This dissertation contributes to the on-going effort to develop a synthetic view of the evolution of the Earth's mantle and its surface systems through analysis of komatiitic volcaniclastic deposits in the 3.5-3.2 Ga Barberton greenstone belt, South Africa. Komatiites are rare ultramafic volcanic rocks that were almost exclusively erupted during the first 2 Ga of Earth's history. They have served a prominent role in interpreting Archean mantle composition and physical conditions but the pervasive alteration affecting all Archean rocks has led to multiple models of their melting conditions and tectonic settings. There have been few reports of komatiitic pyroclastic debris and limited modern volcanological data has been collected on these deposits. Consequently, little is known about the origins of these pyroclastic rocks and their potential contributions towards a more comprehensive understanding of komatiite petrology.
Chapter 1 describes the physical and geochemical characteristics and distribution of numerous current-worked and graded ashfall deposits in the Barberton greenstone belt that have either high refractory element contents or ratios of immobile elements consistent with those of komatiitic flow rocks. Results of this study show that komatiitic volcanism, which is usually described as almost exclusively effusive, could also be highly explosive. The abundance of these tuffs throughout the stratigraphic record reveals that large-scale komatiitic eruptions played a significant role in greenstone belt development for over 200 million years.
Chapter 2 uses the chemical compositions and textures of these explosively fragmented komatiitic tuffs to decipher multiple episodes of post-depositional alteration, which have dramatically affected original element abundances and disguised primary magmatic signatures in many of the samples. Despite this significant metasomatic overprint, primary ratios of Al, Ti, and the high field strength elements are demonstrably retained, allowing for petrogenetic comparisons with the associated komatiitic flow rocks. Most of the tuffs are aphyric, which suggests that these komatiites were erupted as superheated or near-liquidus anhydrous melts and supports their derivation in mantle plumes. Geochemical, stratigraphic, and geochronologic data demonstrate that intervals of komatiitic volcanism in Barberton are highly diverse in terms of eruptive rates, volumes, and magma compositions, suggesting a variety of plume-plate interactions and/or variation in proximity to eruptive centers.
Chapter 3 investigates the physical mechanisms and environmental conditions that enabled multiple episodes of explosive komatiitic volcanism recorded in the Barberton belt. Documentation of bed and clast textures and stratigraphic relationships reveals that explosive komatiitic eruptions and the widespread distribution of ash occurred due to a combination of factors that include phreatomagmatic fragmentation at high magma-water ratios, volatile exsolution, and high discharge rates. Pyroclastic eruptions likely took place during vent shoaling of submarine volcanoes into shallow-water and/or subaerial settings. Prior to shallow-water buildup, komatiitic volcanism was either effusive, forming extensive lava plains, or resulted in moderate to low energy fragmentation, producing thick hyaloclastite lapilli tuffs and lapillistones.
|Subjects||Geology; Petrology; Sedimentary geology|
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