Stable isotope profile across the orthoamphibole isograd in the Southern Marginal Zone of the Limpopo Belt, South Africa

In the Southern Marginal Zone of the Limpopo Belt the transition from granulite- to amphibolite-facies metamorphism is defined by a change from orthopyroxene- to orthoamphibole-bearing assemblages within metasedimentary rocks of the supracrustal Bandelierkop Formation. Oxygen-isotope fractionations among constituent minerals within both amphibolite- and granulite-facies metasedimentary rocks are of similar magnitude implying a similar retrograde thermal history. Temperatures calculated from these fractionations can be arranged in the following order: quartz-garnet ≈ quartz-orthoamphibole ≥ quartz-pyroxene > quartz-plagioclase = quartz-biotite. The observed temperature sequence is consistent with retrograde metamorphism under "closed" system conditions and oxygen closure of garnet, orthoamphibole and orthopyroxene at or close to peak metamorphism while quartz, plagioclase and biotite crystallized from and/or re-equilibrated via a melt/fluid phase down to temperatures of 560°C. Small fluid/rock ratios are implied by the simultaneous closure of quartz, plagioclase and biotite. Given that quartz is the dominant oxygen-bearing phase during retrogressive exchange and assuming garnet closure to oxygen diffusion at the time of formation then the quartz-garnet fractionations may provide minimum peak temperature estimates of 736 ± 52°C. Coarse-grained concordant felsic veins within the metasediments, texturally interpreted as anatectites, are in isotopic equilibrium with the host rock. The "closed" system, defined here only on the basis of oxygen-isotope systematics, does allow for fluid/melt extraction at high temperatures. In contrast to the metasediments, oxygen-isotope systematics for mafic and ultramafic rocks of the Bandelierkop Formation suggest open system behaviour for at least some of the rocks. Such rocks are commonly cross-cut by coarse-grained felsic veins which are not in isotopic equilibrium with the host rock and contain quartz whose δ18O values are indistinguishable from those observed in the metasedimentary rocks. The stable isotope data are consistent with a model involving anatexis of metasedimentary rocks at peak metamorphic temperatures. Partial extraction of the anatectic melts and their crystallization elsewhere in the sequence is suggested by the mineralogy and isotopic composition of large discordant pegmatites together with the isotopic disequilibrium and open system behaviour observed in some mafic and ultramafic rocks. In the pelitic rocks, the release of small amounts of fluid from residual melts and/or somewhat larger amounts from partially collected melts during cooling, may account for many of the retrograde mineralogic features observed for these rocks without perturbing the oxygen-isotope systematics. Such retrograde features include the cordierite + orthopyroxene symplectite after garnet and the incipient hydration of orthopyroxene and cordierite within the granulites and possibly the regional retrogression of granulites to form orthoamphibole gneisses. Retrograde kyanite within these rocks thus suggests an isobaric cooling path. Similar δ13C values of graphite separated from amphibolite- and granulite-facies metasediments together with significant oxygen and carbon isotope heterogeneity among charnockites, metasediments, banded iron formations and most mafic and ultramafic rocks are not consistent with a retrogressive influx of extraneous CO2-rich fluids as the cause of regional retrogression and orthoamphibole formation in the gneisses nor with charnockitization. The data also argue against a pervasive CO2 infiltration as the cause of granulite-facies metamorphism. © 1992.