Age and field relationships of Mahamba orthogneisses and Mkhondo Valley metamorphic suite paragneisses from the Mkhondo river, ancient gneiss complex, Swaziland.
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2013
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Abstract
This study documents for the first time the ages and structural field relationships of banded orthogneisses, and associated highgrade
paragneisses, from the Mkhondo River in the Ancient Gneiss Complex of Swaziland, whose age and regional context has
been disputed. LA-ICP-MS U-Pb zircon dating shows that the orthogneiss unit mapped as Mahamba gneiss is composed of a
3215 ± 12 Ma granodiorite phase that was intruded by a 3162 ± 11 Ma felsic granitic phase. A hornblende-bearing tonalite dated at
2889 ± 13 Ma, which occurs structurally beneath the Mkhondo Valley Metamorphic Suite (MVMS) metasediments, experienced a
metamorphic overprint at 2735 ± 11 Ma that coincided with granulite facies metamorphism and partial melting in the MVMS
(Taylor et al., 2010). Both the MVMS and the Mahamba gneisses are characterised by north-northwest aligned, syn-peakmetamorphic
deformation structures, indicating a period of west-southwest to east-northeast directed crustal shortening. Neither the
Mahamba gneisses, nor the hornblende tonalite are intrusive into the structurally overlying MVMS sediments. This suggests that:
(1) the Mahamba gneisses form part of the exposed basement in the area; and (2) the MVMS is a relatively young, late Archaean
sedimentary succession that is unrelated to ≥ 3.23 Ga high-grade paragneiss units documented to the east and northeast of the
MVMS.
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TAYLOR, J.; STEVENS, G.; LANA, C. de C. Age and field relationships of Mahamba orthogneisses and Mkhondo Valley metamorphic suite paragneisses from the Mkhondo river, ancient gneiss complex, Swaziland. South African Journal of Geology, v. 115, p. 369-384, 2013. Disponível em: <http://sajg.geoscienceworld.org/content/115/3/369>. Acesso em: 20 de jun. 2017.