A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by James A. Kilburg in partial fulfillment of the requirements for the degree of Master of Science, May 1972. Plate 1 referenced in the thesis is also attached to this record.The Upper Precambrian Ely's Peak basalts crop out in a north-south trending, wedge-shaped belt in the area around Nopeming, southwest of Duluth, Minnesota. These Lower Keweenawan flows overlie the basal Upper Precambrian quartzite in the southwestern portion of the Lake Superior basin. There are at least 20 individual flows with a total thickness of about 1,200 feet. The thickest flow is about 125 feet thick while the thinnest is less than 10 feet thick. Many of the flows show considerable lateral continuity; for example, one flow is traceable for about three miles along strike. Chemically, there are two main types of flows as classified by MacDonald and Katsura (1964); these are alkali basalts and tholeiite basalts. The alkali basalts, which have a low silica content, are fine- to medium-grained and form flows which vary in thickness from about 15 to just over 100 feet. Plagioclase, small, anhedral olivine pseudomorphs, and actinolite after augite form a subophitic to poikilitic texture. The opaque minerals ilmenite and magnetite form phenocrysts up to 1.5 mm in diameter in some rocks but are usually found as interstitial microlites in the groundmass. Textures and structures within the flows suggest a low viscosity. Structures within these flows include level, ropy surfaces, bent and straight pipe vesicles, straight vesicle cylinders and vesicular tops. Crude columnar joints were observed in one flow. The tholeiitic basalts are usually fine- to medium-grained, but some thicker flows are coarse-grained. These flows are usually from 30 to 90 feet thick. They include porphyritic, subophitic, and poikilitic types. Augite occurs as phenocrysts in the porphyritic flows and as poikilitic oikocrysts and as intergranular grains in the others. Olivine pseudomorphs are occasionally present, and magnetite and ilmenite generally occur as evenly distributed microlites in the groundmass. Structures within the flows include pillows (only in the basal flow), stretched amygdules, and vesicular tops. The augite-porphyritic basalts which lie at the base of the sequence are distinctive and very rare in the North Shore Volcanic Group. The whole sequence of lavas has undergone hydrothermal metamorphism to the high zeolite-low greenschist facies. Minerals present which demonstrate this are actinolite, chlorite, and epidote. The only zeolite present is wairakite which has been discovered here apparently for the first time in the Lake Superior region. It is the highest-temperature zeolite. Intrusion of the Duluth Complex is thought to be responsible for elevating the geothermal gradient and thus, permitting the formation of wairakite. The gabbro intrusion also contact-metamorphosed the lavas to a medium-grained pyroxene hornfels for a distance of up to one-fifth of a mile from the contact. Pressures of metamorphism are thought to have been between 2,000 and 2,500 bars. This pressure was produced by the weight of up to 30,000 feet of overlying Upper Precambrian lavas and Duluth Complex which underlie the North Shore of Lake Superior. Temperatures are estimated to have been between 290° C and 370° C during the hydrothermal metamorphism. Based on their distinctive petrology and reversed magnetic polarity (Green and Books, 1972), the Ely's Peak basalts appear to correlate with the basal flows at Grand Portage, Minnesota. This implies that the time of deposition at these localities was approximately the same, and the source from which these lavas were derived and the physical conditions of magma generation were the same
