Thesis
Geochemical analysis of Archean lithologies, Yellowstone National Park, WY
Washington State University
Master of Science (MS), Washington State University
2011
Handle:
https://hdl.handle.net/2376/102986
Abstract
Archean units of the northern Yellowstone area consist of biotite gneiss, hornblende gneiss, hornblende-biotite gneiss, quartz monzonite gneiss, mylonite and mica schist. New trace, REE, and isotope ratio data are presented here in order to help constrain the origin of the gneiss protoliths and to elucidate their role as assimilates in younger rhyolites. Average 206Pb/204Pb, 207Pb/204Pb, and 208Pb/206Pb ratios are 15.35, 15.25, and 34.72, respectively. !nd values range from -41.5 to -35.8 for today and -2.2 to 6.7 for 2,700 Ma. Calculated initial 87Sr86Sr ratios range from 0.656 to 0.711. Quartz "18O values vary from 6.5‰ to 9.5‰, plagioclase values vary between 6.5‰ to 8.5‰ and mica values fall between 3.0‰ and 5.5‰. While exact ages for these units remain largely indeterminate, Rb-Sr ages for the gneiss units in the region return average ages of 2.7Ga. Formation of the protoliths in the region most likely occurred along an active continental margin with subduction over an unknown period of time. Petrological and geochemical data do not indicate any fractionation relationships between these units. The protolith magmas for the gneiss units were derived from subduction and partial melting of a hydrothermally altered amphibolite of MORB composition. Crustal contamination and emplacement of the magmas occurred at various crustal levels and generated both positive and negative Eu anomalies, and crustal vs. fluid controlled U/Th and Ba/Th ratios. Incorporation of these gneiss units into the younger Yellowstone rhyolites may occur to a small degree, influencing the observed temporal shifts in radiogenic isotope ratios following caldera-forming eruptions. Sr ratios for the gneiss units are quite variable and may be reflected in the rhyolites. Additionally, the gneiss units may be a significant contributor to the Absarokas volcanics as suggested by trace and REE element data.
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Details
- Title
- Geochemical analysis of Archean lithologies, Yellowstone National Park, WY
- Creators
- Kevin David Tarbert
- Contributors
- Peter B. Larson (Degree Supervisor)
- Awarding Institution
- Washington State University
- Academic Unit
- Environment, School of the (CAHNRS)
- Theses and Dissertations
- Master of Science (MS), Washington State University
- Publisher
- Washington State University; [Pullman, Washington] :
- Identifiers
- 99900525140201842
- Language
- English
- Resource Type
- Thesis