Rapid evolution from sediment to anatectic granulite in an Archean continental collision zone: the example of the Bandelierkop Formation metapelites, South Marginal Zone, Limpopo Belt, South Africa

Abstract

The metamorphic history of the Southern Marginal Zone (SMZ) of the Limpopo Belt, South Africa, possibly provides insight into one of the oldest preserved continental collision zones. The SMZ consists of granitoid gneisses (the Baviaanskloof Gneiss) and subordinate, infolded metasedimentary, metamafic and meta-ultramafic lithologies (the Bandelierkop Formation) and is regarded as the c. 2700 Ma granulite facies reworked equivalent of the Kaapvaal craton basement. The granulite facies metamorphism is proposed to have occurred in response to collision between the Kaapvaal and Zimbabwe cratons. Previous studies have proposed a wide variety of P-T loops for the granulites, with considerable discrepancy in both the shapes of the retrograde paths and the magnitude of the peak P-T conditions. To date, the form of the prograde path and the timing of the onset of metamorphism remain unknown. This study has used a range of different metasedimentary rocks from a large migmatitic quarry outcrop to better constrain the metamorphic history and the timing of metamorphism in the SMZ. Detrital zircon ages reveal that the protoliths to the metasedimentary rocks were deposited subsequent to 2733 AE 13 Ma. Peak metamorphic conditions of 852.5 AE 7.5°C and 11.1 AE 1.3 kbar were attained at 2713 AE 8 Ma. The clockwise P-T path is characterized by heating in the sillimanite field along a P-T trajectory which approximately parallels the kyanite to sillimanite transition, followed by near-isothermal decompression at peak temperature and near-isobaric cooling at~6.0 kbar. These results support several important conclusions. First, the sedimentary rocks from the Bandelierkop Formation are not the equivalent of any of the greenstone belt sedimentary successions on the Kaapvaal craton, as has been previously proposed. Rather, they post-date the formation of the Dominion and Witwatersrand successions on the Kaapvaal craton. From the age distribution of detrital zircon, they appear to have received significant input from various origins. Consequently, at c. 2730 Ma, the Baviaanskloof Gneiss most likely acted as basement onto which the sedimentary succession represented by the Bandelierkop Formation metapelites was deposited. Second, the rocks of the SMZ underwent rapid evolution from sediment to granulite facies anatexis, with a burial rate of~0.17 cm yr À1 . Peak metamorphism was followed by an isothermal decompression to 787.5 AE 32.5°C and 6.7 AE 0.5 kbar and isobaric cooling to amphibolite facies conditions, below 640°C prior to 2680 AE 6 Ma. This age for the end of the high-grade metamorphic event is marked by the intrusion of crosscutting, undeformed pegmatites that are within error the same age as the crosscutting Matok intrusion (2686 AE 7 Ma). Collectively, the burial rate of the sedimentary rocks, the shape of the P-T path, the burial of the rocks to in excess of 30 km depth and the post-peak metamorphic rapid decompression argue strongly that the SMZ contains sediments deposited along an active margin during lateral convergence, and ...

Phase equilibria modelling of residual migmatites and granulites: An evaluation of the melt‐reintegration approach

“…A common approach consists in considering the probable melt productivity of the model protolith composition reconstructed by melt‐reintegration method (e.g. Cai et al., ; Diener et al., ; Guilmette et al., ; Hasalová et al., ; Indares et al., ; Nicoli et al., ; Taylor et al., ; White et al., ; Zhang et al., ). These amounts of melt are a maximum, because melt loss would reduce rock fertility at higher temperature (Korhonen et al., ; Yakymchuk & Brown, ).…”

“…This issue was circumvented by reintegrating a certain amount of melt to the residuum composition and by performing phase equilibria modelling of the new model protolith composition to reconstruct the probable prograde history (see White, Powell, & Halpin, ). The melt‐reintegration approach has become an increasingly routine method among metamorphic petrologists and various ways of calculating and reintegrating the extracted melt have been developed and applied (Anderson, Kelsey, Hand, & Collins, ; Boger, White, & Schulte, ; Cai et al., , ; Chen, Ye, Liu, & Sun, ; Diener, White, & Hudson, ; Diener, White, Link, Dreyer, & Moodley, ; Diener, White, & Powell, ; Dumond, Goncalves, Williams, & Jercinovic, ; Fitzherbert, ; Groppo, Rolfo, & Indares, ; Groppo, Rolfo, & Mosca, ; Groppo, Rubatto, Rolfo, & Lombardo, ; Guilmette, Indares, & Hébert, ; Hallett & Spear, ; Hasalová et al., ; Jiang et al., ; Kelsey & Hand, ; Kohn, ; Korhonen, Brown, Clark, & Bhattacharya, ; Indares, White, & Powell, ; Lasalle & Indares, ; McGee, Giles, Kelsey, & Collins, ; Morrissey, Hand, Kelsey, & Wade, ; Nahodilová, Faryad, Dolejš, Tropper, & Konzett, ; Nicoli, Stevens, Moyen, & Frei, ; Palin et al., ; Redler, White, & Johnson, ; Shrestha, Larson, Guilmette, & Smit, ; Skrzypek, Štípská, & Cocherie, ; Štípská, Schulmann, & Powell, ; Taylor, Nicoli, Stevens, Frei, & Moyen, ; Tian, Zhang, & Dong, ; Tucker, Hand, Kelsey, & Dutch, ; Wang & Guo, ; White et al., ; Yakymchuk et al., ; Yin et al., ; Zhang et al., ; Zou et al., ). Furthermore, the reconstruction of a plausible protolith composition is essential to assess the likely melt productivity of rocks (White et al., ) which, in turn, allows the potential role of loss and redistribution of melt in the evolution of the deeper crust to be explored (Diener & Fagereng, ; Diener et al., ; Korhonen, Saito, Brown, & Siddoway, ; Korhonen et al.,…”

Rapid high‐T decompression recorded by Archean granulites in the northern Wyoming Province: Insights from petrological modelling

“…Brown, ; Chakraborty et al., ; Chambers et al., ; Spear, Pattison, & Cheney, ; and many more), with fewer examples of their application to Archean settings (notable exceptions include e.g. Bhadra & Nasipuri, ; Bhowmik, Wilde, Bhandari, & Basu Sarbadhikari, ; Cutts et al., ; Dumond, Goncalves, Williams, & Jercinovic, ; François, Philippot, Rey, & Rubatto, ; Nicoli, Stevens, Moyen, & Frei, ; White, Palin, & Green, ) to reveal the P−T– t paths that Archean HT rocks took through the crust. Several key questions can be answered from the metamorphic archive and would shed light on heating mechanisms in the Archean crust and Archean tectonic processes.…”

Insights into the complexity of crustal differentiation: K2O‐poor leucosomes within metasedimentary migmatites from the Southern Marginal Zone of the Limpopo Belt, South Africa

“…the basement), known as the Baviaanskloof gneiss. The second most abundant lithology is a metasedimentary unit, which forms part of the Bandelierkop Formation (Du Toit, Van Reenen, & Roering, ; Nicoli et al., ; Taylor et al., ; Van Reenen, Roering, Ashwal, & De Wit, ) that underwent a single granulite facies event at c . 2710 Ma (Figure a,b).…”

“…Note that some of the offsets are not clearly identified. (b) Global P–T–t path followed by the Bandelierkop Formation determined by phase equilibrium modelling on the metasedimentary rock of the Bandelierkop and the Braskpruit quarries (Nicoli et al., ; Taylor et al., ). Gr: granulite facies zone; Amph: amphibolite facies zone.…”