Thesis
Age and petrogenesis of the Matlock ‘Keratophyre’ in northwest Iowa
University of Iowa
Master of Science (MS), University of Iowa
Spring 2024
DOI: 10.25820/etd.007492
Abstract
Mineral exploration cores drilled in 1963 into a magnetic anomaly by the New Jersey Zinc Company in northwest Iowa found the ~2.7 Ga Otter Creek layered mafic intrusion stratigraphically overlain by the ~1.78 Ga Matlock “Keratophyre” (MK). The MK was logged in two of the drilled Matlock cores: C-5 and C-6. The MK was described as a dark gray porphyritic rhyolite unit in the subsurface Precambrian basement of Iowa that grades downward from a crystal-poor tuff (core C-6 and top of core C-5) into a volcanic flow breccia that contains mafic clasts from the underlying 2.7 Ga Otter Creek Layered Igneous Complex (core C-5). This project involved gathering new petrographic and compositional data to investigate the nature and petrogenesis of this highly altered, mixed rhyolite-basalt unit. A PXRF was used to survey compositional variations in the cores to aid sample selection for whole rock geochemistry and U-Pb zircon geochronology (high Zr core sections). Zircon was recovered from two Matlock C-5 core sections, but the sample of crystal-poor tuff (C-6 core section) was barren. Zircons analyzed by LA-ICP-MS at the Arizona LaserChron Center yielded concordia ages of 1803 ± 4 Ma (sample C5 488-9) and 1810 ± 9 Ma (sample C5 1067), suggesting the MK is older than the previous estimate (~1.78 Ga). The ICP-MS data on 24 whole rock samples shows that the MK is not a keratophyre unit but instead, a high potassium rhyolite that has been highly altered in places and variably leached in alkali elements, and that there are three distinct rhyolite units within the MK unit based on incompatible and immobile element ratios, such as Zr/Nb and La/Sm. The pXRF and whole rock geochemical data highlight an unusual amount of mafic clasts in the volcanic flow breccia. These whole rock geochemical data and new zircon U-Pb ages provide additional context about the southern margin of the Superior Craton.
Details
- Title: Subtitle
- Age and petrogenesis of the Matlock ‘Keratophyre’ in northwest Iowa
- Creators
- Nathanael Kilburg
- Contributors
- David Peate (Advisor)William C. McClelland (Committee Member)Raymond Anderson (Committee Member)
- Resource Type
- Thesis
- Degree Awarded
- Master of Science (MS), University of Iowa
- Degree in
- Geoscience
- Date degree season
- Spring 2024
- Publisher
- University of Iowa
- DOI
- 10.25820/etd.007492
- Number of pages
- xi, 129 pages
- Copyright
- Copyright 2024 Nathanael Kilburg
- Grant note
- Geological Society of America for providing financial support in the form of a Graduate Student Research Grant during the 2023-2024 academic year (ii)
- Language
- English
- Date submitted
- 04/17/2024
- Description illustrations
- Illustrations, tables, graphs, charts
- Description bibliographic
- Includes bibliographical references (pages 65-68).
- Public Abstract (ETD)
In 1963, the New Jersey Zinc Company drilled rock cores in northwest Iowa near the town of Matlock. A rock formation was found called the Otter Creek Complex, which is about 2.7 billion years old. Above that, they found a rock formation called the Matlock "Keratophyre" (MK), which is about 1.78 billion years old. This project involved gathering new petrographic and geochemical data to investigate the nature and origin of the MK using mineral and rock samples from two geologic rock cores. A handheld device was initially used to survey element compositional variations in the MK, which helped to select samples for whole rock geochemistry and age dating. It was found that the MK is instead a rhyolite that has been highly altered, and there are three distinct rhyolite units within the MK based on geochemical data. Overall, this new research sheds light on the geological history of Iowa and provides valuable insights into the formation and composition of the MK.
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984647454502771
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