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Petrogenesis of mesoarchean high-K rocks in the Kaapvaal Craton: investigating the Boesmanskop alkaline complex and Salisbury Kop batholith
Thesis ( PhD)--Stellenbosch University, 2025.
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Stellenbosch : Stellenbosch University
2026
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| _version_ | 1867613776764731392 |
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| access_status_str | Open Access |
| author | Leandro, Marcel Vinicius Santos |
| author2 | Stevens, Gary |
| author_browse | Leandro, Marcel Vinicius Santos Stevens, Gary |
| author_facet | Stevens, Gary Leandro, Marcel Vinicius Santos |
| author_sort | Leandro, Marcel Vinicius Santos |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | Thesis ( PhD)--Stellenbosch University, 2025. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/136299 |
| institution | Stellenbosch University (South Africa) |
| language | English |
| last_indexed | 2026-06-10T12:41:31.332Z |
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| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2026 |
| publishDateRange | 2026 |
| publishDateSort | 2026 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/136299 Petrogenesis of mesoarchean high-K rocks in the Kaapvaal Craton: investigating the Boesmanskop alkaline complex and Salisbury Kop batholith Leandro, Marcel Vinicius Santos Stevens, Gary Kisters, Alex Moyen, Jean-Francois Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. Geology, Stratigraphic -- Archaean Syenite Crustal evolution -- Africa, Southern -- Kaapvaal Craton Granite -- South Africa -- Barberton Greenstone Belt Petrogenesis -- Africa, Southern Cratons UCTD Thesis ( PhD)--Stellenbosch University, 2025. Leandro, M. V. S. 2026. Petrogenesis of Mesoarchean high-K rocks in the Kaapvaal Craton: Investigating the Boesmanskop Alkaline Complex and Salisbury Kop batholith. Unpublished doctoral dissertation. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/1d7ce5bd-5dd0-4161-90ae-6c4fd258d269 ENGLISH ABSTRACT: The Boesmanskop Alkaline Complex (BAC) and Salisbury Kop batholith (SB) are two contrasting units of the large ca. 3105 Ma Granodiorite-Monzogranite-Syenogranite (GMS) suite, situated on the eastern margin of the Kaapvaal Craton. The BAC consists of small intrusions at the southern end of the Barberton greenstone belt, recording shear deformation and containing the most potassic and REE-rich rocks of the GMS suite. In contrast, the SB is a homogeneous batholith at the north of the belt with a more sodic character. This study examines the BAC and SB through field observations, petrography, mineral chemistry, whole-rock compositions, and U-Pb zircon geochronology, alongside Rb-Sr and Sm-Nd isotopic data and phase equilibrium modeling for the BAC rocks. The BAC includes coarse-grained syenite, quartz monzonite and leucogranite. Some syenites display porphyritic textures that indicate K-feldspar phenocryst accumulation, with clinopyroxene and Ca-amphibole as intercumulus phases. Field observations indicate that the BAC magmas were emplaced at shallow levels along active shear zones. New U-Pb zircon dating of a quartz monzonite yielded a crystallization age of 3118 ± 9 Ma and inherited cores of ca. 3220 Ma. Zircon, apatite, and titanite compositions reflect high-temperature crystallization from REE-rich felsic magmas, with Ti-in-zircon thermometry recording temperatures up to 915 °C. Additionally, the high K2O+Na2O, low MgO and CaO contents of all the rocks, the presence of inherited zircon cores, and crustal isotopic signatures of the cumulus rocks suggest that they derive from magmas produced by low-degrees of fluid-absent anatexis of the lower crust (including recycled sediments). Phase equilibrium modeling conducted at pressure of 1.5 kbar and temperature range from 800 to 950 °C, consistent with intrusion at shallow depths, demonstrated that a subset of high-alkali content GMS granitic compositions is capable of crystallizing K-feldspar as the first tectosilicate. These experiments support the hypothesis that the syenites and quartz monzonites originated from hot granitic melts that underwent significant crystal accumulation of alkali feldspar. These textural and geochemical features and zircon U-Pb ages advocate that the BAC quartz monzonites and syenites formed earlier than the surrounding granitic bodies, representing the onset of the K-high magmatism responsible for transferring incompatible and heat-producing elements to shallow levels, thereby contributing to the stabilization of the Kaapvaal Craton. Conversely, the leucogranites and granodiorites of the SB lack cumulus textures. U-Pb zircon dating of granodiorites yields a crystallization age of 3094 ± 10 Ma and the titanite dating yields a within error identical age of 3100 ± 11 Ma. The idiomorphic biotite as the only mafic phase suggests that these rocks originated from more evolved and colder magma than the syenites and quartz monzonites. Taken together, the BAC and SB represent two key magmatic complexes that record high-K magmatism associated with the partial melting of the lower crust. The features reported highlight the importance of crustal reworking and lower crustal melting in stabilizing the early cratons, such as the Kaapvaal Craton. The BAC represents the initiation of the magmatic event. RESUMO: O Complexo Alcalino Boesmanskop (BAC) e o batólito Salisbury Kop (SB) são duas unidades contrastantes da grande ca. 3105 Ma Granodiorito-Monzogranito-Sienogranito (GMS) suíte, situadas na margem leste do Cráton do Kaapvaal. O BAC consiste em pequenas intrusões ao sul da suíte, registra a ação de deformação cisalhante e contém as rochas mais potássicas e ricas em ETR da suíte GMS. Em contraste, o SB é um batólito majoritariamente homogêneo ao norte da suíte, com afinidade sódica. Este estudo examina o BAC e o SB por meio de observações de campo, petrografia, química mineral, composições de rocha-total e geocronologia U-Pb em zircão, juntamente com dados isotópicos de Rb-Sr, Sm-Nd e modelagem de fases em equilíbrio para o BAC. O BAC inclui sienitos, quartzo monzonitos e leucogranitos de granulação grossa. Alguns sienitos exibem texturas porfiríticas, indicativas do acúmulo de fenocristais de K-feldspato, com clinopiroxênio e Ca-anfibólio como fases intercumulativas. Observações de campo indicam que os magmas do BAC foram colocados em níveis rasos através de zonas de cisalhamento ativas. A nova datação U-Pb em zircão de um quartzo monzonito revelou uma idade de cristalização de 3118 ± 9 Ma, com núcleos herdados de aproximadamente 3220 Ma. As composições de zircão, apatita e titanita refletem cristalização em altas temperaturas a partir de magmas félsicos ricos em ETR, registrando temperaturas de até 915 °C, baseada no geotermômetro de Ti em zircão. Além disso, os altos teores de K₂O + Na₂O, baixos teores de MgO e CaO observado em todas as amostras, a presença de núcleos de zircão herdados e assinaturas isotópicas crustais das rochas cumulativas sugerem que os magmas iniciais derivam de braixos graus de anatexia na ausência de fluido da crosta inferior (incluindo sedimentos reciclados). A modelagem de fases em equilíbrio realizada a uma pressão de 1,5 kbar e intervalo de temperatura entre 800 e 950 °C, consistente com intrusões colocadas a profundidades rasas, demonstrou que um subconjunto de composições graníticas da GMS suíte, ricas em álcalis é capaz de cristalizar K-feldspato como o primeiro tectossilicato. Esses experimentos apoiam a hipótese de que os sienitos e quartzo monzonitos se originaram de melts graníticos quentes que passaram por acúmulo significativo de cristais de feldspato alcalino. Essas características texturais e geoquímicas e as idades U-Pb em zircão indicam que os sienitos e quartzo monzonitos do BAC se formaram antes dos corpos graníticos ao redor, representando o início do magmatismo de alto K₂O responsável pela transferência de elementos incompatíveis e produtores de calor para níveis superficiais, contribuindo assim para a estabilização do Cráton de Kaapvaal. Por outro lado, os leucogranitos e granodioritos que compõem o SB não apresentam texturas cumulativas. A datação U-Pb em zircão de granodioritos revela uma idade de cristalização de 3094 ± 10 Ma e os cristais de titanita registram uma idade de 3100 ± 11 Ma. A biotita idiomórfica como a única fase máfica sugere que essas rochas se originaram de magmas mais evoluídos e frios do que os sienitos e quartzo monzonitos. Juntos, o BAC e o SB representam dois complexos magmáticos fundamentais que registram o magmatismo de alto potássio associado à fusão parcial da crosta inferior. As características relatadas destacam a importância do retrabalho crustal e da fusão da crosta inferior para a estabilidade da crosta continental primitiva. Enquanto o BAC representa o início do evento magmático, o SB marca seus estágios finais. Doctoral 2026-05-04T08:27:40Z 2026-05-04T08:27:40Z 2025-12 Thesis https://scholar.sun.ac.za/handle/10019.1/136299 en Stellenbosch University 143 pages : illustrations, maps application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Geology, Stratigraphic -- Archaean Syenite Crustal evolution -- Africa, Southern -- Kaapvaal Craton Granite -- South Africa -- Barberton Greenstone Belt Petrogenesis -- Africa, Southern Cratons UCTD Leandro, Marcel Vinicius Santos Petrogenesis of mesoarchean high-K rocks in the Kaapvaal Craton: investigating the Boesmanskop alkaline complex and Salisbury Kop batholith |
| title | Petrogenesis of mesoarchean high-K rocks in the Kaapvaal Craton: investigating the Boesmanskop alkaline complex and Salisbury Kop batholith |
| title_full | Petrogenesis of mesoarchean high-K rocks in the Kaapvaal Craton: investigating the Boesmanskop alkaline complex and Salisbury Kop batholith |
| title_fullStr | Petrogenesis of mesoarchean high-K rocks in the Kaapvaal Craton: investigating the Boesmanskop alkaline complex and Salisbury Kop batholith |
| title_full_unstemmed | Petrogenesis of mesoarchean high-K rocks in the Kaapvaal Craton: investigating the Boesmanskop alkaline complex and Salisbury Kop batholith |
| title_short | Petrogenesis of mesoarchean high-K rocks in the Kaapvaal Craton: investigating the Boesmanskop alkaline complex and Salisbury Kop batholith |
| title_sort | petrogenesis of mesoarchean high k rocks in the kaapvaal craton investigating the boesmanskop alkaline complex and salisbury kop batholith |
| topic | Geology, Stratigraphic -- Archaean Syenite Crustal evolution -- Africa, Southern -- Kaapvaal Craton Granite -- South Africa -- Barberton Greenstone Belt Petrogenesis -- Africa, Southern Cratons UCTD |
| url | https://scholar.sun.ac.za/handle/10019.1/136299 |
| work_keys_str_mv | AT leandromarcelviniciussantos petrogenesisofmesoarcheanhighkrocksinthekaapvaalcratoninvestigatingtheboesmanskopalkalinecomplexandsalisburykopbatholith |