Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by Author "Muhlberg, Moritz"

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    Magmatic and metamorphic evolution of the Stolzburg Block, Barberton Granitoid-Greenstone Terrain
    (Stellenbosch : Stellenbosch University, 2022-12) Muhlberg, Moritz; Stevens, Gary; Moyen, Jean-Francios; Kisters, Alex; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The processes that formed and shaped the first stable continents in the Archean are poorly constrained for various reasons, including the scarcity of well-preserved, unaltered rock sequences and the lack of modern equivalents of lithologies that are typical for the Archean. This has led to the emergence of various geodynamic models that try to explain the unique features of the Archean crust, but too often rely on assumptions on the thermal state of the crust, in particular of the felsic crust. This thesis is an investigation of the igneous and metamorphic history of the Stolzburg Block, an area comprised of tonalite-trondhjemite-granodiorite (TTG) and amphibolite-facies supracrustal rocks, located in the Barberton Granitoid Greenstone Terrain (BGGT) in South Africa. The thermal history of a well-preserved piece of felsic Archean crust is studied through a combination of field work and geochemical, geochronological and petrological analyses on TTG and related rocks, with the aim of providing robust constraints on the thermal state of the Archean crust to improve geodynamic models. Uranium-lead (U-Pb) thermochronology on apatite grains has revealed four distinct groups of 207Pb/206Pb ages that are present throughout the Stolzburg Block. The three oldest groups of U-Pb apatite ages overlap with the time of emplacement of TTG plutons (~3450 Ma), of regional metamorphism (~3230-3200 Ma) and of intrusion of granitic plutons (~3105 Ma), while the ~2820 Ma group indicates a previously undiscovered thermal event. The preservation of the ~3450 Ma apatite ages indicates that the Stolzburg Block has not been heated above ~400- 500 °C after the emplacement of TTG plutons – at least not for a prolonged period. Trace-element and strontium isotope analyses of the dated apatite grains show homogenous compositions regardless of age and 87Sr/86Sr ratios that are within uncertainty identical to the initial 87Sr/86Sr ratio of the respective bulk rock, and these features are being interpreted as primary igneous signatures. This indicates that the different groups of U-Pb apatite ages are not the result of new growth but rather of partial resetting of the U-Pb systematics through heating. Zirconium-in-titanite and titanium-in-zircon thermometry gives temperatures of ~700 °C for magmatic titanite and ~650 °C for metamorphic titanite and zircon from TTG samples, which is in the same range as the reported metamorphic conditions recorded by the associated supracrustal rocks at ~3230-3200 Ma. While no chronological constraints are available for the temperature estimates of the TTG rocks, a shared history between TTG plutons and greenstones can be inferred from the intrusive relationship between the two. The preservation of ~3450 Ma U-Pb signatures in apatite grains shows that, regardless of the timing of heating to ~650 °C, the heating was very short-lived (< 1 million years). This work demonstrates that the felsic Archean crust – at least the section preserved in the BGGT – was relatively cool and stable, and that any heating of the TTG plutons was short- lived. The results of this study argue against a prolonged radiogenic heating of TTG crust and a partial convective overturn of the Archean crust, as proposed by some geodynamic models.