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

A 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 degrees C, followed by a simultaneous pressure decrease and temperature increase to similar to 8/850 degrees C and subsequent retrogression via decompression-cooling to 4-5 kbar at T < 650 degrees 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 similar to 2.6 Ga and DC2 similar to 2.0 Ga) that are separated by an isobaric heating path (similar to 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. (C) 2009 Elsevier Ltd. All rights reserved.