Large Felsic Flows in the Keweenawan North Shore Volcanic Group in Cook County, Minnesota
1988-12
Title
Large Felsic Flows in the Keweenawan North Shore Volcanic Group in Cook County, Minnesota
Authors
Published Date
1988-12
Publisher
Type
Thesis or Dissertation
Abstract
The Devil Track and Kimball Creek felsite sequences are thick and extensive volcanic units in the Keweenawan North Shore Volcanic Group in Cook County, Minnesota. The two units are separated by about 210 m of basalt flows and have a combined total thickness of 625 m, or about 11 [? Text is smudged] of the thickness in the northeast limb of the North Shore Volcanic Group. The major goal of the study was to determine the origin of the units: whether they were lava flows, ash flows or rheoignimbrites. The Kimball Creek felsite sequence has been divided into 4 units on the basis of lithology, chemistry and field evidence. The lowest unit in the Kimball Creek is a porphyritic quartz latite 40 m thick with an extent of at least 7.5 km indicating a very mobile flow. The rock has sparse lineated vesicles, some flow bands and very few visible flattened pumice fragments. The basal quartz latite is tentatively interpreted to be a rheoignimbrite. The next two subunits up-section are icelandites, the lower one 40 m thick, the upper one 200 m thick. The iclandite units contain plagioclase phenocrysts in holocrystalline groundmasses, vesicles increasing in abundance upward and highly vesicular tops. Both units lack primary devitrification features and pyroclastic textures. They are interpreted as lava flows. The highest subunit in the Kimball Creek felsite sequence is a 350-m thick, porphyritic rhyolite that extends inland at least 32 km; it has dimensions more typical of ash flows than of felsic lava flows. Most of the unit is massive and homogeneous with the fine-grained groundmass made up primarily of tabular quartz crystals and subhedral alkali feldspar. Rock near the top and bottom of the unit has flow structures such as lineations and folded bands. The highest known exposure, about 40 m below the present top, contains some fiamme. This unit is interpreted as a rheoignimbrite. The Devil Track felsite sequence has been divided into a lower porphyritic unit and an upper aphyric unit. The lower unit, the Maple Hill rhyolite, is 110 m thick and extends at least 7 km inland from Lake Superior. It has abundant flow structures and highly vesicular rock near the top. This unit is interpreted to have been an extensive rhyolite lava flow. The upper Devil Track unit is a remarkably homogeneous, 200-m thick aphyric rhyolite that extends inland from Lake Superior at least 42 km. It is made up predominantly of fine-grained tabular quartz (tridymite paramorphs) and subhedral alkali feldspar similar to the Kimball Creek rhyolite. It has lineations and bands indicating flowage. The Devil Track is interpreted to be a rheoignimbrite on the basis of its very large extent, low aspect ratio and flow structures, although no pyroclastic textures have been found. Both the Kimball Creek and Devil Track rhyolites are coarser-grained in the middle compared to the upper and lower parts as a result of the slower cooling in the centers of the units. They are each interpreted to have been deposited as single cooling units at unusually high temperatures. The high temperatures and resulting low viscosity of the glass allowed essentially complete collapse of the pyroclastic flows, homogenization of the glass, flowage, and destruction of pyroclasts. High temperatures also facilitated crystallization which further obliterated pyroclastic textures. The Devil Track and Kimball Creek felsite sequences may be part of a magmatic suite that formed by fractional crystallization of a parent magma represented by the icelandite lava flows. Maple Hill magma may have been heated by basaltic magma causing superheating and melting of crystals to produce the aphric Devil Track.
Description
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Thomas Jefferson Fitz III in partial fulfillment of the requirements for the degree of Master of Science, December 1988. Plate 1 referenced in the thesis is also attached to this record.
Related to
Replaces
License
Series/Report Number
Funding information
Isbn identifier
Doi identifier
Previously Published Citation
Suggested citation
Content distributed via the University Digital Conservancy may be subject to additional license and use restrictions applied by the depositor. By using these files, users agree to the Terms of Use. Materials in the UDC may contain content that is disturbing and/or harmful. For more information, please see our statement on harmful content in digital repositories.