Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by Author "Couzinie, Simon Gwenael"

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    Evolution of the continental crust and significance of the zircon record, a case study from the French Massif Central.
    (Stellenbosch : Stellenbosch University, 2018-03) Couzinie, Simon Gwenael; Moyen, Jean-Francois; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The formation of the continental crust is a major consequence of Earth di erentiation. Recent advances on this topic bene ted from improvements of analytical techniques enabling in situ measurements of U Pb Hf O isotope compositions in zircon, a widespread accessory mineral of continental igneous rocks. This study reconstructs the evolutionary path followed by the crust segment today exposed in the eastern part of the French Massif Central (FMC), a portion of the Variscan belt of Western Europe, with the aim to investigate the potential aws of the zircon record of crust evolution. In this scope, the origin and geodynamic signi cance of the constituent FMC lithological units are tackled by combining conventional petrological observations with zircon U Pb Hf O isotope data. Two major inconsistencies exist between the results obtained following this integrated approach and the conclusions drawn solely from zircon isotopic signatures, taken out of their petrological context, as is commonly performed in studies investigating crust evolution. First, zircon Hf model ages point to substantial Mesoproterozoic crust formation in the FMC whereas more than 60% of the crust is actually Neoproterozoic in age. Second, new additions to the continental crust volume during the Variscan orogeny are not recorded even though 5 to 10% of the exposed crust formed at that time. The origin of both discrepancies inherently lies in the mixed isotopic signature carried by many zircon grains. Such equivocal information can only be detected when additional petrological constrains on the zircon host rocks are available and provide guidance in interpreting the zircon record of crust evolution.