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The H 2 O and CO 2 content of cordierite was analysed in 34 samples from successive contact metamorphic zones of the Etive thermal aureole, Scotland, using Fouriertransform infrared spectroscopy (FTIR). The measured volatile contents were used to once free hydrous fluid is exhausted. All sillimanite zone samples record total volatile contents that are significantly lower than modelled H 2 O-CO 2 saturation surfaces,

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ABSTRACT:The Kaapvaal Craton hosts a number of Precambrian sedimentary successions which were deposited between 3105 Ma (Dominion Group) and 1700 Ma (Waterberg Group) Although younger Precambrian sedimentary sequences outcrop within southern Africa, they are restricted either to the margins of the Kaapvaal craton, or are underlain by orogenic belts off the edge of the craton. The basins considered in this work are those which host the Witwatersrand and Pongola, Ventersdorp, Transvaal and Waterberg strata. Many of these basins can be considered to have formed as a response to reactivation along lineaments, which had initially formed by accretion processes during the amalgamation of the craton during the Mid-Archaean.Faulting along these lineaments controlled sedimentation either directly by controlling the basin margins, or indirectly by controlling the sediment source areas.Other basins are likely to be more controlled by thermal affects associated with mantle plumes. Accommodation in all these basins may have been generated primarily by flexural tectonics, in the case of the Witwatersrand, or by a combination of extensional and thermal subsidence in the case of the Ventersdorp, Transvaal and Waterberg. Wheeler diagrams are constructed to demonstrate stratigraphic relationships within these basins at the first-and second-order levels of cyclicity, andcan be used to demonstrate the development of accommodation space on the craton through the Precambrian.

This paper discusses geological events with an approximately global preservational scale which can aid inter-cratonic correlations and contribute to postulates of supercontinents for a set of chosen Precambrian cratons. The chronological scale of such events is highly variable, and most event types detailed (supercontinent-, mantle plume-, orogenic-, chemostratigraphic-, glacial events and major unconformities) have durations concomitant with the large scale interaction of mantle thermal and plate tectonic processes that were largely responsible for their genesis, i.e. 10's to 100's of millions of years. Geologically instantaneous events of global compass (e.g., impact or major eruptive events) provide important chronological markers for interpreting the longer term events. The same interplay of tectono-thermal geodynamic processes that drives the evolution of the Earth and the operation of its supercontinent cycles is also, ultimately, responsible for and of comparable duration to first-and second-order sequence stratigraphic cyclicity. This paper thus serves to introduce these concepts and discuss the problems in their application to specific Precambrian cratons, in relation to the aim of this special issue, of providing a set of accommodation curves for many of these ancient crustal terranes.

Published literature argues that the Limpopo Belt can be subdivided into three zones, each with a distinctive geological character and tectono-metamorphic fingerprint. There are currently two contrasting schools of thought regarding the tectono-metamorphic evolution of the CZ. One camp argues that geochronological, structural and prograde pressure-temperature (P-T) evidence collectively indicate that the CZ underwent tectono-metamorphism at ca. 2.0 Ga which followed a clockwise P-T evolution during a transpressive orogeny that was initiated by the collision of the Kaapvaal and Zimbabwe cratons. Deformation and metamorphism consistent with this scenario are observed in the southern part of the NMZ but are curiously absent from the whole of the SMZ. The opposing view argues that the peak metamorphism associated with the collision of the Kaapvaal and Zimbabwe cratons occurred at ca. 2.6 Ga and the later metamorphic event is an overprint associated with reactivation along Archean shear zones. Post-peakmetamorphic conditions, which at present cannot be convincingly related to either a ca. 2.6 or 2.0 Ga event in the CZ reveal contrasting retrograde paths implying either nearisothermal decompression and isobaric cooling associated with a 'pop-up' style of exhumation or steady decompression-cooling linked to exhumation controlled by erosion. Recent data argue that the prograde evolution of the ca. 2.0 Ga event is characterised by isobaric heating prior to decompression-cooling. Contrasting P-T paths indicate that either different units exist within the CZ that underwent different P-T evolutions or that some P-T work is erroneous due to the application of equilibrium thermobarometry to mineral assemblages that are not in equilibrium. The morphology of the P-T path(s) for the ca. 2.6-2.52 Ga event are also a matter of dispute. Some workers have postulated an anticlockwise P-T evolution during this period whilst others regard this metamorphic event as following a clockwise evolution. Granitoid magmatism is broadly contemporaneous in all three zones at ca. 2.7-2.5 suggesting a possible causal geodynamic link. P-T contrasts between and within the respective zones prevent, at present, the construction of a coherent and inter-related tectonic model that can account for all of the available evidence. Detailed and fully-integrated petrological and geochronological studies are required to produce reliable P-T-t paths that may resolve some of these pertinent issues.

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.

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