Identifier

etd-06192008-092427

Degree

Master of Science (MS)

Department

Geology and Geophysics

Document Type

Thesis

Abstract

The 3.29 Ga Pioneer Ultramafic Complex (WPC) is a 1 km thick east-northeast trending tilted (90° dip) section of layered ultramafic rock in the Weltevreden Formation in the west-northwest region of the Barberton Greenstone Belt (BGB). It was originally interpreted as an intrusive complex with slaty zones that have a linear horizontal fabric produced by shear. However, a recent study identifies these slaty zones as komatiitic tuffs and attributes their horizontal fabric to plane and cross bedding. The present study suggests the non-slaty zones are extrusive komatiitic flows with abundant pyroxene spinifex tops, normal volcanic textures and crystal sizes, and no indication of intrusive cross cutting. A geologic map is presented for WPC komatiites along with a stratigraphic section. The stratigraphic section includes 9 flows or flow sets (numbered WP 1 – 9). WP 1, WP 4, and the lower WP 7 are relatively undifferentiated massive olivine cumulate flows with minor interstitial pyroxenes. WP 2 and 3 are layered komatiitic flows with dunitic to peridotitic bases and increasing pyroxene content and changing pyroxene morphology toward pyroxene spinifex flow tops. Pyroxene spinifex komatiitic basalts are dominant in WP 6 – 8. Each flow type has substantially differing rock and mineral chemistry. Layered komatiitic flows are likely to represent more ponded lavas. WPC komatiites are mineralogically well preserved for Archean komatiites with abundant fresh olivine, orthopyroxene, pigeonite, augite, and chromite. The WPC komatiites are of the Al-undepleted geochemical type with rock and mineral Al2O3/TiO2 very similar to other units of the Weltevreden Formation. This geochemical signature is commonly attributed to the depth of partial melting in plumes, with Al-undepleted komatiites representing shallower partial melting (<450km), and also indicates that WPC komatiites may have a magmatic source related to that of the previously studied Weltevreden Formation komatiites. However, WPC komatiites differ from previously studied Weltevreden Formation komatiites in that they are somewhat less magnesian (maximum olivine Mg# 92.9 vs. 95.6), indicating a lower eruption temperature, but have much more complex and variable lithologic layering.

Date

2008

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

Gary R. Byerly

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