Intraplate volcanism is a worldwide phenomenon producing volcanoes away from active plate boundaries, a process that cannot yet be sufficiently explained by plate tectonic processes, and thus is still a missing piece in the understanding of the dynamics and evolution of our planet. Models for the formation of intraplate volcanism are dominated by mantle plumes, but alternative explanations, such as adiabatic decompression triggered by lithospheric delamination, and edge driven convection (EDC), could be responsible for magmatism. This dissertation explores intraplate volcanic locations that do not fit the mantle plume model, and presents geochemical evidence for lithospheric delamination and edge driven convection for the cause of volcanism.
I studied an Eocene volcanic swarm exposed in the Appalachian Valley and Ridge Province of Virginia and West Virginia, which are the youngest known igneous rocks along the Eastern North American Margin (ENAM). These magmas provide the only window into the most recent deep processes contributing to the post-rift evolution of this margin. This study presents the first high precision 40Ar/39Ar ages along with new geochemical data, and radiogenic isotopes that constrain the melting conditions and the timing of emplacement. Modeling of the melting conditions suggests that melting occurred under conditions slightly higher than average mantle beneath mid-ocean ridges. Asthenosphere upwelling related to localized lithospheric delamination is a possible process that can explain the intraplate signature of these magmas that lack evidence of a thermal anomaly.
The Virginia-West Virginia region of the ENAM also preserves a second post-rift magmatic event in the Late Jurassic. By studying both the Late Jurassic and Eocene magmatic events we can better understand the post-rift evolution of passive margins. This study presents a comprehensive set of geochemical data that includes new 40Ar/39Ar ages, major and trace-element compositions, and analysis of radiogenic isotopes to further constrain their magmatic history. Modeling suggests that the felsic volcanics from both the Late Jurassic and Eocene events are consistent with fractional crystallization. Lithospheric delamination is the best hypothesis for magmatism in Virginia/West Virginia, due to tectonic instabilities that are remnant from the long-term evolution of this margin, resulting in a 'passive-aggressive' margin that records multiple magmatic events long after rifting ended.
Finally, Bermuda is an intraplate volcano that has been historically classified as mantle plume related but evidence to support the plume model is lacking. Instead, geophysics have argued that EDC is the best model to explain Bermuda volcanism. This study presents the first geochemical analysis of Bermuda volcanism, and found that Bermuda was built by two different magmatic processes: melting of carbonated peridotite to produce silica under-saturated, trace element enriched volcanics and melting of an enriched upper mantle component that produced silica saturated volcanics. We attribute the cyclicity of silica under-saturated and silica saturated volcanics to EDC melting. / Ph. D. / Intraplate volcanoes are found away from active plate boundaries and cannot be explained by plate tectonics. Most introductory geology textbooks attribute intraplate volcanism to the mantle plume model, where hot material rises buoyantly through the Earth’s mantle from depths near the core-mantle boundary. The associated volcanoes are then found in a linear track, due to plate motion over the stationary mantle plume. The mantle plume model is valid for some locations, such as Hawaii, but cannot explain all intraplate volcanoes. Other localized models such as lithospheric delamination and edge driven convection are needed to explain intraplate volcanism. Lithospheric delamination is a process where the base of the lithosphere (crust and upper mantle) pulls away from the top of the lithosphere due to density contrasts. The delamination of the base of the lithosphere allows for the warmer asthenosphere (mantle beneath the lithosphere) to upwell and produce melts by decompression. Edge driven convection is a process where temperature differences in thick, cold continental crust and thin, warm oceanic crust creates a localized convecting cell in the mantle. This convecting cell is associated with down-welling beneath the continental crust and upwelling beneath the oceanic crust, and associated volcanism would be found on the oceanic crust.
In Virginia-West Virginia there are two pulses of intraplate volcanic activity. Chapter 2 of this dissertation explores the geochemistry of the youngest volcanoes of Eastern North America, which are 48 million years old. Combining the geochemistry with the regional geophysics I proposed that lithospheric delamination is a plausible mechanism for these volcanic rocks. Chapter 3 further examines these volcanoes and adds a second pulse of magmatism that occurred 152 million years ago. Lithospheric delamination can also explain the 152 million year old volcanics.
Bermuda is an extinct volcanic island found in the Atlantic Ocean, and has been historically explain by the mantle plume model. However, there has been no geochemical data to support the mantle plume model and the geophysical evidence supports edge driven convection. I present the first geochemical analysis of Bermuda’s volcanic pedestal and find that edge driven convection is a more plausible mechanism to account for volcanism.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/83554 |
| Date | 21 December 2016 |
| Creators | Mazza, Sarah Elizabeth |
| Contributors | Geosciences, Gazel, Esteban, Caddick, Mark J., Bodnar, Robert J., Bizimis, Michael, Johnson, Elizabeth Baedke |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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