P–T record of two high-grade metamorphic events in the Central Zone of the Limpopo Complex, South Africa

Abstract

Many Precambrian granulite facies terrains experienced two regional high-grade events, with the first one having occurred in the Neoarchaean and the subsequent overprints in the Palaeoproterozoic. Since structural and geochronological studies often are unable to discriminate between these two tectono-metamorphic events (D/M), we used the configuration of a composite P–T path as a signature of two granulite facies metamorphisms. The path has a forked lightning shape, composed of two high-grade decompression cooling (DC) branches: a higher-pressure (DC1) and a lower pressure (DC2) trajectory, which are connected by an isobaric heating (IH) path, indicating the start of the second high-grade metamorphism. This approach was used to distinguish different D/M events in the Central Zone of the Limpopo high-grade (HG) terrain, which previously had been considered to mainly reflect evidence for a Palaeoproterozoic event (∼ 2 Ga). We developed a method that allowed us to discriminate between two different HG events based on strong chemical heterogeneity of coexisting minerals in metapelitic gneisses. Using this method we have studied Al-rich gneisses from key geological structures in different parts of the Central Zone, for which we provided evidence for the first Neoarchaean HG event D2/M2 (∼ 2.6 Ga), that is mostly reflected by DC1. Evidence for a subsequent Palaeoproterozoic HG tectono-metamorphic event, D3/M3 (∼ 2 Ga) in the same rocks is reflected by both the IH and relatively low-pressure DC2 segments of a the composite P–T path. Although all studied rock samples preserve records of both the HG metamorphic events, the D3/M3 strongly sheared gneisses contain lesser amounts of the D2/M2 relics compared to rocks less affected by shearing. The results of our studies and a review of published data allow a correct interpretation of questionable isotopic ages and structural data with the following conclusion: the CZ of the Limpopo Complex was subjected to three superimposed tectono-metamorphic events: the earliest D1/M1 (∼ 3.33 Ga) is related to greenstone metamorphic conditions ( Kreissig et al., 2000), followed by two high-grade events that respectively occurred at ∼ 2.6 Ga (D2/M2) and ∼ 2 Ga (D3/M3).