Natural Resources Research Institute Report of Investigations

The GRB is a large composite, calc-alkaline batholith comprising plutons ranging in composition from diorite, tonalite, granodiorite, quartz monzonite to hornblende syenite. However, two-mica quartz monzonite appears to be the most abundant rock type (Prince and Hanson, 1972; Sims & Viswanathan, 1972). Biotite and hornblende are the two principal ferromagnesian minerals in the batholith, but in some of the more mafic rocks and a few hornblende granites, the hornblendes have cores of clinopyroxene (Sims & Viswanathan, 1972). Some two-mica granites and aplites contain garnet; probably reflecting their very fractionated compositions. Although graywacke and greenstone sequences to the north are contact metamorphed up to the middle amphibolite facies by intrusion of the batholith, petrographic reports of the GRB rocks from west of Babbitt consistently describe primary igneous mineral assemblages. Typically, plagioclase + K-feldspar + quartz + biotite, and either hornblende or muscovite. Commonly described secondary minerals are, chlorite, epidote and muscovite, e.g., Sims & Viswanathan (1972), and this suggests that the original, igneous crystallization mineral assemblages have been partially retrogressed by no more than low grade (maximum of greenschist facies) regional metamorphism. Another commonly reported characteristic of the GRB is that the rocks are foliated, and the textures are variously described as “granulated”, “cataclastic” or “mortar-textured”. Typically, the foliation is subvertical and trends to the SE and, consequently, has been related to the same regional deformation event active in the country rocks at the time of pluton emplacement, or shortly afterwards, since it is generally parallel to the foliation in the older Archaean rocks to the north. In some places this SE-trending foliation in the GRB may have formed in the magmatic or submagmatic state. However, a subvertical, NE-trending foliation in some of the GRB rocks 2 developed after the batholith rocks had crystallized; this foliation is related to faulting at relatively low temperatures. Some of these faults now place batholith rocks directly in contact with older Archaean rocks to the north, cutting out rocks of the contact metamorphic aureole. Green (quoted in Sims and Viswanathan, 1972) reported that rocks of the GRB east of Babbitt are thermally metamorphosed by the Duluth Igneous Complex. Textures there are recrystallized and new mineral assemblages, e.g., hornblende + augite + hypersthene + biotite + magnetite, replace the original igneous mineral assemblages typically found in the GRB farther to the west. Hypersthene has exsolution lamellae, suggesting inversion from pigeonite. Green observed that in some rocks hypersthene was retrograded to biotite + actinolite and that copper sulphides were introduced in some rocks. Green attributed the low variance mineral assemblages east of Babbitt as resulting from partial equilibration between the original igneous assemblage and the amphibolite, hornblende amphibolite or pyroxene-hornfels facies assemblages formed by contact metamorphism. He suggested that thermal metamorphism of the GRB caused by intrusion of the Duluth Igneous Complex reached temperatures of 600 to 675o C, at an assumed pressure of 1 to 2.5 kbars.Funded by Minnesota Technology, Inc., the Universite du Quebec a Chicoutimi, Chicoutimi , Quebec G7H 2B1 Canada; Wallbridge America, Inc.; and the Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442