A petrographic and geochemical study of mafic sills within the Transvaal Supergroup, east of the Bushveld Igneous Complex
| dc.contributor.advisor | Klausen, Martin | en_ZA |
| dc.contributor.author | Netshidzivhe, Thendo | en_ZA |
| dc.contributor.other | Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. | en_ZA |
| dc.date.accessioned | 2017-11-20T08:57:56Z | |
| dc.date.accessioned | 2017-12-11T10:35:01Z | |
| dc.date.available | 2018-12-31T03:00:07Z | |
| dc.date.issued | 2017-12 | |
| dc.description | Thesis (MSc)--Stellenbosch University, 2017. | en_ZA |
| dc.description.abstract | ENGLISH ABSTRACT: The marginal sills that intruded into the Transvaal Supergroup have long been used to infer the parental magmas of the Rustenburg layered suite (RLS) due to their geochemical similarities which they share with the different zones of the RLS. New geochemical and petrographical data on mafic-ultramafic sills hosted within the sedimentary rocks across most of the Transvaal Supergroup, as well as below it (i.e., into the Archean basement) reveal a large group of ‘boninitic norites’ (BN) with primary mineral assemblage of euhedral orthopyroxene, subhedral plagioclase and interstitial plagioclase as well as tholeiitic dolerites (TDs) that have a primary mineral assemblage of euhedral plagioclase, clinopyroxene, and accessory Fe–Ti oxides. There are also some ultramafic sills that clearly formed through the accumulation of mainly olivine crystals, suggesting in situ fractional crystallisation within the sills. The slightly ultramafic and orthopyroxene-rich BN-sills are not believed to be cumulates, however, as testified by ‘spinifex-textured’ examples of similar compositions that may be regarded as quenched magmas. In many cases both BN and TD type sill samples are severely altered or even metamorphosed into mainly amphibole-rich mineral assemblages (as well as serpentinized harzburgites), where normative calculations become the best way to interpret protolith modal compositions. It is not obvious to classify more altered/metamorphosed sills as being pre-Bushveld and more pristine samples as synto post-Bushveld, because local alteration, as opposed to regional contact metamorphism, may also have played a role. Truly boninitic sills have relatively high MgO, Si, LREE and lower Fe-Ti and were emplaced throughout the sedimentary sequence, together with graphic quartzfeldspar dominated norites, which through their geochemistry are believed to have been more differentiated magmas within the same BN-suite. A comparison of geochemical signatures confirms a link between the BN rocks and the early B1 magmas of the Bushveld Complex. However, the wider stratigraphical distribution of BN sills argue against these clearly Bushveld related magmas to only have been injected from the large magma chamber that formed the RLS, but rather that many BN sills also were injected prior to the magma chamber’s establishment as well as replenished it during the deposition of the RLS. The geochemical characteristics of the tholeiitic dolerites (high Ti, low Si, Mg and LILE) do not match other proposed Bushveld-related B2 and B3 parental magmas, These TD sills also seem to cluster within the Silverton Formation, the Archean basement, as well as the Malmani Group (mainly cutting the Uitkomst Complex), and could therefore have formed during an entirely different magmatic event. A geochemical match suggests that most basement sills were fed by an intersecting Rykoppies dyke swarm, during the early deposition of the Wolkberg Group. A similar geochemical comparison cannot be made with overlying Transvaal Supergroup hosted lavas, where it seems plausible that many of the Silverton-hosted TD sills may have formed during the deposition of the Machadodorp Formation. However, geochemical similarities with the ~1.1 Ga Diabete dyke swarm also opens up for many of the TD suite being post- Bushveld (especially those that cut throughout the Uitkomst Complex). | en_ZA |
| dc.description.abstract | AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar | af_ZA |
| dc.embargo.terms | 2018-12-31 | |
| dc.format.extent | 158 pages : illustrations, maps | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/10019.1/102636 | |
| dc.language.iso | en_ZA | en_ZA |
| dc.publisher | Stellenbosch : Stellenbosch University | en_ZA |
| dc.rights.holder | Stellenbosch University | en_ZA |
| dc.subject | Sedimentary rocks | en_ZA |
| dc.subject | Petrographical data | en_ZA |
| dc.subject | Geochemical data | en_ZA |
| dc.subject | Sills (Geology) | en_ZA |
| dc.subject | UCTD | en_ZA |
| dc.title | A petrographic and geochemical study of mafic sills within the Transvaal Supergroup, east of the Bushveld Igneous Complex | en_ZA |
| dc.type | Thesis | en_ZA |
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