Paleo- to Mesoarchean polymetamorphism in the Barberton Granite-Greenstone Belt, South Africa: Constraints from U-Pb monazite and Lu-Hf garnet geochronology on the tectonic processes that shaped the belt: Discussion

Cutts et al. (2014) combined thermodynamic modeling of compositional change in metamorphic garnet with metamorphic peak and retrograde pressure-temperature (P-T ) data derived from quantitative P-T pseudosections to determine clockwise P-T paths for several samples from the Tjakastad schist belt. The Tjakastad schist belt is located in the central portion of the Stolzburg domain at the southwestern end of the Barberton Granite-Greenstone Belt (BGGB). The Stolzburg domain is part of the older southern terrane in the BGGB, which lies south of the Saddleback-Inyoka fault system—an inferred suture (de Ronde and de Wit, 1994). The Stolzburg domain is bounded on the west by the Inyoni shear zone, to the north by the Komati fault, on the east by the Motjane schist belt, and to the south by the younger Mpululzi batholith (see Cutts et al., 2014, their figure 1). Cutts et al. (2014) correlated P-T paths determined for samples from the Tjakastad schist belt with those established by Moyen et al. (2006) for samples from the Inyoni shear zone <15 km to the west (see Cutts et al., 2014, their figure 1). These authors confirmed an earlier observation by Diener et al. (2005) that there is a trend of increasing peak pressure away from the Komati fault, which they interpreted to indicate that the Stolzburg domain represents an exhumed orogenic core. Cutts et al. (2014) concluded that their results are more consistent with the late Paleoarchean regional metamorphism being the result of subduction-accretion processes rather than due to gravitational redistribution by sagduction of the greenstone belt cover and doming of the tonalite-trondhjemite-granodiorite (TTG) basement. By contrast, the gravitational redistribution interpretation, as proposed by Van Kranendonk (2011), was emphasized in a recent article by Van Kranendonk et al. (2014) published after the Cutts et al. (2014) article had appeared in print. These contrasting views of the evolution of the BGGB are typical of differences in the interpretation of the geology and inferences about the tectonics of Archean granite-greenstone terrains worldwide. Therefore, it is important to discuss the evidence in this case and evaluate it against predictions from each of the models, particularly because the BGGB is one of the type localities for Archaean granite-greenstone tectonics. In principle, the quantitative P-T paths determined by Cutts et al. (2014) for several samples from the Tjakastad schist belt represent data that might permit discrimination between the alternative tectonic scenarios and allow an independent assessment of each model. However, although the general sense of dT/dP with time for sample paths is likely to be correct, the P-T paths themselves are subject to intrinsic uncertainties, as discussed later. Thus, in practice it is unclear whether P-T paths are diagnostic of one particular geodynamic driver over another as the cause of burial and exhumation of supracrustal rocks. Over and above this problem, there is inherent doubt about the geodynamics of the lithosphere-asthenosphere system during the Archean Eon due to secular change, as discussed below. For this reason alone, all models for the evolution of Archean lithosphere, whether based on uniformitarian principles or not, must be considered suspect and should be subjected to rigorous testing of predictions against observations. In the commentary that follows, I review briefly Archean geodynamics. Then, I use the full range of data available from the published literature to argue that the results given by Cutts et al. (2014) are not only compatible with gravitational redistribution, but also on a warmer Earth, may be better explained by such a model rather than by uniformitarian plate boundary processes. The following is a list of points of contention: (1) That the Saddleback-Inyoka fault system and its extension along the Inyoni shear zone is a suture resulting from the tectonic juxtaposition of two separate tectonic blocks—the northern and southern BGGB terranes—by plate boundary processes. (2) That the age of intrusion of the TTG plutons in the Stolzburg domain of ca. 3.50– 3.45 Ga is a significant obstacle to an interpretation that the dome-and-keel architecture was the result of greenstones sinking between rising domes of TTG crust at ca. 3.23 Ga, coeval with the regional metamorphism.