Browsing by Subject "Geological Sciences"
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Item The effects of contact metamorphism by the Duluth complex on proterozoic footwall rocks in Northeastern Minnesota.(2010-06) Frost, Shelby JeanneThe 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.Item Geology and petrogenesis of the Tuscarora Intrusion of the Duluth Complex, Gillis Lake 7.5’ quadrangle, northeastern Minnesota(2010-09) Costello, Daniel EdwardThe Tuscarora Intrusion is a layered mafic intrusion located at the base of the 1.1 Ga Duluth Complex in northeastern Minnesota. Detailed field mapping (1:12,000) and follow-up petrographic and geochemical analyses were conducted to evaluate the troctolitic igneous stratigraphy of the Tuscarora Intrusion, as well as to understand the petrologic relationship between the troctolitic cumulates and the plagioclase-rich gabbros of the overlying Anorthositic Series. Previous studies have interpreted the two lithologies as being interlayered, a relationship that would be unique within the Duluth Complex. Field mapping has identified two lithologically distinct stratigraphic zones within the Tuscarora Intrusion. The lower zone is composed of olivine gabbro to augite troctolite that is typically heterogeneous in mode and texture and locally displays modal layering and foliation. Most notably, it contains abundant, large (>100m) basaltic hornfels inclusions. The upper zone is composed of homogenous, foliated troctolite to leucotroctolite, which grades upward from melatroctolite at the base of the zone. Inclusions in the upper zone are mostly large, often elongate blocks of anorthositic rocks and are especially abundant near the contact with the Anorthositic Series. No field evidence was found for an interlayered relationship between the two lithologies and geochemical studies imply distinct parent magma compositions. This study concludes that the Tuscarora Intrusion and Anorthositic Series are two separate intrusive suites, as found in other areas of the Duluth Complex. Furthermore, the lower and upper zones of the Tuscarora are interpreted to have formed successively from major emplacement episodes of moderately evolved tholeiitic mafic magma. The emplacement model proposed has plagioclase porphyritic magmas intruding at some level above the base of the North Shore Volcanic Group to create the Anorthositic Series. This was followed by emplacement of lower zone magmas closer to base of the volcanic pile. This resulted in the incorporation of abundant volcanic inclusions and in strong contamination of the magma by interaction with pre-rift rocks of the footwall. Emplacement of upper zone magmas occurred above the newly crystallized Lower Zone and below the Anorthositic Series cap, which contributed anorthositic inclusions.Item High Resolution geochemical XRF data from Elk Lake, Minnesota: A Holocene paleoclimate record from varved lacustrine sediments.(2010-09) Rush, Robert AllenThe study of Holocene climate change is vital to understanding present and future climate conditions in the Upper Midwest region of the United States. Varved sediments from Elk Lake, Clearwater County, Minnesota provide an archive of multiple climate sensitive proxies and past climate conditions, particularly related to the balance of precipitation and evaporation (available moisture) for the North Central United States. Studies conducted in the past using Elk Lake sediments have established large scale and long term changes in the climate history of the region, but were done at a resolution that only allowed for a discussion of events on time scales of hundreds to thousands of years. Scanning XRF is a new analytical technique that allows for much higher resolution, geochemical data to be gathered from sediment cores for the characterization of climate variability with resolution on the order of decades to inter-annual changes. This study seeks to repeat, using new analytical and higher resolution methods, the work done by previous researchers. One centimeter resolution XRF scans were used to describe changes that occurred during the Holocene, and 200 micron scans were used to identify the nature of varve deposition during major periods in the Holocene and to characterize the timing and relationships between the laminations that make up individual varves. With higher resolution data with which to work, time series analysis provides insight into high frequency cycles during the Holocene record including El Nino Southern Oscillation (ENSO) and solar activity cycles. With the addition of the first known geophysical data set from Elk Lake, this study also illustrates the usefulness of obtaining multiple records from an individual lake. Through the use of both geochemical and geophysical data, it is shown that events seen separately in each data set can be correlated to one another and an accurate estimate for the timing of major climate events can be obtained.Item A postglacial record of climate change from East Crooked Lake and Tofte Lake, Minnesota.(2010-09) Wendt, Kelly MichaelThe postglacial Holocene climate history of Minnesota is characterized by a cool postglacial period beginning about 10 ka and lasting until about 8 ka when dryer conditions ushered in a Prairie Period which lasted until about 4 ka at which time moisture increased. From about 4 ka to the present climate conditions have remained relatively stable. This history of Minnesota climate has been observed by Dean and colleagues in numerous studies at Elk Lake, Minnesota. In these studies the researchers looked at pollen, geochemistry, diatoms, magnetic properties, and isotopes. This study uses sediment cores from nearby East Crooked Lake to see if scanning X-ray fluorescence results are comparable to classic inorganic geochemistry results from Dean at Elk Lake. A sediment core from Tofte Lake near Ely, MN is also compared to see if the results from Elk Lake and East Crooked Lake are regional in extent. The results confirm that scanning XRF is comparable to the classic, time consuming geochemistry methods used by Dean at Elk Lake and that the postglacial Holocene history recorded at Elk Lake was also recorded at East Crooked Lake. The timing of events at Elk Lake and East Crooked Lake are adjusted to correlate with the more accurate dating of events at Steel Lake, MN. The findings do not support that the climate events at Elk Lake and East Crooked Lake are regional in their extent to Tofte Lake. Tofte Lake did not experience a dryer Prairie Period.Item Potential for slumps, sediment volcanoes, and excess turbidity in the Nemadji River Basin.(2010-06) Mossberger, Irvin GeraldBeginning in the early 1990‘s, a portion of Deer Creek, a tributary of the Nemadji River in the Lake Superior basin, experienced the formation of sediment volcanoes in its creek bed, along with related slumping, enhanced erosion rates, and turbidity in excess of state total maximum daily load (TMDL) limits. Slumping near the stream has increased erosion rates and may eventually threaten the stability of nearby structures. The resulting excess turbidity negatively impacts aquatic wildlife, and increases sedimentation rates and dredging costs in downstream navigable waters. Slumping and formation of sediment volcanoes at Deer Creek were likely caused when dynamite was used to destroy a nearby beaver dam on the creek. Some combination of rapid pond drainage and/or disturbance from the explosives may have led to fracturing of a glacio-lacustrine clay confining layer over a locally extensive aquifer. A sediment volcano and associated slumping are also present along nearby Mud Creek. The sediment volcano areas at Deer and Mud Creeks both occur at the toe of 10-meter high slumps. The failure planes of these slumps may facilitate formation of sediment volcanoes by providing pathways for groundwater to reach the surface. Predicting slump locations should then also help predict the location of potential sediment volcanoes. The stratigraphic and hydrologic conditions in Deer and Mud Creeks are similar to those throughout nearby areas in the Nemadji River basin. This project examines the relationship between the slumps and sediment volcanoes, and develops a predictive model of the potential for slope failure in the lacustrine clay portions of the basin. A 3-D model of stratigraphy and hydraulic potential from more than 300 wells is used, along with slope stability analysis with the stress-slope and Mohr-Coulomb equations in a GIS. Results of the modeling found higher susceptibility for slumping in areas of high slope and high potentiometric surface. The model correlated well at a 92% rate with a data set of 322 inventoried slumps, and included both volcano areas, without overpredicting high-risk slump areas. Another model, SINMAP 2.0, was run to test the veracity of the original model‘s results. Both models were in good agreement with each other. This project provides a reasonable approximation of slope stability and can be used to assist in land use planning to help reduce erosion and its consequences.Item Structural, Kinematic, and Hydrothermal Fluid Investigation of the Gold-Bearing Murray Shear Zone, northeastern Minnesota(2009-10) Johnson, Thomas KennethGold mineralization within Archean granite greenstone terrains of northeastern Minnesota continues to be enigmatic despite its known association to shear zones, faults, and folds. As products of deformational events shear zones, faults, and folds localize fluid transportation and ultimately gold mineralization. Their kinematic history and structural architecture contain important clues about the genesis, characteristics, and mechanics of ore-bearing fluids. The Murray Shear Zone of northeastern Minnesota hosts subeconomic gold in a unique structural setting. Evidence exists in support of shear zone-hosted gold mineralization that coincides with established models for well-known and profitable gold deposits of other Archean greenstone terrains. This thesis presents a structural, kinematic, and hydrothermal fluid investigation of the Murray Shear Zone with implications for gold mineralization. Field and laboratory research such as structural mapping, analysis of thin sections for microstructures and deformation mechanisms, and geochemical study was conducted to provide a comprehensive look at gold mineralization processes of the Murray Shear Zone.Item U-Pb geochronology of the Duluth Complex and related hypabyssal intrusions: investigating the emplacement history of a large multiphase intrusive complex related to the 1.1 Ga Midcontinent Rift.(2010-07) Hoaglund, Steven ArthurPaces and Miller (1993) precisely established the main intrusive periods that created the Duluth Complex and related intrusions associated with the 1.1 Ga Midcontinent Rift in NE Minnesota. They did not, however, resolve differences in emplacement ages within and between major intrusive units due to a small number of dated samples. New high-precision U-Pb baddeleyite and zircon ages from five mafic intrusions related to the Duluth Complex builds on the work of Paces and Miller (1993). Ages from three intrusions that span the entire range of Duluth Complex layered series intrusive activity indicate that the layered series and anorthositic series were synchronously emplaced around 1098 Ma. Moreover, zircon ages obtained from the perceived oldest (Partridge River, 206Pb/238U age 1095.94 ± 0.18 Ma) and youngest (Bald Eagle, 206Pb/238U age 1095.64 ± 0.19 Ma) layered series intrusions suggest the bulk of layered series activity occurred within a period of 670 ka. The Tuscarora intrusion (early layered series) has a distinctly older 207Pb/206Pb baddeleytie age than the Partridge River intrusion (1098.81 ± 0.32 and 1097.98 ± 0.37 Ma, respectively). However, in light of new information regarding the incompatibility between zircon and baddeleyite ages, its temporal relationship with other layered series intrusions remains unclear. Zircon ages obtained from two early Beaver Bay Complex (BBC) intrusions indicate Duluth Complex and BBC magmatism likely overlap. The Houghtaling Creek troctolite is indistinguishable from the other layered series intrusions. Moreover, the Wilson Lake ferrogabbro, long interpreted to be older than the Houghtaling Creek, is distinctly younger (207Pb/206Pb zircon ages 1095.75 ± 0.92 and 1098.62 ± 0.50 Ma, respectively). There still exists an approximate 2 Ma gap between early Beaver Bay and Duluth Complex, and late BBC. A reasonable estimate based on the geometry of layered series intrusions indicate that over 16,000 km3 of mafic magma intruded during layered series emplacement (excluding unknowable amounts of erosion). With over 16,000 km3 of mafic magma being intruded over a period of 670 ka, emplacement rates were at least 0.024 km3/yr, which is similar to estimates for other large continental flood basalt provinces.