Differentiation and stabilisation of the Archean continental crust, the example of the northern edge of the Kaapvaal craton, South Africa

[摘要] ENGLISH ABSTRACT: The Earth is one of the only solid celestial bodies displaying, at present day, an internal activity significant enough to modify its surface. This activity is mostly pictured by plate tectonic motions, which triggers the magmatic activity of mid-ocean ridges as well as subduction zones and mountain ranges development. These geodynamics are the locus of the creation of the oceanic crust, the continental crust and the reworking of older crustal material respectively that deeply modify both physical and chemical properties of the Earth's lithosphere. One of the main consequences of these processes is the chemical layering of the continental crust, composed of granodioritic to granitic upper levels while lower crustal levels are more mafic. Two crustal processes are assumed to be responsible for such a structure: (i) the crustal growth taking place at subduction zones and (ii) the crustal differentiation taking place within collision and accretionary orogens.The PhD work presented in this manuscript focuses on the recognition and the manifestation of Archean crustal differentiation processes. The Archean eon which represents 1/3 of the geological record is featured by both lithologies unrecognized in younger eons and cryptic geodynamics. Most of investigations concentrate on the characterisation of aluminium-rich lithologies that allow an accurate determination of the pressure-temperature evolution underwent by crustal materials during crustal thickening geodynamics. However, aluminium-rich lithologies - mainly represented by metasediments - account for only 10 % on average of Archean terranes whereas orthoderived gneisses (TTG + granite) - which also testify for crustal differentiation processes - form around 75 % of these terranes. The following contribution depicts an Archean composite grey gneiss complex located at the northern edge of the Kaapvaal craton is South Africa. Results carried out during this PhD study have major consequences on Archean geodynamics.Firstly, the Uranium-Lead isotope study of zircon grains from igneous lithologies of the northern Kaapvaal craton indicates an uninterrupted recording from ca. 2.97 Ga to ca. 2.68 Ga, namely during more than 300 Ma (which is the time that separates the Carboniferous/Permian transition from present day). This protracted recording sheds light on Archean tectonics and questions the ways and means of such geodynamics. Our isotope investigation, associated with field investigation and whole rock major and trace elements chemistry, are assumed to picture accretionary orogen geodynamics that contribute to a moderated crustal thickening.Secondly, the Lutetium-Hafnium isotope signature of the same zircon grains displays unprecedented results. Our analyses indicate that igneous lithologies from the northern Kaapvaal craton evolve on a single crustal trend through time. This is interpreted as the absence of exotic material involved in the accretionary orogen, i.e. the crustal block evolved in a thermodynamically closed system.It is therefore crucial to monitor the evolution of the grey gneiss complex forming phases through time. Geochemical analyses of dated phases show a noteworthy sequence of events with:. Emplacement of a basic complex (produced by partial melting of a depleted mantle) at ca. 2.97 Ga, which is subsequently partially melted, implying the,. Production and emplacement of TTGs during 100 Ma, from ca. 2.95 Ga to ca. 2.85 Ga, themselves subsequently differentiated into,. Granites which intrude the continental crust from ca. 2.85 Ga to ca. 2.75 Ga.These geodynamics are also marked by the emplacement of greenstone belts and associated sedimentary basins. The burying of supracrustal lithologies is recognized while ongoing accretionary orogen, however, no partial melting feature is described. After the intrusion of granites at ca. 2.85-2.75 Ga, partial melting reactions are recognized within supracrustal formations. Therefore, this study aims to demonstrate the key role of a rheologically strong granitic crust for the onset of an efficient burying process. Finally, the burying of supracrustal formations towards lower crustal levels triggers mantle metasomatism, ensued partial melting, production of mantle derived granite and therefore stabilisation of the entire crustal block.The PhD work presented here shed light on the Archean tectonics. This eon is perceived as a time where geodynamics were faster than present day due to a hotter and thus a more active Earth. Nevertheless, our results show that composite grey gneiss complexes may be built over a protracted time span, achieved through self-refinement of crustal materials. Grey gneiss complexes are only moderately investigated even though information enclosed in these lithologies is complementary with those from aluminium-rich rocks. Therefore, deeper investigations of these geological objects must be a central scope in order to improve the knowledge of the Archean eon and appears necessary for the building of even more realistic geotectonic models.

[发布机构] Stellenbosch University