High-grade metamorphism of ironstones in the Mesoarchaean of southwest Swaziland

Abstract

A sequence of high-grade rocks collectively referred to as Mkhondo Valley Metamorphic Suite is exposed around the Nhlangano Gneiss Dome in southwest Swaziland and also less extensively further north. Meta-ironstones are part of this sequence and have been studied specifically with two main objectives in mind, firstly, to test the hypothesis of the high-grade metamorphic sequence correlating with lower-grade Pongola-age rocks nearby, and secondly, to assess the potential of these meta-ironstones for retrieving P-T conditions of metamorphism. The studied ironstones contain substantial Al due to terrigenous clastic input and also include Mn-rich varieties. Three compositional groups of meta-ironstones can be distinguished: garnet-orthopyroxene-bearing ones, garnet-bearing, orthopyroxene-free ones, and a garnet-poor to garnet-free type. A comparison of bulk rock compositions between these high-grade meta-ironstones and low-grade Mn-poor to Mn-rich ironstones from the Mozaan Group supports a stratigraphic correlation. Pseudosection modelling demonstrates that aluminous meta-ironstones provide useful P-T constraints at high peak temperatures as they do not tend to produce significant amounts of melt, if any. The presence of Al is critical for introducing garnet to amphibolite and granulite-facies assemblages, which also allows to apply conventional thermobarometers. The combination of pseudosection modelling and conventional thermobarometry yielded peak metamorphic conditions of 850–890 °C at 7 to 8 kbar. These data are compatible with the ones obtained from associated metapelitic gneisses, except that the latter indicate decompression to lower pressure granulite-facies conditions. The petrological, structural and geochronological data from southwest Swaziland confirm major orogenic activity in the eastern Kaapvaal Craton during the Neo-Archaean.

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Acknowledgments

P. Horváth acknowledges the support of an NRF postdoctoral scholarship. We would like to thank Noah Nhleko, Swaziland Geological Survey, for showing us the outcrops of low-grade Pongola Fe- and Mn-rich metasediments, Johann Diener for discussions on amphibole modelling, Thomas Fockenberg at the University of Bochum for performing the Fe2+ analytical tests to cross-check our own results, and the director and staff of the Geological Survey and Mines Department, Swaziland, for their hospitality and support. Armin Zeh and an anonymous reviewer are thanked for their critical comments that helped to improve the manuscript.

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Correspondence to Jürgen Reinhardt.

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Horváth, P., Reinhardt, J. & Hofmann, A. High-grade metamorphism of ironstones in the Mesoarchaean of southwest Swaziland. Miner Petrol 108, 589–605 (2014). https://doi.org/10.1007/s00710-013-0307-1

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Keywords

  • Magnetite
  • Kaapvaal Craton
  • Phase Diagram Calculation
  • Transvaal Supergroup
  • Peak Assemblage