Browsing by Author "Macey, P"

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    Lithostratigraphy of the Mesoproterozoic Twakputs Gneiss
    (Bureau for Scientific Publications, 2021) Doggart, S; Macey, P; Frei, D
    The Twakputs Gneiss is a garnetiferous, K-feldspar megacrystic, biotite granite-granodiorite orthogneiss. It represents a major unit in the Kakamas Domain of the Mesoproterozoic Namaqua-Natal Metamorphic Province extending about 250 km between Riemvasmaak in South Africa and Grünau in southern Namibia. The Twakputs Gneiss occurs as foliation-parallel, sheet-like bodies tightly infolded together with granulite-facies paragneisses into which it intrudes along with a variety of other pre-tectonic granite and leucogranite orthogneisses. These rocks were subsequently intruded by late-tectonic garnet-leucogranites, granites and charnockites. The Twakputs Gneiss is a distinctive unit characterised by large ovoid to elongate megacrysts of twinned perthitic K-feldspar, set in a coarse-grained matrix of garnet, biotite, quartz and feldspar. It contains a penetrative foliation defined by the alignment of K-feldspars and streaks of biotite that developed during the main phase D2 of the Namaqua Orogeny (~1.2 to 1.1 Ga). The foliation and an accompanying elongation lineation are more intensely developed along lithological contacts, especially at the margins of the mega-scale F3 domes and basins that refold the regional fabrics. U-Pb zircon dating of the Twakputs Gneiss has yielded concordia ages of between ~1192 and 1208 Ma. Wholerock geochemistry shows consistent major, trace and REE elemental trends, and thus reflect chemical variability from a single fractionating magma. The Twakputs Gneiss has a granitic to granodiorite composition and is strongly peraluminous. The geochemistry and the ubiquitous presence of garnet and pelitic xenoliths indicate an S-type granite protolith. The Twakputs Gneiss is the most voluminous and widespread member of the Eendoorn Suite which comprises seven textural variants of garnetiferous, K-feldspar-megacrystic granitoid orthogneiss of the same age.
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    Lithostratigraphy of the Naros Granite (Komsberg Suite), South Africa and Namibia
    (Bureau for Scientific Publications, 2021) Thomas, R; Macey, P; Frei, D
    The Naros Granite occurs as a large, northwest-trending ovoid batholith roughly 30 km long and 15 km wide straddling the Orange River border between South Africa and Namibia, 25 km northeast of Onseepkans. It consists mainly of a leucocratic to mesocratic grey, coarse-grained equigranular hornblende-biotite granite-granodiorite that is locally mildly feldspar porphyritic. Small, ovoid mafic autoliths are common and characteristic of the Naros Granite. The composition of the unit varies from granite to granodiorite with a minor leucogranitic phase observed along the southern margin of the batholith. Hornblende and biotite are ubiquitous mafic minerals but small amounts of orthopyroxene occur locally. The Naros Granite has yielded tightly-constrained U-Pb zircon ages between 1 114 Ma and 1 101 Ma. The Naros Granite is generally unfoliated to weakly deformed with only localised shearing along contacts with the surrounding country rocks giving rise to orthogneissic fabrics. It has an intermediate to felsic composition (mean SiO2: 63.9 ± 2.2 wt.%) and is strongly metaluminous. This, together with its biotite-hornblende ± orthopyroxene mineral assemblage and the abundance of mafic autoliths, suggests it is an I-type granitoid, with the source magma produced by partial melting of older igneous rocks that had not undergone any significant chemical weathering. The Naros Granite is the youngest and most evolved member of the ~1.11 Ga Komsberg Suite, a collection of late-to post-tectonic I-type metaluminous, intermediate to felsic, biotite ± hornblende granitoids and their charnockitic equivalents that have intruded the older pre-tectonic gneisses of the Kakamas Domain of the Namaqua Metamorphic Sector