Lithostratigraphy of the Mesoproterozoic Twakputs Gneiss
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Date
2021
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Publisher
Bureau for Scientific Publications
Abstract
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.
Description
Keywords
Twakputs Gneiss, Lithostratigraphy, biotite granite-granodiorite orthogneiss
Citation
Doggart, S, P. H Macey, and D Frei. “Lithostratigraphy of the Mesoproterozoic Twakputs Gneiss.” South African journal of geology 124.3 (2021): 783–794. Web.