Petrology and Geochronology of the Basement Rocks in Eastern Botswana: Implications for the Evolution of the Limpopo Complex

概要

This study conducted integrated investigations on petrography, mineral chemistry, phase equilibria modeling, and geochronology of high-grade metamorphic rocks from the eastern part of Botswana to constrain pressure–temperature (P–T) conditions, timing of metamorphism, and correlations with adjacent crustal blocks. Such data are useful for better understanding of the tectonics of the Limpopo Complex as well as the Neoarchean- Paleoproterozoic crustal evolution in Southern Africa.

High-pressure garnet-bearing amphibolite and mafic granulite occur in the Maope area of the southern Motloutse Complex in eastern Botswana. The geochemical signatures of the mafic granulite indicate possible subduction-related volcanic arc affinity. The peak metamorphic condition of the garnet-bearing amphibolite was constrained as 850–910°C and 10.0–10.5 kbar based on phase equilibrium modeling and geothermobarometry. Zircon grains from the garnet-bearing amphibolite with lower Th/U ratios of 0.02–0.23 yielded a weighted-mean 207Pb/206Pb age of 2020.2 ± 4.9 Ma as the timing of high-grade metamorphism. In contrast, oscillatory-zoned zircons from the mafic granulite with higher Th/U ratios of 0.44–0.86 recorded a magmatic age of 2648.8± 9.8 Ma. As the age is consistent with previously reported age of a tonalite gneiss (2645 Ma) from the same region, Neoarchean bimodal arc magmatism is inferred. The results of this study indicate that the southern Motloutse Complex underwent high-pressure granulite-facies metamorphism with post-peak decompression along a single clockwise P–T evolution during Paleoproterozoic Era (~2.02 Ga). The results of this study suggest that the southern Motloutse Complex is nearly equivalent to the Beit Bridge Complex (BBC) as a possible western extension of a collisional suture (the Central Zone of the Limpopo Complex) between the Archean Kaapvaal and Zimbabwe Cratons.

Sapphirine-bearing Mg–Al-rich and pelitic granulites occur in a metasupracrustal unit (the BBC) in the eastern part of Botswana. Textural observations and phase equilibria modeling of the peak mineral assemblages of the granulites recorded a peak metamorphic condition of >930°C and >10.3 kbar, suggesting high-pressure (HP) and ultrahigh- temperature (UHT) metamorphism. Post-peak retrograde conditions were obtained based on the stability of sapphirine + cordierite symplectite assemblage in the Mg–Al-rich rocks (820°C–860°C and 6.4–7.0 kbar) and orthopyroxene + cordierite symplectite assemblage in the pelitic granulite (830°C–930°C and 4.3–5.8 kbar), indicating near-isothermal decompression after the peak HP–UHT metamorphism along a clockwise P–T path. Similar HP–UHT metamorphism and near-isothermal decompression were also reported from different regions of the BBC in South Africa and Zimbabwe. Monazite grains show isochron ages of 2115 ± 77 Ma, 2058 ± 40 Ma, and 2033 ± 34 Ma. Similar ages were obtained from zircon U–Pb geochronology as 2141 ± 24 Ma and 2010 ± 19 Ma (Th/U = 0.05–0.26). REE and Y contents of the monazites indicate that the older ages from higher Y monazites (2.1 Ga) correspond to the prograde metamorphic event, whereas the younger ages (2.0 Ga) indicate the near-peak high-grade event. The P–T conditions and zircon/monazite ages suggest Paleoproterozoic long-lived (>100 million years) metamorphism associated with intracontinental transpressional tectonics related to Paleoproterozoic orogenic events around the united Zimbabwe-Kaapvaal Craton.

Detrital zircons grains in quartzite samples from the Beit Bridge, Phikwe, and Motloutse Complexes show a similar age pattern of discordant and concordant zircon grains, suggesting Eoarchean to Mesoarchean (ca. 3800 – 3100 Ma) ages. Younger ages of ca. 3000 – 2400 Ma have been regarded as rejuvenated ages. The depositional age of the sediments is constrained as 3.1–2.6 Ga based on the youngest detrital zircon age and the oldest metamorphic age. The provenance of the >3.1 Ga detrital zircons has been regarded as orthogneisses and granitoids of the southern Zimbabwe Craton. The 3.8 Ga age is the oldest detrital zircon reported in this study suggesting for the presence of an unknown older crust in the proximal area. The 2.6 Ga and 2.0 Ga ages obtained from homogenous rims of zircon grains are consistent with metamorphic events reported in the entire Central Zone. Therefore, the Neoarchean collision of the Kaapvaal and Zimbabwe Cratons is inferred. The Central Zone could be an ancient accretionary complex formed by subduction of an oceanic lithosphere underneath the proto-Zimbabwe Craton, and the final collision and cratonization of the Kalahari Craton at 2.6 Ga.

Keywords: Eastern Botswana, High-pressure granulite, Ultrahigh temperature, Phase equilibria modeling, Detrital zircon, Geochronology.