Paper Number: 695
High-grade metamorphism and crustal melting at ca. 3.2 Ga in the eastern Kaapvaal craton, southern Africa
Kröner, A1,2, Nagel, Th.3, Hoffmann, J.E.4, Xie, H.1, Wong, J.5, Geng, H.5, Hegner, E.6, Liu, X.7, Hofmann, A.8, Liu, D.1, Yang, J.9
1Beijing SHRIMP Centre, Chinese Academy of Geological Sciences, China, kroener@uni-mainz.de
2Institut für Geowissenschaften, Universität Mainz, D-55099 Mainz, Germany
3Department of Geosciences, Aarhus University, Høegh Guldbergs Gade 2, DK-8000 Åarhus, Denmark
4Institut für Geologische Wissenschaften, Abt. Geochemie, Freie Universität Berlin, Germany
5Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China
6Department für Geo- und Umweltwissenschaften und GeoBio Center, Universität München,
Theresienstrasse 47, 80333 München, Germany
7State Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China
8Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, South Africa
9State Key Laboratory of Lithospheric Evolution, Institute of Geology & Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing, China
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The Ancient Gneiss Complex (AGC) of Swaziland is a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton and consists of 3.66-3.2 Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. High-strain deformation has led to structural parallelism at almost all contacts, making it virtually impossible to map different gneiss generations in the field. We report on a 3.2 Ga granulite-facies assemblage in central Swaziland (cooling path from ~800 to 600 °C at pressures between 7 and 9 kbar) and relate the occurrence of numerous ca. 3.2 Ga granitoid gneisses in the AGC to a major tectono-thermal event that not only affected the AGC but also the neighbouring Barberton granitoid-greenstone terrane. Our SHRIMP zircon ages, in combination with Hf-in-zircon and whole-rock Hf-Nd isotopic data, suggest that extensive melting of Palaeoarchaean granitoid crust, in combination with variable but relatively small additions of mantle-derived material, was responsible for the generation of the 3.2 Ga granitoid suite.
Previous models have related the 3.2 Ga event in the eastern Kaapvaal craton to subduction and collision processes [1,2] but we see no evidence for long, narrow belts and metamorphic facies changes reflecting lithospheric suture zones, and there is no unidirectional asymmetry in the thermal structure across the entire region from Swaziland to the southern Barberton granite-greenstone terrane as is typical of Phanerozoic and Proterozoic belts [3]. Instead, we envision a major underplating event at ca. 3.2 Ga, giving rise to widespread and extensive melting in the lower crust and mixing with mantle-derived under- and intraplated mafic magma to generate the voluminous granitoid assemblages now observed in the AGC and the southern Barberton terrane. This is compatible with large-scale crustal reworking during a major thermo-magmatic event, and the apparent lack of a mafic lower crust in the Kaapvaal and a sharp and flat Moho [4].
References:
[1] Moyen J-F et al. (2006) Nature 442: 559-562.
[2] Schoene B and Bowring SA (2010) Geol Soc America Bull 122: 408-429.
[3] Brown M (2015) Geol Soc America Bull 127: 1550-1557.
[4] Niu F and James DE (2002) Earth Planet Sci Lett 200: 121-130.