- journal_title：South African Journal of Geology
- Contributor：J. TAYLOR ; G. STEVENS ; C. LANA
- Publisher：Geological Society of South Africa
- Date：2012-09-01
- Format：text/html
- Language：en
- Identifier：10.2113/gssajg.115.3.369
- journal_abbrev：South African Journal of Geology
- issn：1012-0750
- volume：115
- issue：3
- firstpage：369
- section：Articles
This study documents for the first time the ages and structural field relationships of banded orthogneisses, and associated high-grade 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-peak-metamorphic 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 (Taylor et al., 2012).