Two Keweenawan Basaltic Dike Swarms in the Duluth Area, Minnesota
1987-09
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Two Keweenawan Basaltic Dike Swarms in the Duluth Area, Minnesota
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1987-09
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Abstract
Two Keweenawan mafic dike swarms crop out around and within the city of Duluth, Minnesota. The older Carlton County swarm is reversely polarized and crops out in an area from Thomson Dam to Ely's Peak. The dikes strike rather uniformly N 30° E with steep or vertical dips and widths of dikes range from <1 meter to approximately 18 meters. Major primary minerals present include plagioclase, augite, magnetite, ilmenite, and olivine. The samples range from aphanitic to medium-grained and common textures include intergranular, subophitic and quench textures. Alteration is generally slight. Results from a textural flow direction analysis are not definitive, but may indicate a southwestward magma transport direction, implying that the magma chamber which fed these dikes was located to the northeast of the samples studied. Chemically, the rocks range in composition from basalt to basaltic andesite, with quartz tholeiite being the most abundant rock type. These samples are subalkaline and fall within the tholeiitic field on an AFM diagram. They show fairly consistent trends on MgO variation diagrams, with the various elements exhibiting either a positive or negative correlation with MgO values. These trends seem to indicate that either fractional crystallization and/or magma mixing could account for the range in chemical compositions observed. As a whole, the swarm is somewhat evolved, being high in Fe, Kand Ti, and shows large dips at Sr on spidergrams, indicating plagioclase fractionation. Rare-earth element patterns however, do not show an orderly increase in elemental abundances from primitive to more evolved samples and several of the sample patterns cross one another, implying that simple fractionation alone cannot account for these trends. Chemical modeling of fractionation and/or magma mixing for the samples of this swarm achieves inconclusive results. The best results were achieved for several samples by modeling of major elements for fractionation of plagioclase, augite and pigeonite. However, trace-element modeling of this process sheds doubt as to the possibility of this process alone producing the observed variations in chemistry. The Duluth swarm, which is younger than the Carlton County swarm, is normally polarized and crops out in an area from Ely's Peak northeastward to Lakewood. The strikes are much more variable, with a general N-S direction, and dips are generally steep to vertical. Widths range from <l meter to 18 meters, but the majority of the dikes are <4 meters thick. Grain size varies from aphanitic to medium-grained, and intergranular, subophitic and ophitic textures are common. The major primary minerals include olivine, plagioclase, augite, magnetite i and ilmenite. Alteration is generally slight. Results from the flow direction analysis indicate a -variable flow direction. This may indicate either a centrally located magma chamber or that more than orie magma chamber was feeding the dikes of this area. Chemically, the rocks range in composition from basalt to basaltic andesite. The most common rock type is an olivine tholeiite. This swarm is subalkaline and plots within the tholeiitic field. It shows fairly coherent trends on MgO variation diagrams indicating that perhaps fractional crystallization and/or magma mixing could account for the chemical variations seen. It is chemically more primitive than the Carlton County swarm, but has large Sr anomalies on spidergrams, indicating extensive plagioclase fractionation. Rare earth element patterns are more orderly, with a progressive increase in element abundances from primitive to more evolved samples. Chemical modeling of the processes which produced the range in chemistry for this swarm yielded good results for major elements for fractionation of olivine, plagioclase, augite and pigeonite. However, trace element modeling of this process gives inconsistent results for compatible elements for many of the samples, implying that fractionation alone did not produce all the chemical variation within this swarm.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Jayne Anne Reichhoff in partial fulfillment of the requirements for the degree of Master of Science, September 1987. There are 5 supplementary files also attached to this record, which contain Plates 1-5 referenced in the thesis.
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