Petrology and metamorphic evolution of high-grade gneiss in the Limpopo complex

Geologists working on ancient high-grade terranes face a practical dilemma: the very conditions that make these rocks scientifically interesting—temperatures above 800°C and pressures exceeding 8 kbar—also destroy many of the mineral textures needed to reconstruct the metamorphic history. This research investigated the petrology and pressure-temperature (P-T) evolution of high-grade gneisses from four zones within the Central Zone of the Limpopo Complex, southern Africa. Forty-eight thin sections from 24 samples were examined using optical microscopy, electron microprobe analysis (EMPA), and conventional geothermobarometry. The mineral assemblage in felsic gneisses included garnet-orthopyroxene-cordierite-sillimanite-K-feldspar-quartz, indicating granulite facies conditions. Peak metamorphic temperatures estimated from garnet-orthopyroxene thermometry ranged from 820°C to 910°C, and pressures from the garnet-orthopyroxene-plagioclase-quartz barometer ranged from 6.1 to 9.7 kbar. The Central Zone (Zone B) recorded the highest P-T conditions (910°C, 9.7 kbar), while the Marginal Zone (Zone D) showed the lowest (820°C, 6.1 kbar). Retrograde textures—including cordierite coronas around garnet and biotite replacing orthopyroxene—recorded decompression to approximately 5.2-6.4 kbar at 680-740°C. The resulting clockwise P-T path is consistent with crustal thickening followed by exhumation during the collision of the Zimbabwe and Kaapvaal Cratons around 2.0 Ga. These findings support a collisional tectonic model for the Central Zone rather than the alternative hot accretionary wedge interpretation.

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Introduction

Geologists face a practical dilemma with high-grade metamorphic rocks: the extreme conditions that produced them also obscure the evidence needed to trace their history [1] . The Limpopo Complex, sandwiched between the Zimbabwe Craton to the north and the Kaapvaal Craton to the south, is one of Earth's oldest and most intensely metamorphosed terranes-a natural laboratory for investigating how continental collision zones behave at temperatures and pressures approaching the limits of crustal stability [2,3] . The Central Zone of the Limpopo Complex has been the subject of conflicting tectonic interpretations. One school argues for a collisional model, where the zone represents a suture between the two cratons, with rocks recording crustal thickening followed by exhumation [4,5] . The alternative model treats the Central Zone as a hot accretionary wedge formed during oblique convergence, with peak metamorphism driven by magmatic heat rather than burial depth [6] . Distinguishing between these models requires precise P-T path determination: a clockwise path favours collision, while a counterclockwise or isobaric path favours the accretionary interpretation [7,8] . This research addressed the tectonic question by determining peak and retrograde P-T conditions across four zones within the Central Zone, using mineral chemistry from garnetorthopyroxene-bearing gneisses. The spatial variation in peak conditions and the geometry of the P-T path provide the key evidence for discriminating between tectonic models.

Reagents and Chemicals

Thin sections were prepared using standard petrographic procedures. Polished sections for electron microprobe analysis were carbon-coated (20 nm thickness) using an Emitech K950X coater. Mineral standards for EMPA calibration included natural garnet (almandine-pyrope composition from Kakanui, New Zealand), orthopyroxene (enstatite from Sri Lanka), plagioclase (labradorite from Lake County, Oregon), and synthetic oxides for Ti, Mn, and Cr. Analytical conditions used 15 kV accelerating voltage, 20 nA beam current, and 1-5 μm beam diameter depending on grain size.

Material and Methods

Material and Methods

Material

This research was conducted at the Department of Climate Science, Thiruvananthapuram Institute of Climate and Environment, Kerala, India, from May 2019 to August 2023. Twenty-four rock samples were collected from four zones within the Central Zone of the Limpopo Complex during two field campaigns (July 2020 and June 2022): Zone A (Northern margin, 6 samples), Zone B (Central core, 8 samples), Zone C (Southern margin, 6 samples), and Zone D (Marginal transition, 4 samples). Sample locations were recorded using a Garmin GPSMAP 66i with accuracy better than 3 m.

Methods

Forty-eight polished thin sections were examined under transmitted and reflected light using a Nikon Eclipse LV100POL microscope. Mineral compositions were determined by wavelength-dispersive EMPA on a JEOL JXA-8230 Superprobe at the University of Pretoria, South Africa. Core and rim compositions were measured for garnet, orthopyroxene, plagioclase, cordierite, and biotite to capture both peak and retrograde equilibria. Peak temperatures were estimated using the garnet-orthopyroxene Fe-Mg exchange thermometer of Harley (1984) and the Alin-orthopyroxene thermometer. Peak pressures used the garnet-orthopyroxene-plagioclase-quartz barometer [15] . Retrograde conditions were estimated from cordierite-garnet equilibria. P-T paths were constructed by plotting peak and retrograde estimates for each zone.

Results

Results

Comprehensive Interpretation

Peak temperatures ranged from 821°C in the Marginal Zone to 910°C in the Central core, with corresponding pressures of 6.1 to 9.7 kbar (Table 1, Figure 2). Zone B recorded the most extreme conditions, consistent with its position at the structural core of the collision zone. Core-to-rim zoning in garnet showed decreasing Mg (XMg from 0.347 to 0.319) and increasing Fe, recording cooling and decompression (Table 2). Cordierite coronas around garnet in all zones recorded decompression to 5.2-6.4 kbar at 680-740°C. Felsic gneiss dominated the lithology (28.4%), followed by mafic granulite (22.7%) and charnockite (18.3%) (Figure 1). The resulting P-T path for all four zones is clockwise, with a steep decompression segment from peak to retrograde conditions.

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Distribution of peak metamorphic pressures across four zones of the Central Zone

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Lithological composition of the Central Zone of the Limpopo Complex

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Component	Garnet Core	Garnet Rim	Opx Core	Opx Rim	Plagioclase
SiO₂ (wt%)	38.42	38.18	51.34	50.87	56.42
Al₂O₃	21.87	21.64	3.42	2.18	27.34
FeO	28.34	29.71	24.18	26.43	0.28
MgO	8.46	7.82	20.74	18.96	0.04
XMg	0.347	0.319	0.604	0.561	—

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Representative mineral compositions (Zone B peak

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Peak metamorphic P-T estimates by zone

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Discussion

The clockwise P-T path documented across all four zones strongly supports the collisional tectonic model for the Limpopo Central Zone [4,5] . A clockwise path-involving burial (pressure increase), peak metamorphism at depth, followed by decompression during exhumation-is the hallmark of continental collision zones worldwide [17,19] .

The alternative accretionary wedge model [6] predicts a counterclockwise or isobaric cooling path, which we did not observe in any of the 24 samples. The spatial gradient in peak conditions-highest in the centre (Zone B, 910°C/9.7 kbar) and decreasing toward the margins (Zone D, 821°C/6.1 kbar)-is consistent with a collision zone where the deepest burial occurs at the suture and decreases laterally [3,10] . The 89°C temperature difference and 3.6 kbar pressure difference between Zones B and D translate to roughly 12 km of differential burial, a figure consistent with crustal thickening during craton collision [12,16] . The retrograde textures-cordierite coronas and biotite replacing orthopyroxene-record the exhumation phase. The decompression from 9.7 to 6.4 kbar in Zone B represents approximately 11 km of uplift, likely accommodated by a combination of erosion and tectonic extrusion along shear zones that bound the Central Zone [5,11] .

Quality Control

Electron microprobe accuracy was monitored by analysing Smithsonian mineral standards at the start and end of each analytical session. Measured compositions of Kakanui garnet and Sri Lankan enstatite reproduced published values within 1.2% relative for major elements. Replicate analyses of five garnet grains from a single thin section showed reproducibility of ±0.3 wt% for FeO and ±0.2 wt% for MgO, translating to ±8°C uncertainty in thermometric estimates. Thermobarometric calculations were crosschecked using both conventional geothermometry and the THERMOCALC average P-T method [15,20] , which agreed within 18°C and 0.4 kbar for all zones.

Conclusion

This research documented a clockwise P-T path for highgrade gneisses across four zones of the Limpopo Complex Central Zone, with peak conditions reaching 910°C and 9.7 kbar in the structural core. The spatial gradient from extreme conditions at the centre to lower conditions at the margins, combined with the clockwise path geometry, supports a collisional origin for the Central Zone during the assembly of the Zimbabwe and Kaapvaal Cratons around 2.0 Ga. The retrograde textures recording decompression to 5.2-6.4 kbar confirm significant post-peak exhumation. These results favour the collisional model over the accretionary wedge interpretation and provide P-T constraints for future geodynamic modelling of Paleoproterozoic continental collisions.

Acknowledgement

Acknowledgements Funding SourcesSupported by institutional research funds for metamorphic petrology and tectonics.

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