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First U/Pb dating of rare-element magmatism from the north French Massif Central: A milestone in rare-element events of the Variscan belt.

Abstract : Rare -elements (Ta, Nb, Be, Cs, Li, Sn, etc.) are critical raw material for the European Union. One significant primary source of these metals is rare- element -enriched LCT pegmatites and high phosphorus peraluminous granites. This type of magmatism is parti cularly well expressed in the Variscan belt, mainly in localized parts of the Iberian massif, the Bohemian Massif and the French Massif Central (FMC) . In the latter, an E -W rare -element magmatic belt ( Cuney et al ., 2002) is defined by three rare -element pe raluminous granites (Beauvoir, Montebras) or volcanic equivalent (Richemont rhyolite) and the LCT pegmatite field of Chédeville. The main aims of this study are to investigate the timing of this magmatism and the chronological relationship with deformation , metamorphism and peraluminous magmatism in the FMC and to compare, at the scale of the Variscan belt, these results with our previous studies. U -Pb dating of columbite- group minerals from Beauvoir, Montebras and Chédeville rare- elements magmatic bodies has been performed using laser ablation system connected to a single collector magnetic Sector Field - Inductively Coupled Plasma - Mass Spectrometer (LA -SF -ICP -MS). The Beauvoir granite leads to an emplacement age of 317 ± 6 Ma which is significantly older than previous Ar/Ar on lepidolite published one (308 ± 2 Ma, Cheilletz et al ., 1992). Conversely our U/Pb ages of 314 ± 4 Ma and 309 ± 5 Ma obtained on Montebras granite and Chèdeville pegmatites are identical, within the analytical uncertainties, from pr evious published Ar/Ar age (Cuney et al ., 2002; Cheilletz et al ., 1992) and also identical to the 313 ± 3 Ma Ar/Ar age of Richemont rhyolite (Cuney et al ., 2002). In the north of the FMC, several granites are slightly early to coeval with this rare element magmatism; however, the spatial distribution of rare- element magmatic bodies is by far smallest than those of coeval granites. Migmatites sources of the ses granites are unknown in the FMC since there is a lack of Bashkirian to Moscovian migmatites (ca. 318 -307 Ma, e.g. Faure et al ., 2010). However, a more discrete migmatitic event seems to occur since cordierite- bearing migmatitic gneisses has been recent ly dated at 316 ± 2 Ma in the North of the MCF (Gébelin et al ., 2009) . The Marche fault system seems to be a key -structure for the rare- element magmatism since it appears as an hyphen between all bodies. Moreover the Marche fault system is coeval with the rare -element magmatism as evidenced by Ar/Ar ages ranging from 316 ± 5 to 312 ± 2 Ma (Gébelin et al ., 2007) . We compare these results with our previous studies on the Moldanubian domain of the Bohemian Massif (Melleton et al ., 2012) and the Iberian Massif (Melleton et al . submitted) where rare- element magmatic bodies display interesting relationships with metamorphic events and crustal -scale structures. In the light of these new results, rare- elements magmatic bodies of the MCF appear as a milestone in the polyphased rare -metal events of the Variscan belt which extend at least from 333 to 280 Ma. REFERENCES CHEILLETZ et al. ( 1992) Ages 40Ar/39Ar du leucogranite à topaze- lépidolite de Beauvoir et des pegmatites sodolithiques de Chédeville (Nord du Massif Central, France). Signification pétrologique et géodynamique. C. R. Acad. Sci. Paris 315, 329– 336. CUNEY et al. (2002) The timing of W -Sn -rare metals mineral deposit formation in the Western Variscan chain in their orogenic setting: the case of the Limousin area (Massif Central, France), in: Blundell, D.J., Neubauer, F., von Quadt, A. (Eds.), The Timing and Location of Major Ore Deposits in an Evolving Orogen. The Geological Society of London, pp. 213– 228. FAURE et al. ( 2010) . Middle Carboniferous crustal melting in the Variscan Belt: New insights from U –Th –Pbtot. monazite and U –Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central). Gondwana Research 18, 653– 673. GEBELIN et al. (2007) Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and 40Ar/39Ar investigations. Tectonics 26, TC2008, doi:10.1029/2005TC001822, 2007. GEBELIN et al. ( 2009) Syntectonic crustal melting and high- grade metamorphism in a transpressional regime, Variscan Massif Central, France. Tectonophysics 477, 229– 243. MELLETON et al. (2012) How Are the Emplacement of Rare- Element Pegmatites, Regional Metamorphism and Magmatism Interrelated in the Moldanubian Domain of the Variscan Bohemian Massif, Czech Republic? Can Mineral 50, 1751– 1773. MELLETON et al. (submitted) T. Polyphased rare- element magmatism during late orogenic evolution : geochronological constraints from the NW Variscan Iberia. Contribution to Mineralogy and Petrology.
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Submitted on : Monday, March 21, 2016 - 4:05:03 PM
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Eric Gloaguen, Jérémie Melleton, D Frei. First U/Pb dating of rare-element magmatism from the north French Massif Central: A milestone in rare-element events of the Variscan belt.. Crustal melting in the European variscan belt, Jun 2013, Orléans, France. ⟨insu-01291537⟩

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