The Pennian (~ 245 Ma) Jwaneng kimberlite, situated in southern Botswana, comprises three steep-sided pipes that coalesce approximately 100m below the present day surface to fonn a 54ha body. These pipes have been labelled the South, Centre and North Lobes. The kimberlites intruded a thick sequence of Proterozoic shales, dolomites and sandstones and a thin veneer of consolidated to poorly consolidated mudstones and siltstones of the Karoo Supergroup. Although the shapes of these pipes are comparable to other southern African pipes, they are filled predominantly with crater facies volcaniclastic kimberlite. No tuffisitic kimberlite breccia, the characteristic rock type of the diatremes of other southern African pipes, has yet been identified. The Jwaneng kimberlite thus represents an exception to the standard model for southern African kimberlites, implying that different processes need to be invoked to explain its fonnation. The present study involves a detailed volcanological and sedimentological analysis of the volcaniclastic fill of the Jwaneng South Lobe. Two principal and distinct lithofacies have been identified: the quartz-free RVK facies and the quartz-bearing QRVK facies. Both facies include fine to coarse grained, predominantly massive and subordinate chaotically bedded deposits. The volcaniclastic rocks have been classified as resedimented volcaniclastic kimberlite (RVK) , since their deposition is ascribed to mechanisms dominated by mass flow processes. Based on certain characteristics and differences between the two principal facies, and their spatial distribution within the pipe, they are interpreted as being the products of at least two separate eruption episodes. Certain characteristics (e.g. }hape, granularity~ of the juvenile· magma clasts III the volcaniclastic kimberlite suggest complete crystallisation and devolatilisation of the magma at depth prior to explosive fragmentation. A scenario in which this might have occurred, and which led to catastrophic explosive eruption and pipe excavation is proposed. Explosive eruption and associated tuff cone formation is followed by resedimentation of the material back into the pipe by mass flow processes. Mass flow processes are dominated by debris flow, with lesser grain flow, hyperconcentrated flow and subaqueous mud flow and suspension settling of muddy kimberlitic sediments. Geochemical analyses of the latter indicate a high degree of contamination and weathering, and mixing between pristine kimberlite and silicic shale/mud compositions. Failure and collapse of parts of the underlying pipe walls yielded megablocks of poorly consolidated Permian Karoo mudstone in the peripheral zone of the pipe. This source of the megablocks is supported by their bulk chemical composition. Minor phreatic/phreatomagmatic eruptions are suggested by the presence of rare accretionary and armoured lapilli within both the QRVK and RVK facies. Subsidence of the volcaniclastic pipe fill, inferred mainly from the oversteepened dips of the bedded QRVK and RVK facies, may be related to gravity-induced compaction, late-stage phreatomagmatic eruptions or eruption ofthe adjacent Centre Lobe.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:rhodes/vital:4995 |
| Date | January 2001 |
| Creators | Machin, Kimberley |
| Publisher | Rhodes University, Faculty of Science, Geology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Masters, MSc |
| Format | 188 p., pdf |
| Rights | Machin, Kimberley |
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