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U-Pb dating, Wolframite, Tungsten deposits, French Massif Central, Variscan belt
Abstract:
We present U-Pb thermal ionization mass spectrometer (TIMS) ages of wolframite from several granite-related hydrothermal W±Sn deposits in the French Massif Central (FMC) located in the internal zone of the Variscan belt. The studied wolframite samples are characterized by variable U and Pb contents (typically <10 ppm) and show significant variations in their radiogenic Pb isotopic compositions. The obtained U-Pb ages define three distinct geochronological groups related to three contrasting geodynamic settings: (i) Visean to Namurian mineralization (333–327 Ma) coeval with syn-orogenic compression and emplacement of large peraluminous leucogranites (ca. 335–325 Ma), (ii) Namurian to Westphalian mineralization (317–315 Ma) synchronous with the onset of late-orogenic extension and emplacement of syn-tectonic granites (ca. 315–310 Ma) and (iii) Stephanian to Permian mineralization (298–274 Ma) formed during post-orogenic extension contemporaneous with the Permian volcanism in the entire Variscan belt. The youngest ages (276–274 Ma) likely reflect the reopening of the U-Pb isotopic system after wolframite crystallization and may correspond to late hydrothermal alteration (e.g. ferberitization). Our results demonstrate that W(±Sn) mineralization in the FMC formed during at least three distinct hydrothermal events in different tectono-metamorphic settings over a time range of 40 Ma.
