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Title: The petrogenesis of the Mesoproterozoic mafic dykes and sills of Zimbabwe
Author: Ward, Susan E.
ISNI:       0000 0001 3563 2938
Awarding Body: University of Portsmouth
Current Institution: University of Portsmouth
Date of Award: 2002
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The assembly of the Mesoproterozoic super-continent Rodinia of southern Africa is evidenced by the fonnation of supracrustal belts and related granitic magmatism mainly between 1400 and 900 Ma and known collectively throughout Africa as the Kibaran event. As these new crustal segments were being fonned, the ancient cratons were flexed and, in some regions, fractured. The associated intracratonic mafic magmatism is preserved as mafic dyke swanns in certain parts of southern Africa. Three dyke swarms, namely the Kamativi, Mutare and Guruve are believed to be of "Kibaran" age (although their age constraints are, as yet, unknown). The Umkondo sills are also of Kibaran age having been dated at 1105 ± 2Ma. The geochemistry of these three dyke swanns indicates that whilst they are ostensibly unremarkable continental tholeiites, they do in fact comprise dykes of rather different compositions. Three distinct subgroups are identified within the Kamativi dykes, six subgroups within the Mutare dykes and six subgroups within the Guruve dykes. The recognition of these subgroups suggests that the term "swarm" cannot be used when referring to these intrusive suites until the age constraints of the different subgroups within a suite are defmed. The Umkondo sills have a unifonn geochemistry and are very different and easily recognisable from the older (~1.8 Ga) Mashonaland sills in that they have a higher Si02 content, lower Fe203 and Ti02, content with lower TilZr and TiN ratios than the Mashonaland sills. The differences in the geochemistry of the Kamativi, Mutare and Guruve subgroups and the Umkondo sills has led to the recognition of three distinct magma types. A relatively enriched, high-Ti source termed type I, a depleted low-Ti source tenned type II and type III which is a relatively depleted low-Ti source with selective trace element, subduction-related enrichment. The similarities between the type III dykes and the Umkondo sills indicate a craton-wide igneous event with melting from similar source mantle compositions. Distinct plume characteristics are not found within the geochemistry of the Zimbabwe intrusives, but this does not necessarily preclude a plume source. Crustal contamination may overprint plume signatures and result in the increase 87Sr/86Sr and corresponding decrease in 144Ndl143Nd ratios as seen in these dykes. As collisional tectonics along the western and southern margins of the Kalahari craton are indicated in the Namaqua-Natal Belt of South Africa, compressional stresses along the southern and western margins of the Kapvaal craton may have caused extension within the Zimbabwe craton. This resulted in the upwelling of hotter mantle material which induced melting of the subduction modified SCLM. This research has shown the importance of the geochemistry of dykes, in establishing whether they constitute separate "swanns", or relatively small batches of melt from different sources intruded penecontemporaneously.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available