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[SMP26-P06] Paleoproterozoic (ca. 1.92 Ga) monazite age from the 2.58 Ga Great Dyke: Implications for regional thermal overprinting along the southern margin of the Zimbabwe Craton
Keywords:Zimbabwe Craton, Great Dyke, Paleoproterozoic, monazite Th-U-total Pb geochronology
The 2575 Ma Great Dyke is a layered mafic-ultramafic intrusion that occurs across the Zimbabwe Craton, extending for about 550 km and 1 to 11 km in width. It cuts all Paleo- to Neoarchean lithologies of the Zimbabwe Craton including tonalitic-trondhjemitic-granodioritic (TTG) gneisses, greenstone belts, and intrusive granitoids. Therefore, the intrusion has been considered to mark the last major thermal event of the Zimbabwe Craton (Oberthür et al., 2002). The Great Dyke accompanies satellite dykes (e.g., East Dyke, Umvimeela Dyke) which are distributed approximately parallel to the length of the main ‘dyke’. Among them, the Southern Dyke continues further south to the northern margin of the Limpopo Complex which is a Neoarchean (ca. 2.7-2.6 Ga) orogeny established by the collision of the Zimbabwe Craton to the north and the Kaapvaal Craton to the south.
This study examined the NNE-SSW-trending Southern Dyke that crosscuts the regional ENE-WSW-trending foliation of the Northern Marginal Zone of the Limpopo Complex. It dominantly consists of norite with euhedral and phenocrystic orthopyroxene and clinopyroxene as well as anhedral plagioclase (anorthite-rich) that fills the matrix of the pyroxenes. Rounded olivine occurs as inclusions in the phenocrystic minerals. The orthopyroxene is partly mantled by fine- to medium-grained aggregates of albite-rich plagioclase, biotite, K-feldspar, clinopyroxene, apatite, and rare quartz. They are texturally late-stage minerals, therefore thermal recrystallization probably took place during a post-magmatic stage. This study applied Th-U-total Pb dating technique to monazites associated with the recrystallized minerals to evaluate the timing of the thermal overprinting and its tectonic implications.
The norite contains small (less than 45 microns) monazite grains predominantly associated with the recrystallization minerals. The application of monazite Th-U-total Pb geochronology on these grains yielded a Paleoproterozoic weighted-mean age of 1923 ± 28 Ma, except one grain with a weighted-mean age of 2560 ± 12 Ma. The older age is consistent with the reported crystallization age of the Great Dyke (2575 Ma, Oberthür et al., 2002). Paleoproterozoic (ca. 2.0 Ga) thermal events are reported from the Central Zone of the Limpopo Complex (e.g., Jaeckel et al., 1997) and shear zones (e.g., Kamber et al., 1995) that divide different geological units of the complex. Tsunogae and Belyanin (2020) reported Ar-Ar ages of 2020 ± 8 Ma to 1968 ± 10 Ma from amphibolites of the Buhwa Greenstone Belt from the southern margin of the Zimbabwe Craton along the Limpopo Complex-Zimbabwe Craton boundary. The new monazite age from the Southern Dyke is slightly younger, but is consistent with the age range. The Paleoproterozoic thermal event could be related to the activity of ca. 2.0 Ga superplume that probably gave rise to the activity of large igneous provinces in Australia and South Africa (e.g., Bushveld Igneous Complex) (Fiorentini et al., 2020).
References
Fiorentini, M.L., O’Neill, C., Giuliani, A., Choi, E., Maas, R., Pirajno, F., Foley, S., 2020. Bushveld superplume drove Proterozoic magmatism and metallogenesis in Australia. Scientific Reports, 10, 19729.
Höss, A., Klemd, R., Bolhar, R., Haase, K.M., Ramos, J., Geiger, E., S Brandt, S., Graupner, T., 2024. Magmatic evolution of the Schiel Alkaline Complex, Bushveld large igneous province, South Africa. Lithos, 466-467, Article 107464.
Jaeckel, P., Kröner, A., Kamo, S.L., Brandl, G., Wendt, J.I., 1997. Late Archaean to early Proterozoic granitoid magmatism and high-grade metamorphism in the central Limpopo belt, South Africa. Journal of the Geological Society, 154, 25-44.
Kamber, B.S., Kramers, J.D., Napier, R. Cliff, R.A., Rollinson H.R., 1995. The Triangle Shearzone, Zimbabwe, revisited: new data document an important event at 2.0 Ga in the Limpopo Belt. Precambrian Research, 70, 191–213.
Tsunogae, T., Belyanin, G.A., 2020. P-T and Ar-Ar age constraints on low- to high-grade metabasites from the Buhwa Greenstone Belt, southern Africa: Implications for Neoarchean to Paleoproterozoic thermal evolution along the Limpopo Complex-Zimbabwe Craton boundary. Journal of African Earth Sciences, 162, Article 103722, 1–17.
Oberthür, T., Davis, D.W., Blenkinsop, T.G., Höhndorf, A., 2002. Precise U–Pb mineral ages, Rb–Sr and Sm–Nd systematics for the Great Dyke, Zimbabwe – constraints on late Archean events in the Zimbabwe craton and Limpopo belt. Precambrian Research, 113, 293–305.
This study examined the NNE-SSW-trending Southern Dyke that crosscuts the regional ENE-WSW-trending foliation of the Northern Marginal Zone of the Limpopo Complex. It dominantly consists of norite with euhedral and phenocrystic orthopyroxene and clinopyroxene as well as anhedral plagioclase (anorthite-rich) that fills the matrix of the pyroxenes. Rounded olivine occurs as inclusions in the phenocrystic minerals. The orthopyroxene is partly mantled by fine- to medium-grained aggregates of albite-rich plagioclase, biotite, K-feldspar, clinopyroxene, apatite, and rare quartz. They are texturally late-stage minerals, therefore thermal recrystallization probably took place during a post-magmatic stage. This study applied Th-U-total Pb dating technique to monazites associated with the recrystallized minerals to evaluate the timing of the thermal overprinting and its tectonic implications.
The norite contains small (less than 45 microns) monazite grains predominantly associated with the recrystallization minerals. The application of monazite Th-U-total Pb geochronology on these grains yielded a Paleoproterozoic weighted-mean age of 1923 ± 28 Ma, except one grain with a weighted-mean age of 2560 ± 12 Ma. The older age is consistent with the reported crystallization age of the Great Dyke (2575 Ma, Oberthür et al., 2002). Paleoproterozoic (ca. 2.0 Ga) thermal events are reported from the Central Zone of the Limpopo Complex (e.g., Jaeckel et al., 1997) and shear zones (e.g., Kamber et al., 1995) that divide different geological units of the complex. Tsunogae and Belyanin (2020) reported Ar-Ar ages of 2020 ± 8 Ma to 1968 ± 10 Ma from amphibolites of the Buhwa Greenstone Belt from the southern margin of the Zimbabwe Craton along the Limpopo Complex-Zimbabwe Craton boundary. The new monazite age from the Southern Dyke is slightly younger, but is consistent with the age range. The Paleoproterozoic thermal event could be related to the activity of ca. 2.0 Ga superplume that probably gave rise to the activity of large igneous provinces in Australia and South Africa (e.g., Bushveld Igneous Complex) (Fiorentini et al., 2020).
References
Fiorentini, M.L., O’Neill, C., Giuliani, A., Choi, E., Maas, R., Pirajno, F., Foley, S., 2020. Bushveld superplume drove Proterozoic magmatism and metallogenesis in Australia. Scientific Reports, 10, 19729.
Höss, A., Klemd, R., Bolhar, R., Haase, K.M., Ramos, J., Geiger, E., S Brandt, S., Graupner, T., 2024. Magmatic evolution of the Schiel Alkaline Complex, Bushveld large igneous province, South Africa. Lithos, 466-467, Article 107464.
Jaeckel, P., Kröner, A., Kamo, S.L., Brandl, G., Wendt, J.I., 1997. Late Archaean to early Proterozoic granitoid magmatism and high-grade metamorphism in the central Limpopo belt, South Africa. Journal of the Geological Society, 154, 25-44.
Kamber, B.S., Kramers, J.D., Napier, R. Cliff, R.A., Rollinson H.R., 1995. The Triangle Shearzone, Zimbabwe, revisited: new data document an important event at 2.0 Ga in the Limpopo Belt. Precambrian Research, 70, 191–213.
Tsunogae, T., Belyanin, G.A., 2020. P-T and Ar-Ar age constraints on low- to high-grade metabasites from the Buhwa Greenstone Belt, southern Africa: Implications for Neoarchean to Paleoproterozoic thermal evolution along the Limpopo Complex-Zimbabwe Craton boundary. Journal of African Earth Sciences, 162, Article 103722, 1–17.
Oberthür, T., Davis, D.W., Blenkinsop, T.G., Höhndorf, A., 2002. Precise U–Pb mineral ages, Rb–Sr and Sm–Nd systematics for the Great Dyke, Zimbabwe – constraints on late Archean events in the Zimbabwe craton and Limpopo belt. Precambrian Research, 113, 293–305.