The effects of contact metamorphism by the Duluth complex on proterozoic footwall rocks in Northeastern Minnesota.

Abstract

The 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.