A probe into the deeper structure of the Saldania Belt: pressure-temperature-time signatures of Malmesbury group xenoliths hosted in the Darling Batholith
| dc.contributor.advisor | Stevens, Gary | en_ZA |
| dc.contributor.advisor | Kisters, Alex | en_ZA |
| dc.contributor.advisor | Taylor, Jeanne | en_ZA |
| dc.contributor.author | Cilliers, Kirstin | en_ZA |
| dc.contributor.other | Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. | en_ZA |
| dc.date.accessioned | 2019-02-25T16:20:37Z | |
| dc.date.accessioned | 2019-04-17T08:05:53Z | |
| dc.date.available | 2022-04-30T03:00:12Z | |
| dc.date.issued | 2019-04 | |
| dc.description | Thesis (MSc)--Stellenbosch University, 2019. | en_ZA |
| dc.description.abstract | ENGLISH ABSTRACT: The Saldania belt (SB) in the Western Cape Province of South Africa forms the southernmost extension of Pan-African orogenic belts in southern Africa. Much of the belt is underlain by the Ediacaran Malmesbury Group (MG) that is intruded by granites of the 550-510 Ma Cape Granite Suite (CGS). The sparse outcrop in the region, relatively uniform low-grades of metamorphism and the seemingly monotonous composition of the supracrustal rocks leave the depositional environments and overall geodynamic setting of the rocks controversial. The S-type Darling Batholith (DB) of the CGS hosts abundant metasedimentary xenoliths of greenschist to granulite-facies grade. Villaros et al. (2009) has reported P-T conditions of 10 ± 2 kb and 850 °C for a granulitic xenolith hosted in the DB. This study presents a detailed petrographic and geochronological study on the metasedimentary xenoliths, which allows better constraints on the deeper structure of the Saldania belt and the age and nature of rocks that are not exposed anywhere in the belt. Peak metamorphic assemblages are syn-tectonic, with high-strain fabrics and isoclinally transposed bedding that is invariably truncated by the granite host rock. Thus these fabrics are the products of regional deformation and metamorphism prior to magmatic intrusion. U-Pb dating, of detrital zircon grains separated from the metasedimentary xenoliths, indicates dominant age peaks between 1200 and 574 Ma, constraining the maximum age of deposition to ca. 574 Ma. These ages indicate the presence of Pan-African rocks at depth that compare well with previous age estimates for the MG at surface. The xenoliths are therefore identified as deeper structural equivalents of the MG. The majority of the sediment that makes up the MG is very young compared with the age of deposition. This indicates that sediment influx was dominated by volcanic-arc material, which, in turn, indicates deposition along a convergent margin as opposed to back arc basin. Metamorphic monazite in the xenoliths records ages of ca. 540 Ma. This shows that peak metamorphism is, within error, identical to the main phase of 540-535 Ma S-type granitic magmatism in the SB, but fabric truncation confirms that the xenoliths record a regional deformation that preceded the emplacement of the CGS. This study focuses on one type of high-grade xenolith, Grt + Pl + Bt + Sil + Hc + Cor + Ilm granulite- facies metapelites. Phase-equilibrium modelling results in estimates of peak conditions a 6.3 - 6.9 kb and 825 - 850 °C for these granulites. The metasedimentary xenoliths thus provide a P-T window as deep as 23- 26 km into the Saldania Belt. Previous studies of the CGS that focussed on inherited zircon core U-Pb ages, Hf isotopes and δ18O values support the model that the CGS magmas were sourced from partial melting of the MG metasediments. The granulitic MG xenoliths are all quartz- depleted and highly peraluminous, indicating that they underwent significant partial melting, which likely contributed to magma genesis of the CGS. P-T estimates of the melt-depleted granulite xenolith thus characterise the crustal depth of magma genesis of the CGS. This study provides an upper bound of 23 km, and Villaros et al. (2009) the lower bound of + 37 km for CGS magma genesis. Equilibration pressures of the xenoliths also imply a considerable structural thickness for MG rocks of 23- 26 km, which indicates that the MG sediments were most likely deposited and deformed along an active convergent margin as accretionary melánge, which can typically become up to 40 km thick. These thickness estimates contrast markedly with similarly old but much thinner (2-3 km) sequences to the immediate north such as the Gariep Supergroup in the Gariep Belt, for which most studies assume a back-arc depositional environment. Hence, correlations between the MG and Gariep Supergroup no longer seem tenable. | en_ZA |
| dc.description.abstract | AFRIKAANSE OPSOMMING: Die Saldania-band (SB) in die Wes-Kaap Provinsie van Suid-Afrika vorm die suidelike uitbreiding van Pan-Afrika-orogeniese bande in Suider-Afrika. Baie van die band is ondergeskik deur die Ediacaran Malmesbury Groep (MG) wat deur graniete van die 550-510 Ma Kaapse Graniet Suite (KGS) ingedring word. Die yl uitsak in die streek, relatief uniforme lae grade metamorfose en die oënskynlik eentonige samestelling van die supracrustale gesteentes laat die afsettingsomgewings en die algemene geodinamiese omgewing van die rotse omstrede bly. Die S-tipe Darling Batoliet (DB) van die KGS bied oorvloedige metasimentêre xenoliete van groenskis tot granuliet-fasies- metamorfose. Villaros et al. (2009) het P-T toestande van 10 ± 2 kb en 850 °C gerapporteer vir 'n granulitiese xenoliet wat in die DB gehuisves word. Hierdie studie bied 'n gedetailleerde petrografiese en geochronologiese studie oor die metasimentimentêre xenoliete wat beter beperkings op die dieper struktuur van die Saldania-band en die ouderdom en aard van keldergesteentes wat nie in die band blootgestel word nie, toelaat. Piek-metamorfe samestellings is sintetonies met hoëspanningindikators en isoklinaal-transponerende beddegoed, wat altyd deur die graniet-gasheer afgekap word. Hierdie materiale is die produk van streeksdeformasie en metamorfisme voor magmatiese indringing. U-Pb-dateering van detritale sirkoonkorrels geskei van die metasimentimentêre xenoliete dui op dominante ouderdompieke tussen 1200 en 574 Ma, wat die maksimum ouderdom van afsetting beperk tot ca. 574 Ma. Hierdie ouderdomme dui die teenwoordigheid van Pan-Afrika-rotse aan die diepte aan, wat goed vergelyk met vorige ouderdomskattings vir die MG op die oppervlak. Die xenoliete word dus as dieper strukturele ekwivalente van die MG geïdentifiseer. Die meerderheid van die sediment wat die MG uitmaak, is baie jonk in vergelyking met die ouderdom van afsetting. Dit dui daarop dat die instroming van die sediment oorheers is deur vulkaniese boogmateriaal, wat op sy beurt 'n aanduiding is van afsetting langs konvergente plaatgrense. Metamorfe monasiet in die xenoliths rekordeer ouderdomme van ca. 540 Ma. Dit toon dat piekmetamorfose binne foutgrense identies is aan die hooffase van 540-535 Ma S-tipe granietmagmatisme in die band, maar afkorting van streeksmetamorfiese teksture bevestig dat die xenoliete 'n streeksdeformasie opteken wat die KGS indringing voorafgegaan het.Stellenbosch University https://scholar.sun.ac.za v Hierdie studie fokus op een tipe hoëgraadse xenoliet, 'n Grt + Pl + Bt + Sil + Hc + Cor + Ilm granuliet-fasies metapeliet. Fase ewewigsmodellering skat piek toestande van 6.3 tot 6.9 kb en 825 tot 850 ° C vir die granuliete. Die metasedimentêre xenoliete bied dus 'n P-T-venster so diep soos 32- 26 km in die Saldania-band. Vorige studies van die KGS wat gefokus het op geërfde zirkoonkern U-Pb- ouderdomme, Hf-isotope en δ18O -waardes, ondersteun dat die KGS-magmas afkomstig is van die gedeeltelike smelting van die MG-sedimente. Die granulitiese MG-xenoliete is kwarts uitgeput en hoogs peralumineus, wat aandui dat hulle beduidende smelting ondergaan het wat tot die magma- generasie van die KGS bygedra het. P-T-ramings van die smelt-uitgeputte granuliet-xenoliet karakteriseer dus die korsdiepte van die magma-generasie van die KGS. Hierdie studie bied die boonste perk van 23 km en Villaros et al. (2009) die ondergrens van + 37 km vir KGS magma- generasie. Ewewigsdrukke van die xenoliete impliseer ook 'n aansienlike strukturele dikte vir MG-rotse van 23 tot 26 km, wat daarop dui dat die MG-sedimente langs 'n aktiewe konvergente marge gestort en vervorm is, as 'n aanvullende melange wat tipies tot 40 km dik kan word. Hierdie dikte ramings kontrasteer merkbaar met die soortgelyke ou, maar baie dunner (2-3 km) sedimentêre opeenvolgings tot die onmiddellike noorde soos die Gariep Supergroep in die Gariep-band, waarvoor die meeste studies 'n terugboë-afsettingsomgewing aanneem. Dus lyk korrelasies tussen die MG en Gariep Supergroep nie meer geldig nie. | af_ZA |
| dc.description.version | Masters | |
| dc.embargo.terms | 2022-04-30 | |
| dc.format.extent | xxiii, 165 pages : illustrations, maps | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/10019.1/105635 | |
| dc.language.iso | en_ZA | en_ZA |
| dc.publisher | Stellenbosch : Stellenbosch University | en_ZA |
| dc.rights.holder | Stellenbosch University | en_ZA |
| dc.subject | Saldania belt -- South Africa -- Western Cape | en_ZA |
| dc.subject | Granite -- South Africa -- Darling | en_ZA |
| dc.subject | Malmesbury Batholith (South Africa) | en_ZA |
| dc.subject | Xenoliths -- South Africa -- Malmesbury | en_ZA |
| dc.subject | Pressure -- Temperature | en_ZA |
| dc.subject | Metasedimentary rocks | en_ZA |
| dc.subject | Granulite -- Africa Southern | en_ZA |
| dc.subject | UCTD | en_ZA |
| dc.title | A probe into the deeper structure of the Saldania Belt: pressure-temperature-time signatures of Malmesbury group xenoliths hosted in the Darling Batholith | en_ZA |
| dc.type | Thesis | en_ZA |