Geochronology and isotopic characterisation of LCT pegmatites from the Orange River Pegmatite Province

Doggart, Shane William (2019-04)

Thesis (MSc)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: The pegmatites of the Orange River Pegmatite Belt (ORPB) form an extensive, ca. 450km long continuous W-E trending belt extending from Vioolsdrif to Kenhardt in South Africa, with the northern extent reaching into the southern Karas region of Namibia. This study attempts to explain the temporal and geochemical relationship of the pegmatites in the ORPB and the rocks of the Namaqua sector of the NNMP. In order to understand the evolution of the ORPB pegmatites this study uses: 1) in-situ high spatial resolution monazite (LA-Q-ICP-MS) geochronology to obtain pegmatite emplacement ages, and 2) in situ LA-MC-CP-MS isotopic tracing of monazite grains to determine the Sm-Nd isotope compositions and 2-stage depleted model ages for the pegmatites. The geochronological data yielded U-Pb dates ranging from ~960 Ma to ~1038 Ma suggesting that the pegmatites of the ORPB were emplaced over a ~80 M.yr period. Additionally, the fact that the ORPB pegmatites are observed in the field to intrude metasediments of the Namaqua-Natal Metamorphic Belt, might be used to suggest that they were sourced via partial melting mechanism during late-stage D4 deformation of these metasediments. This is hard to envisage owing to the fact that the pegmatites that are emplaced into tectonic domains with vastly different metamorphic histories from greenschist- and amphibolite-facies in the ~1.9 Ga Richtersveld Magmatic Arc (RMA) to the poly-metamorphic granulite-facies Mesoproterozoic Kakamas Domain (KD). The question is posed: how did these late-stage, highly fractionated granitic rocks originate? What was their emplacement history? If they are not linked to any late Namaqua age granitic plutons, is the source of the parental melts sourced via local anatexis of the metasedimentary country rocks of the RMA and KD? Radiogenic data obtained give ƐNd (1000) values of ~ -15 to ~ – 1, with the lowest values attributed to pegmatites within the RMA with ƐNdt values increasing where pegmatites intrude into the more juvenile Mesoproterozoic KD. T2DM model ages suggest that the pegmatites have similar sources to their host country rocks. However, metamorphic grades within the RMA never attained conditions conducive to partial melting and in the KD had reached peak, granulite-facies, metamorphic conditions ~200 Ma prior, and were hence refractory to further melting at ~1.0 Ga during the regional pegmatite emplacement. The findings from this study indicate that the pegmatites within the RMA (T2DM: ~2.7 – 2.1 Ga) were possibly sourced from anatexis occurring at depth of fertile metasediments in the tectonically underlying Bushmanland Subprovince (BSP; T2DM: ~2.9 – 2.3 Ga) at ~1.0 Ga. Sm-Nd geochemistry from this study also suggests that the parental melts responsible for the pegmatites in the KD (T2DM: ~~1.8-1.7 Ga) are interpreted to have been sourced via anatexis of retrogressed granulite-facies KD country rocks at deeper crustal levels at around the same time, and possibly aided by fluid from anatexis of the BSP.

AFRIKAANSE OPSOMMING: Die pegmatiete van die Oranje Rivier Pegmatiet Gordel (ORPG) vorm ’n uitgebreide ca. 450 km-lange kontinue W–O-strekkende gordel vanaf Vioolsdrif tot Kenhardt in Suid-Afrika, met die noordelike uitbreiding wat die suidelike Karas area van Namibië bereik. Hierdie studie poog om die temporele en geochemiese verwantskap tussen die pegmatiete in die ORPG en die gesteentes van die Namakwa sektor van die NNMP te verklaar. Om die evolusie van die ORPG pegmatiete te verstaan, gebruik hierdie studie: 1) in situ hoë ruimtelike resolusie monasiet (LA-Q-ICP-MS) geochronologie om die inplasingsouderdomme te verkry en 2) in situ LA-MC-CP-MS isotoop natrekking van monasietkorrels om die Sm-Nd isotoop samestellings en 2-fase verarmde model ouderdomme van die pegmatiete vas te stel. Die geochronologiese data het U-Pd datums wat vanaf ~960 Ma tot ~1038 Ma varieer, wat sou voorstel dat die pegmatiete van die ORPG ingeplaas is oor ’n tydperk van ~80 M.yr. Hierbenewens mag die feit dat die ORPG pegmatiete in die veld gesien word om die metasedimente van die Namakwa-Natal Metamorfe Gordel binne te dring, gebruik word om voor te stel dat hulle ontstaan het as gevolg van ’n gedeeltelike smeltingsmeganisme gedurende die laat-stadium D4 deformasie van hierdie metasedimente. Dit is moeilik om voor te stel aangesien die pegmatiete ingeplaas is in tektoniese domeine wat ingrypend verskillende metamorfe geskiedenisse het as die groenskis en amphibolite fasies in die 1.9 Ga Richtersveld Magmatiese Boog (RMB) tot die polimetamorfe granuliet fasies Mesoproterosoïese Kakamas Domein (KD). Die vraag ontstaan nou: hoe het hierdie laat-stadium, hoogs gefraksioneerde granitiese gesteentes ontstaan? Wat was hulle inplasingsgeskiedenis? Indien hulle nie verwant is aan enige laat-stadium Namakwa-ouderdom granitiese plutone nie, het die stam smeltsels ontstaan via die lokale anatekse van die metasedimentêre newegesteentes van die RMB en die KD? Beskikbare radiogene data het εNd (1000) waardes vanaf ~ -15 tot ~ – 1 gelewer, met die laagste waardes toegeken aan die meer juveniele Mesoproterosoïse KD. T2DM model ouderdomme blyk voor te stel dat die pegmatiete ’n soortgelyke oorsprong as hulle newegesteentes sou hê. Nietemin, metamorfe grade binne die RMD het nooit toestande bereik wat gunstig sou wees vir gedeeltelike smelting nie en het in die KD piek granuliet-fasies metamorfe toestande bereik te ~200 Ma vooraf en was dus vuurvas met betrekking tot verdere smelting om en by ~1.0 Ga ten tye van die regionale pegmatiet inplasing. Die bevindings van hierdie studie dui aan dat die pegmatiete binne die RMB (T2DM: ~2.7 – 2.1 Ga) moontlik hul oorsprong vanuit anatekse mag hê wat plaasgevind het in ’n diep omgewing te midde van die vrugbare metasedimente in die tektonies onderliggende Boesmanland Subprovinsie (BSP; T2DM: ~2.9 – 2.3 Ga) te ~1.0 Ga. Sm-Nd geochemie vanuit hierdie studie sou ook aandui dat die stamsmeltsels wat aanleiding gegee het tot die pegmatiete in die KD (T2DM: ~~1.8-1.7 Ga) geïnterpreteer sou kon word as afkomstig vanuit die anatekse van retrogressiewe granuliet facies KD newegesteentes in dieper kors lae omtrent terselfde tyd, en moontlik aangehelp is deur vloeistof vanuit die anatekse van die BSP.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105944
This item appears in the following collections: