Granitoid-greenstone relationships in the eastern Kaapvaal craton and implications for early crustal evolution
Final Report Abstract
Key findings from this proposal were that the Eoarchean tonalites-trondhjemites-granodiorites (TTGs) from the Itsaq Gneiss Complex were generated from melting of typical Eoarchean tholeiitic metabasalts that occur within the Isua Supracrustal Belt. For the first time, a combined thermodynamic-geochemical modelling approach was used to place constraints on the precursor rocks of TTGs. Isotopic and geochemical analyses revealed that the oldest granitoids of the Ancient Gneiss Complex of Swaziland were not juvenile contributions to the early continental crust. They rather formed by melting of older crustal sources, likely by interaction with mantlederived basaltic material. A second generation of TTGs within the Ancient Gneiss Complex likely formed by magma-mixing processes of underplated basaltic magma and melts of older felsic crust, followed by fractional crystallization. This suite of rocks were previously thought to represent typical TTGs. Contrasting to TTGs, which follow a trondhjemitic melting trend, this suite of rocks follow a calc-alkaline differentiation trend. By studying felsic volcanic, ultramafic and mafic rocks within the Sandspruit and Theespruit Formations of the Barberton Greenstone belt, it was found that both formations are indistinguishable in age and chemical composition and should be treated as one coherent unit. Geochemical modeling showed that parts of the amphibolites were crustally contaminated with older crust. This is also suggested by heterogeneous Hf isotope compositions of zircons from interlayered felsic volcanic rocks from both formations. The first coherent dataset for µ142Nd values for different rocks types, covering ages from 3.64 to 3.20 Ga, was produced during the course of this project. The results reveal that samples, independent of rock type and age, preserve either a modern-like µ142Nd compositions or represent slightly enriched values of ca. -6. This indicates that an enriched µ142Nd signature was reworked, inherited from a mantle source that experienced an early differentiation event within the first 500 Ma of Earth’s history, or that an enriched signal from a Hadean protocrust was reworked throughout the Archean in the eastern Kaapvaal craton. By analyzing the Hf isotopic and trace element composition of the ultra-depleted 3.33 Ga Commondale komatiites the most depleted mantle sources for Paleoarchean rocks worldwide were found. Interestingly, the Hf-Nd-Os isotope systems revealed coherent ages but documented decoupled isotope compositions. It was shown by isotope and trace element modeling that the decoupling either originated from a subduction-derived fluid overprint of strongly depleted mantle sources or by inheritance of an isotopic signature from delaminated garnet-pyroxenite restites in the mantle source. Based on analyses of amphibolites of the Pongola Supergroup found in the Kubuta region, a refined model for the formation of the Pongola volcanic suite was proposed. All magmas experienced crustal contamination by AFC processes involving >3.5 Ga crust of the Ancient Gneiss Complex and were initially derived from komatiitic melts that were likely generated in a mantle plume. Based on the major and trace element compositions of the Kubuta Pongola volcanics were identified as the most mafic compositions yet published from the supergroup. As indicated using the Nb/Yb vs. Th/Yb diagram and trace element modelling a progressing source depletion was obtained and a refined model for the Pongola volcanism was proposed in the PhD thesis.
Publications
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(2012). Generation of the Eoarchean tonalitetrondhjemite-granodiorite (TTG) suite from thickened mafic arc crust. Geology 40, 375–378
Nagel, T., Hoffmann, J.E., Münker, C.
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(2012). Hafnium isotope evidence for a transition in the geodynamics of continental growth after 3.2 Ga. Nature 485, 627–630
Næraa, T., Scherstén, A., Rosing, M.T., Kemp, A.I.S., Hoffmann, J.E., Kokfelt, T.F., Whitehouse, M.J.
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(2013). Generation of early Archaean felsic greenstone volcanic rocks through crustal melting in the Kaapvaal craton, southern Africa. Earth and Planetary Science Letters 381, 188-197
Kröner, A., Hoffmann, J.E., Xie, H., Wu, F., Münker, C., Hegner, E., Wong, J., Wan, Y., Liu D.
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(2014). Constraints on the process of Eoarchean TTG formation in the Itsaq Gneiss Complex, southern West Greenland. Earth and Planetary Science Letters 388, 374-386.
Hoffmann, J.E., Nagel, T.J., Münker, C., Næraa, T., Rosing, M.T.
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(2014). Paleo- to Mesoarchean polymetamorphism in the Barberton Greenstone Belt: constraints from U-Pb monazite and Lu-Hf garnet geochronology on the tectonic processes that shaped the belt. GSA Bulletin 126, 251-270
Cutts, K.A., Stevens, G., Hoffmann, J.E., Buick, I., Frei, D., Münker, C.
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(2015). Archaean granulitic paragneisses from Central Swaziland: Inferences on Paleoarchaean crustal reworking and a complex metamorphic history. Journal of the Geological Society of London 172, 139-152
Suhr, N., Hoffmann, J.E., Kröner, A., Schröder, S.
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(2014). Generation of early Archaean grey gneisses through crustal recycling in the eastern Kaapvaal craton, southern Africa. Precambrian Research 255, 833-846
Kröner, A., Hoffmann, J.E., Xie, H., Münker, C., Hegner, E., Hofmann, A., Wang, Y.
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(2014). Just another drip: structural analysis of a proposed Mesoarchean suture from the Barberton Mountain Land, South Africa. Precambrian Research 254, 19-35
Van Kranendonk, M.J., Kröner, A., Hoffmann, J.E., Nagel, T.J., Anhaeusser, C.
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(2016). Chronology of the oldest supracrustal sequences of the Barberton Greenstone Belt, South Africa and Swaziland. Precambrian Research 279, 123-143
Kröner, A., Anhauesser, C.R., Hoffmann, J.E., Hegner, E., Wong, J., Geng, H., Xie, H., Yang, J., Liu, D.
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(2016). Source composition, fractional crystallisation and magma mixing processes of the 3.48-3.43 Ga Tsawela tonalite suite (Ancient Gneiss Complex, Swaziland) – implications for Palaeoarchean geodynamics. Precambrian Research 276, 43-66
offmann, J.E., Kröner, A., Hegner, E., Viehmann, S., Xie, H., Iiacheri, L.M., Schneider, K.P., Hofmann, A., Wong, J., Geng, H., Yang, J.H.
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(2017) The origin of highly radiogenic Hf isotope compositions in 3.33 Ga Commondale komatiite lavas (South Africa). Chemical Geology 455, 6-21
Hoffmann, J.E., Wilson, A.H.
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(2018). Coexistence of Archean igneous rocks with enriched and non-enriched 142Nd signatures in the eastern Kaapvaal Craton, southern Africa. Earth and Planetary Science Letters 487, 54-66
Schneider, K.P., Hoffmann, J.E., Boyet, M., Münker, C., Kröner, A.
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(2018). High-temperature metamorphism and crustal melting at ca. 3.2 Ga in the eastern Kaapvaal craton, southern Africa. Precambrian Research 317, 101-116
Kröner, A., Nagel, T.J., Hoffmann, J.E., Xie H., Wong, J., Geng, H., Hegner, E., Hofmann, A., Kasper, H.-U., Liu, D., Yang, J.
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(2018). The oldest granite clast in the Moodies conglomerate, Barberton greenstone belt, South Africa, and its likely origin. South African Journal of Geology 121(1), 43-50
Kröner, A., Wong, J., Xie, H.
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(2019). The Ancient Gneiss Complex of Swaziland and environs: an updated record of early Archaean crustal evolution in southern Africa. In: Earth’s oldest rocks (2nd ed.). (Eds. Van Kranendonk, M.J., Bennett, V.C., Hoffmann, J.E.). p. 553-567. Elsevier. Amsterdam, Oxford, Cambridge
Hoffmann, J.E., Kröner, A.
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(2019). The formation of tonalitestrondhjemites-granodiorites in early continental crust. In: Earth’s oldest rocks (2nd ed.). (ed. Van Kranendonk, M.J., Bennett, V.C., Hoffmann, J.E.). p. 133-168. Elsevier. Amsterdam, Oxford, Cambridge
Hoffmann, J.E., Zhang, C., Moyen, J.-F., Nagel, T.J.
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.(2019): Archaean Crystalline Rocks of the Eastern Kaapvaal Craton. In: Alfred Kröner und Axel Hofmann (Hg.): The Archaean Geology of the Kaapvaal Craton, Southern Africa. Cham: Springer International Publishing (Regional Geology Reviews), S. 1–32
Schmidt, H-P, Spokas, K, Wrage-Mönnig, N
