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Les granites à métaux rares : origine, mise en place et mécanismes de la transition magmatique-hydrothermale

Abstract : Rare metal granites result from the crystallization of highly evolved magmas enriched in volatiles and fluxing elements (F, Li, P, B) and in metals such as Sn, Nb, Ta and W. In the Western Europe Variscan belt, these granites are attributed to a late orogenic episode of rare metal magmatism. Since the 60’s, numerous studies have focused on the specific geochemical signature of rare metal granites and several models have been proposed for their genesis. However, several questions remain and, in particular, the context and mechanisms of magma emplacement have been only rarely addressed. In order to build a comprehensive understanding of magmatic, hydrothermal and tectonic processes and mechanisms responsible for (i) rare metal magma generation, (ii) magma emplacement and geometry, (iii) specific geochemical signature and metal enrichment, a multi-approach and multi-scale study has been performed on the example of the rare metal granite of Argemela (Central Iberian Zone, Portugal). It has been complemented by an experimental study on the genesis of rare metal magmas. Results of the structural analysis highlighted the special structural context of emplacement of the Argemela intrusion. Several criteria suggest a fast vertical and channelled magma ascent up to shallow subvolcanic levels. The petrological and geochemical study of the granite and related mineralization allow the identification of key events and processes involved during the magmatic-hydrothermal transition. Mechanisms of concentration/transportation and deposition of metals have been also specified. The experimental study indicates that a single step low degree partial melting is not efficient enough to produce rare metal magmas, since most are sequestered into the restite. However, remelting of enriched restitic assemblages, a mechanism which remains to be tested, could well produce the required rare metal concentrations. Finally, this study demonstrates the necessity to combine several approaches, since processes and mechanisms are interrelated in these mineralized systems.
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Julie Michaud. Les granites à métaux rares : origine, mise en place et mécanismes de la transition magmatique-hydrothermale. Sciences de la Terre. Université d'Orléans, 2019. Français. ⟨NNT : 2019ORLE3002⟩. ⟨tel-02967703⟩

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