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 almost-equal-to quartz-orthoamphibole
greater-than-or-equal-to 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-degrees-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-degrees-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 delta-O-18
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 delta-C-13 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.