Earth and Planetary Sciences ETDs

Publication Date

7-10-2013

Abstract

The ~75 wt % SiO2 Basal Rhyolite is one of seven chemically distinct units of the Dullstroom Formation, the oldest of four formations that comprise the 4-6 km thick siliceous Rooiberg Group of the 2.057 Ga Bushveld Complex, South Africa. The discovery of composite zircon-baddeleyite grains and chromite in the Basal Rhyolite demands a reassessment of proposed scenarios for its petrogenesis. These scenarios include: 1) crustal melting caused by, and differentiation of, intrusive ultramafic magmas of the 9 km thick Rustenberg Layered Suite and 2) crustal melting initiated by an impact of one or more bolides. Although evidence for shock metamorphism, key to verifying an impact origin, has not been found, it has been proposed that unique quench textures in the Basal Rhyolite formed during quenching from impact-induced superheat (T > 1686° C). The presence of coexisting zircon (ZrSiO4) and baddeleyite (ZrO2) has been documented in several impact structures as a result of the reversible reaction: ZrSiO4 = ZrO2 + SiO2 at ~1686°C. Therefore, the presence of zircon and baddeleyite in the Basal Rhyolite provides a test for an impact origin of the Basal Rhyolite. Electron microprobe and transmission electron microscope studies highlight several characteristics of the baddeleyite that are inconsistent with their formation by impact processes. The size, shape, chemistry and configuration of chromites in the Basal Rhyolite also suggest that they once existed in the interstices to olivine or pyroxene in a mafic cumulate prior to entrainment. Since no cumulates are known to exist underneath the Bushveld Complex, the parent lithology of the chromite and baddeleyite is likely a mafic magma associated with the Rustenberg Layered Suite. Chemical similarities between chromites from this study and chromites of a baddeleyite-bearing chill sequence to the Bushveld Complex provide the grounds for future comparative studies of these minerals. At this point, endogenous processes more readily explain the presence and characteristics of composite zircon-baddeleyite grains and chromite. Their presence in the Basal Rhyolite suggests that a dynamic magmatic setting existed during the early stages of Rooiberg Group extrusion.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Brearley, Adrian

Second Committee Member

Newsom, Horton

Third Committee Member

Jones, Rhian

Language

English

Keywords

Bushveld Complex, Rooiberg Group, magma mixing, zirconia, baddeleyite, chromite, chrome spinel, siliceous igneous rocks, impact

Document Type

Thesis

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