{"id":{"repo_id":"cape-town","oai_identifier":"oai:open.uct.ac.za:11427/20270"},"canonical_url":"https://search.dev.ndltd.org/etd/cape-town/oai:open.uct.ac.za:11427/20270","repository":{"repo_id":"cape-town","name":"University of Cape Town","base_url":"https://open.uct.ac.za/oai/request"},"display":{"title":"A low - δ 18 O intrusive breccia from the Koegel Fontein complex, South Africa","abstract":"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","abstract_html":"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 &lt;1.0 wt %) and is consistent with interaction with fluids at a relatively high temperature (&gt; 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 &#x27;Snowball Earth&#x27; 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","abstract_has_math":false,"creators":["Olianti, Camille Andrea Elisa"],"institution":"Department of Geological Sciences","degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Harris, Chris"],"committee_chairs":[],"committee_members":[],"year":2016,"date_issued":"2016","date_published":"2016","updated_at":"2026-07-22T22:23:22Z","subjects":[],"languages":["eng"],"rights":[],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"http://hdl.handle.net/11427/20270","outbound_label":"Handle","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Harris, Chris"]},{"key":"dc:creator","label":"Author","values":["Olianti, Camille Andrea Elisa"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.accessioned","label":"Dc Date Accessioned","values":["2016-07-08T11:45:03Z"]},{"key":"dc:date.available","label":"Dc Date Available","values":["2016-07-08T11:45:03Z"]},{"key":"dc:date.issued","label":"Date","values":["2016"]},{"key":"dc:publisher.department","label":"Dc Publisher Department","values":["Department of Geological Sciences"]},{"key":"dc:publisher.institution","label":"Dc Publisher Institution","values":["University of Cape Town"]},{"key":"dc:type","label":"Dc Type","values":["Master Thesis"]},{"key":"dc:type.qualificationlevel","label":"Dc Type Qualificationlevel","values":["Masters"]},{"key":"dc:type.qualificationname","label":"Dc Type Qualificationname","values":["MSc"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language.iso","label":"Language (ISO)","values":["eng"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["http://hdl.handle.net/11427/20270"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["Includes bibliographical references"]},{"key":"dc:description.abstract","label":"Abstract","values":["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"]},{"key":"dc:title","label":"Title","values":["A low - δ 18 O intrusive breccia from the Koegel Fontein complex, South Africa"]}]}],"canonical_facts":{"dc:contributor.advisor":["Harris, Chris"],"dc:creator":["Olianti, Camille Andrea Elisa"],"dc:date.accessioned":["2016-07-08T11:45:03Z"],"dc:date.available":["2016-07-08T11:45:03Z"],"dc:date.issued":["2016"],"dc:description":["Includes bibliographical references"],"dc:description.abstract":["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"],"dc:identifier.uri":["http://hdl.handle.net/11427/20270"],"dc:language.iso":["eng"],"dc:publisher.department":["Department of Geological Sciences"],"dc:publisher.institution":["University of Cape Town"],"dc:title":["A low - δ 18 O intrusive breccia from the Koegel Fontein complex, South Africa"],"dc:type":["Master Thesis"],"dc:type.qualificationlevel":["Masters"],"dc:type.qualificationname":["MSc"]},"updated_at":"2026-07-22T22:23:22Z"}