Polyphase W-Sn mineralization and rare metal magmatism in relation to the late-Variscan tectono-metamorphic evolution of the southeastern French Massif Central
Résumé
The southeastern French Massif Central represents an ideal area to study the linkage between regional metamorphism, crustal partial melting, emplacement of granitic magmas, and hydrothermal Sn-W mineralization in a polyphase tectono-metamorphic setting related to the late-Variscan orogeny. Here, we describe the mineralogical, structural, geochemical, and geochronological characteristics of cassiterite–wolframite-bearing quartz veins at St-Mélany, a small uneconomic Sn-W occurrence located in the North Cévennes area. The veins show evidence of ductile deformation (boudinage, asymmetric folding, dynamic recrystallization) consistent with a synkinematic emplacement during the regional low-pressure–medium-temperature metamorphism at ca. 320–315 Ma. This dominantly water-fluxed melting event reaching muscovite breakdown conditions (T < 750 °C, P ≈ 0.6 GPa) was synchronous to the emplacement of the syntectonic Rocles peraluminous granite, which is interpreted as a proximal source for the mineralizing fluids at St-Mélany. The U–Pb LA-ICP-MS dating of coexisting wolframite and cassiterite from a mineralized quartz vein yielded lower-intercept ages of 318.4 ± 2.2 Ma and 311.4 ± 1.0 Ma (2σ), respectively. These results suggest a temporal decoupling of W and Sn mineralization with a time gap of 4–10 Myr, but additional work is needed to confirm this interpretation. A weighted mean 40Ar/39Ar date of 304.5 ± 4.8 Ma (2σ) was obtained for muscovite from the selvage of a mineralized vein, interpreted as a recrystallization age related to metamorphic re-equilibration or hydrothermal overprinting. Dikes of aplites and pegmatites cut the Sn-W-mineralized veins and were emplaced at 305.9 ± 3.9 Ma (2σ) based on U–Pb LA-ICP-MS dating of magmatic cassiterite. The dikes have highly evolved compositions typical of peraluminous high-phosphorus rare metal granites with Li-F-Ta > Nb-Sn-Be enrichments. Emplacement of the granitic dikes was coeval with the regional low-pressure–high-temperature metamorphism at ca. 305–300 Ma, reaching biotite dehydration melting conditions (T > 800 °C, P ≈ 0.4 GPa), which led to the formation of the Velay anatectic dome possibly linked to lower crust granulitization. We conclude that polyphase emplacement of W-Sn-mineralized veins at ca. 320–310 Ma and rare metal granitic dikes at ca. 305 Ma results from contrasting crustal melting conditions, in relation to the late-Carboniferous orogenic evolution of the southeastern French Massif Central, and possibly related to delamination of the subcontinental lithospheric mantle.
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