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Innovative and multi-disciplinary approach for discussing the emplacement of Variscan LCT-pegmatite fields

La mise en place des champs de pegmatites Varisques de type LCTe: Approche multi-disciplinaire et innovante

Abstract : In order to define new metallogenic guides for Li, Nb and Ta supply, three Variscan LCT pegmatite fields are investigated. A multidisciplinary approach is favoured to discuss both the genetic and emplacement models for LCT-type deposits. The spatial statistical analyses demonstrate the high clustering rate of Li-rich pegmatites and the spatial relationships between Li-pegmatites location and surrounding damage fractured zone. The Li-isotopes analyses on micas suggest the absence of genetic continuum between surrounding granites and pegmatites since δ 7 Li values (‰) are similar through pegmatite field and inside granite samples. To refine these previous results, preliminary numerical models are developed to constrain the ascent of these low viscosity pegmatite-forming melts and determine key criterion of their uncoupling from parental magma source. According to these results, the high permeability seems to play a crucial role in the pegmatite melts collection and during their ascent. The combination of geostatistical, geochemical and numerical tools allows suggesting an alternative model of direct crustal anatexis to the common parental granite model. These high permeability damage shear zones are considered to favour the low volume of pegmatite-forming melt pumping and their escape from their deeper crustal magma source.