Identifier

etd-02052010-141640

Degree

Master of Science (MS)

Department

Geology and Geophysics

Document Type

Thesis

Abstract

The Saw Mill Complex (SMC) is a 1275 m thick layered komatiitic sequence in the 3.3 Ga Weltevreden Formation, uppermost stratigraphic unit of the Onverwacht Group in the northern part of the Barberton Greenstone Belt, South Africa. A series of ultramafic complexes in the Weltevreden Formation have been interpreted as layered ultramafic intrusions, consisting of thick, layered ultramafic units of peridotite, pyroxenite, dunite, and gabbro. However, recent work on the Pioneer complex of the Weltevreden Formation has demonstrated an extrusive origin of komatiites and tuffs. The Weltevreden Formation has been less studied than other Onverwacht Group units. It is likely composed of a number of individual tectonically juxtaposed terrains brought in by thrust faulting during deposition of Fig Tree Group sediments and felsic volcanics in a magmatic arc setting. It is currently regarded as a product of plume-based eruptive centers. This study represents an ongoing effort to elucidate the structure, stratigraphy, and petrogenesis of the Weltevreden Formation. A new 1:1000 scale map and stratigraphic section of the SMC reinterpret the igneous complex as extrusive, with lithologic units interpreted as layered komatiitic flows and interbedded tuffs. The SMC was rapidly emplaced, not allowing any sediment other than tuff to be deposited. Flows preserved in the SMC dominantly formed as open flow pathways, allowing for large quantities of olivine to accumulate in the lower portions of flows. Some flows formed as closed systems, possibly as a result of deforming tuffs below the flows to form lava lakes. Large cm-sized lapilli, including those with aerodynamic shapes, suggest some subaerial explosions, possibly near-vent. Cross-bedding in the tuffs represent shallow marine environments. Geochemically, the SMC is very similar to Weltevreden Formation komatiites recently studied in the Pioneer complex. They have Al2O3/TiO2 near 30, Gd/Lu normalized values between 0.8 and 1.2, maximum liquid compositions of approximately 33% MgO, and olivines up to Fo94. Oxygen fugacity of the mantle source is determined from V to have -0.18 ∆NNO. MELTS modeling suggests eruption temperature in excess of 1615ºC, representing some of the hottest volcanism experienced on the early Earth.

Date

2010

Document Availability at the Time of Submission

Student has submitted appropriate documentation to restrict access to LSU for 365 days after which the document will be released for worldwide access.

Committee Chair

Byerly, Gary

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