Potassic late orogenic Stephanian volcanism in the South West french Massif Central (Decazeville, Figeac, Lacapelle-Marival basins): an example for mantle metasomatism along strike-slip faults?

Henriette Lapierre (1) , Christophe Basile (1) , Thomas Berly (1) , Emmanuel Canard (1)

1 LGCA - Laboratoire de Géodynamique des Chaines Alpines

In the Southwestern part of the French Massif central (Decazeville basin, at the Sillon Houiller fault termination; Figeac and Lacapelle-Marival basins along the Argentat fault), Stephanian volcanism exhibits shoshonitic affinities. Their chondrite-normalized Rare Earth Element (REE) patterns are enriched in light REE, but almost flat for heavy REE, with marked negative Eu anomalies. Primitive mantle-normalized element spectra show negative Nb, Ta, P, Sm, Ti, and positive Th, U, Pb anomalies, respectively. εNd values are negative and homogeneous (-6 to -4). This volcanism shares the same geochemical patterns as the late-orogenic Stephanian-Permian magmatism from the southern part of the Variscan belt (Pyrénées, Alps, Sardinia). We explain these geochemical characteristics as resulting from the partial melting of a metasomatised mantle. We propose a new mechanism to explain this melting process: horizontal displacement along the main late-orogenic strike-slip faults might bring into contact a hydrated lower crust with the lithospheric mantle. Mantle metasomatism within the strike-slip fault zone may then induce partial melting

Mots clés: late orogenic magmatism; shoshonitic; Sillon Houiller fault; Variscan

Domaines: Tectonique; Géochimie

Henriette Lapierre, Christophe Basile, Thomas Berly, Emmanuel Canard. Potassic late orogenic Stephanian volcanism in the South West french Massif Central (Decazeville, Figeac, Lacapelle-Marival basins): an example for mantle metasomatism along strike-slip faults?. Bulletin de la Société Géologique de France, 2008, 179 (5), pp.491-502. ⟨hal-00138515v2⟩