Masters Degrees (Earth Sciences)

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Browsing Masters Degrees (Earth Sciences) by Author "Cilliers, Kirstin"

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    A probe into the deeper structure of the Saldania Belt: pressure-temperature-time signatures of Malmesbury group xenoliths hosted in the Darling Batholith
    (Stellenbosch : Stellenbosch University, 2019-04) Cilliers, Kirstin; Stevens, Gary; Kisters, Alex; Taylor, Jeanne; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    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.