Variscan lamprophyres of the South Armorican Domain and comparison with lamprophyres of the Western European Variscan belt
Résumé
Late to post-orogenic lamprophyres of the European Variscides attest variable compositions of the mantle beneath the structural zones of the belt. These compositions resulted from different contributions of mantle components involving geotectonic processes during the orogeny, such as oceanic subduction of mafic crust and sediments, continental subduction, collision with mantle input, and delamination of overriding plates. For documenting these processes, we have surveyed three sites of lamprophyre intrusions in the Vendean part of the South Armorican tectonic Zone with spessartite sills and minette dykes, and a fourth site in the West-Armorican kersantite swarm. The age of spessartite is estimated between 320 and 315 Ma on the base of structural relationships with the dated neighbouring granite. Dykes of minette share similar intrusive setting along the post-orogenic NW-SE dextral shear zones. One dyke is dated at 286.2 ± 6.6 Ma (Early Permian) by K/Ar method. The Western Brittany kersantite swarm is Middle to Late Carboniferous in age. All these rocks display common mineral and chemical compositions of lamprophyres. A review of the Variscan European lamprophyres is conducted in order to document their geochemical fingerprints compared with those of the studied samples.