The Keweenawan Lavas in the City of Duluth
1984-04
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The Keweenawan Lavas in the City of Duluth
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1984-04
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Abstract
The area of study extends from Mesaba avenue in downtown Duluth, Minnesota, to the Lester River in the east end of the city. The bedrock consists of intrusive and extrusive igneous rocks, along with a small amount of interflow sedimentary rocks. The emphasis of this study is on the extrusive rocks. This sequence of rocks is Keweenawan in age (1100 m.y.), and occurs near the base of the North Shore Volcanic Group. The extrusive rocks are underlain (and intruded) by the Duluth Complex, and are cut by five thick sills (Messabi, Endion, Northland, Lakeside, and Lester River) with a total thickness of 1125 m, and by 36 dikes with a total thickness of 161 m. Thirteen thin (a few cm to four meters) and one thick (34.5 m) interflow sandstone beds (for a total thickness of about 50 m) form the sedimentary part of the sequence. Extrusive rocks are clearly tholeiitic, with a higher volume of felsic rocks, a higher percentage of potassium and commonly a higher Ti and Fe content than other major tholeiitic series described in the literature. Seventy-six volcanic units were distinguished and mapped: a cross-section profile yields a thickness of 2920 m for these units. Four ignimbrites form 531 m of this thickness, and lava flows comprise the rest. Compositions of the ignimbrites range from rhyolite to icelandite, and they are characterized by a lack of bedding, poor sorting, considerable thickness and great lateral extent of units. Lava flows of the Duluth area include felsic, intermediate and mafic rocks. Six felsic flows form 637 m of the sequence and range in composition from rhyolite to icelandite, and very commonly contain phenocrysts of feldspar, altered Fe-silicates and magnetite. Intermediate flows are of andesitic composition and include 32 units forming a total thickness of 1032 m; phenocrysts of plagioclase are common and locally comprise up to 60% of a flow. Mafic lava flows include olivine tholeiite and quartz tholeiite. Olivine tholeiites were formed from a very fluid magma and form nineteen units, with a total thickness ranging from a minimum of 363 m to a maximum of 1086 m. Dark color, mottled appearance and columnar jointing are their field characteristics. Quartz tholeiites form 15 units with a total thickness of 345 m, and commonly have a brecciated (aa type) top. The units are commonly thicker than olivine tholeiite flows. Plagioclase phenocrysts, although not abundant, are present in the majority of quartz tholeiite flows. The volcanic rock& in the area have undergone varying amounts of hydrothermal alteration, burial and contact metamorphism. Hydrothermal products are manifested as epidote, chlorite, calcite, quartz, feldspar and zeolites in the flows, and have filled the vesicles (now amygdules), joints and other cavities. This sequence of rocks has also been subjected to contact metamorphism of up to pyroxene-hornfels facies, and burial metamorphism (hydrothermal alteration) which locally approaches greenschist facies. The strike (azimuth) of the volcanic units changes from 140 degrees in the southwestern part of the area to 30 degrees in the northeastern part. Dip is about 30 degrees east in the southwestern part; it changes rather irregularly, but generally decreases toward the east. This implies some subsidence of the Lake Superior basin both during and after volcanic activity, as other investigators have found. A few faults were recognized, generally showing southeast-northwest trends with a nearly vertical dip. Duplication of flows due to faulting was not observed.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Shoaullah Motamedi in partial fulfillment of the requirements for the degree of Master of Science, March 1984. There are 2 supplementary files also attached to this record, which contain two variations of Plate 1 referenced in the thesis.
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