Includes bibliographical references / The ~135 Ma Koegel Fontein complex, 350 km north of Cape Town, has rocks with abnormally low δ 18 O values, as low as - 5.2 ‰. The rocks with the lowest δ 18 O values belong to an intrusive breccia and have a fine - grained, black matrix. The silica content of these breccia rocks ranges between 44 - 68 wt %, their whole - rock δ 18 O values vary between - 5.2 ‰ and +1.8 ‰ , and their water content is between 0.2 - 1.1 wt %. The major and trace element composition of the breccia rocks is consistent with them containing variable proportions of bentonite, alkali basalt, gneiss, and epidote - and xenolith - rich material. Mapping indicates that the initial intrusion was an alkali basalt dyke and a bostonite dyke that were then intruded by two breccia plugs that extended along the planes of weakness created by the pre - existing dykes, forming two sub - parallel breccia dykes. The water content of the breccia rocks is low (average <1.0 wt %) and is consistent with interaction with fluids at a relatively high temperature (> 300°C). These fluids interacted with the rocks from both dykes and are responsible for their 18 O - depletion. On the basis of the O - and H - isotope composition of the breccia rocks, the δ 18 O value calculated for the fluids in equilibrium with the breccia rock s is approximately - 10.8 ± 0.2 ‰. This low δ 18 O value of the meteoric fluids is too low for the low latitude and warm climate at ~135 Ma and indicates that 18 O - depletion at Koegel Fontein predates the intrusion of the complex in the Cretaceous. It is possible that low - δ 18 O fluids circulated through the country rock during the Pan African when crustal reworking enabled deep circulation of surface water. This period of crustal reworking coincides with a 'Snowball Earth' event and would have been accompanied by 18 O - depleted meteoric water. In the Cretaceous, it is assumed that a mantle plume heated the crust, causing dehydration before partial melting of the crust. The initial low - δ 18 O fluids are effectively metamorphic fluids in equilibrium with the rock. It i s proposed that the initial loss of these fluids became explosive. These fluids migrated along the pre - existing alkali basalt and bentonite dykes, incorporating fragments of alkali basalt, bostonite, and country rock gneiss. This xenolith - rich fluidised material was then emplaced rapidly into the crust
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/20270 |
| Date | January 2016 |
| Creators | Olianti, Camille Andrea Elisa |
| Contributors | Harris, Chris |
| Publisher | University of Cape Town, Faculty of Science, Department of Geological Sciences |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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