research is to understand crustal conditions associated with emplacement of a large mafic igneous massif such as the Duluth Complex, and develop a better idea of the thermal state of the crust during the time of rifting. To constrain crustal conditions we must determine the effects that intrusion of the Duluth Complex had on adjacent wall rocks, in particular the Ely’s Peak basalts of the North Shore Volcanic Group, and the Thomson and Virginia formations of the Animike Group. These effects include the extent and grade of metamorphism. At the time of intrusion, metamorphic pressure was approximately 2.5 kbar and temperature was 600-700°C, based on the presence of metamorphic minerals such as orthopyroxene and wollastonite. The contact metamorphic aureole extends from the Duluth Complex into the Animikie Group for approximately 200 m and into the Ely’s Peak basalts for approximately 100 m. This is consistent with estimates made by Severson (1995), Duchesne (2004) and Kilburg (1972). The criteria used to define the aureole include textures such as mortar texture and spotty slate texture in the Animikie Group, and granoblastic texture in the Ely’s Peak basalts. Porphyroblasts of metamorphic indicator minerals such as cordierite, wollastonite, garnet, and pyroxene also help define the aureole. A simple 1D thermal conduction model reproduces the conditions in the contact aureole determined by petrographic relations, and helps explain why the aureole is thin. Despite the high temperature of the intrusion, the aureole is thin because the wall rocks were fairly cool when the Duluth Complex intruded, having equilibrated over about 600 m.y. at shallow crustal levels to a typical continental geotherm. The thin contact aureole also indicates that multiple intrusions may have occurred instead of one large intrusion. This would have led to insulation of the younger intrusions and a thinner aureole. Liberation of fluids in the wall rocks by the intrusion may have also played a role in keeping the contact aureole relatively thin by lowering reaction temperatures.Item Guidebook 11. Field Trip Guidebook for the Keweenawan (Upper Precambrian) North Shore Volcanic Group, Minnesota(Minnesota Geological Survey, 1979) Green, John C.The Upper Precambrian plateau lavas of the Lake Superior region were produced in response to tensional rifting of North America roughly 12001120 x 106 years ago (White, 1972a). Geological and geophysical evidence shows that they consist of a group of about eight separate volcanic accumulations which partly overlap in time and space as the locus and activity of rifting changed along what is now the Midcontinent Gravity High (Green, 1977). Nearly all of these lava accumulations ("plateaus") contain preserved sections which range in thickness from 2.5 to 7 km and are made of hundreds of individual flows. They have subsided centrally during and/or after eruption. Basalt of various compositions predominates; the most common type is nonporphyritic olivine tholeiite with unusually high Al content and ophitic texture. Transitional to weakly alkaline basalt is also common. Quartz tholeiite, basaltic andesite and rhyolite are moderately abundant in most of the plateaus, and icelandite (intermediate quartz latite) is found in some.