U-Pb dating of columbite-tantalite from Variscan rare-elements granites and pegmatites
Résumé
Rare-elements (Li, Cs, Ta, Be, Sn...) magmatism is well expressed in the European Variscan belt with mainly pegmatites and granites. These bodies are known in most of the different realms of the belt, but they are particularly abundants in localized parts of the Iberian massif, the French Massif Central and the Bohemian Massif. U-Pb dating of columbite-tantalite from selected Variscan rare-elements granites and pegmatites of these three massifs has been performed using laser ablation system connected to a single collector magnetic SectorField - Inductively Coupled Plasma - Mass Spectrometer (LA-SF-ICP-MS). The main aims of this study are to investigate the timing of this magmatism in the Variscan belt and the chronological relationship with the surrounding granitoid suites. Our results highlighted the existence of several emplacement episods : - in the Moldanubian domain of the Bohemian Massif, rare-elements pegmatites emplaced at around 340-330 Ma. - in the North of French Massif Central, the Montebras and Beauvoir granites and the Chèdeville pegmatite lead to emplacement age at 315-310 Ma. - in the North of Iberian massif, three different events have been recognized, with the emplacement of the Argemela granite at 326 ± 3 Ma, a first group of pegmatites emplaced at 310 ± 5 Ma, a second at 301 ± 3 Ma. In this context, rare-elements magmatism do not look to be related to lower crust processes (i.e. granulitic metamorphism) and rare metal magmatism event appears diachroneous and polyphased.