Conflicting P–T paths within the Central Zone of the Limpopo Belt: A consequence of different thermobarometric methods?

Abstract

a b s t r a c tA single metapelitic sample from the Verbaard locality, near Messina was investigated in order to construct a P-T path and moreover, highlight pertinent contradictions in the current P-T database. Interpretations based on P-T pseudosections, garnet isopleth thermobarometry and mineral mode/isopleth modelling indicate that the mineral assemblages, textures and zonations developed in the metapelite formed along a single clockwise P-T path. The metamorphic evolution is characterized by an early high-pressure phase at 10-11 kbar/800°C, followed by a simultaneous pressure decrease and temperature increase to $8/850°C and subsequent retrogression via decompression-cooling to 4-5 kbar at T < 650°C. Growth zoning in garnet provides evidence for an earlier, prograde history, however, as potential melt-loss was not accounted for this must be deemed speculative. The results of this study agree entirely with that of [Zeh, A., Klemd, R., Buhlmann, S., Barton, J.M. 2004. Pro-and retrograde P-T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa); constraints from quantitative phase diagrams and geotectonic implications. Journal of Metamorphic Geology 22, 79-95], who adopted a similar approach to thermobarometry i.e. pseudosections. The results are, however, inconsistent with recent publications that argue for a twofold, metamorphic history defined by two decompression-cooling paths (DC1 $2.6 Ga and DC2 $2.0 Ga) that are separated by an isobaric heating path ($2.0 Ga). The disparity in the results obtained from different workers can be explained by an examination of the thermobarometric methods employed. The methodology employed to derive the twofold, polymetamorphic P-T path appears to be erroneous. At present, the most reliable and robust method for determining P-T paths is the pseudosection approach to thermobarometry. Future modelling of Limpopo Belt granulites should adopt this strategy and ensure potential melt-loss is taken into account. Alternatively, this potential problem can be avoided altogether by investigating rocks of mafic composition.

Meso-Archaean and Palaeo-Proterozoic sedimentary sequence stratigraphy of the Kaapvaal Craton

“…Boshoff et al 2006;van Reenen et al, 2008;Millonig et al, 2008) and a final ~2.03 Ga metamorphic overprint (e.g. Boshoff et al, 2006;Zeh et al, 2007;Perchuk et al, 2008;Rigby et al, 2008b;van Reenen et al, 2008;Rigby, 2009;Rigby & Armstrong, 2010). Conversely, the Southern Marginal Zone appears to have an Archaean-only history, with no evidence of 2.0 Ga metamorphism (Eriksson et al, in press).…”

Events in the Precambrian history of the Earth: Challenges in discriminating their global significance

“…They postulated that the later metamorphic overprint produced the same mineral assemblage as the first and that the only change was the composition of the pre-existing minerals. However, Rigby (2009) argued that a reliance on compositional zoning profiles from polymetamorphic samples may lead to erroneous P-T estimates and subsequently a misinterpretation of the tectonic evolution.…”

Orthopyroxene + Sillimanite Predating Sapphirine + Quartz: A Rare Case of Ultrahigh-Temperature Metamorphism From the Central Zone, Limpopo Complex, South Africa

“…(1) Holland & Powell (1998); (2) Kelsey et al (2004) investigation for the last two decades (e.g., Tsunogae & Miyano 1989, Perchuk et al 2000, Van Reenen et al 2004, Zeh et al 2004, Rigby 2009), the ultrahigh temperature event was not reported until recently. Tsunogae & Van Reenen (2006) obtained ultrahigh temperature conditions of ~930 °C at P = 9-10 kbar from orthopyroxene + sapphirine and garnet + staurolite assemblages from a gedrite-garnet rock from the Central Zone in Zimbabwe.…”