Department of Earth Sciences

Permanent URI for this community

https://scholar.sun.ac.za/handle/10019.1/380

Browse

Browsing Department of Earth Sciences by browse.metadata.advisor "Buick, Ian S."

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Geochronology and isotopic characterisation of LCT pegmatites from the Orange River Pegmatite Province
    (Stellenbosch : Stellenbosch University, 2019-04) Doggart, Shane William; Buick, Ian S.; Macey, Paul H.; Frei, Dirk; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    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.