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 french Massif Central (Decazeville basin, at the Sillon Houiller fault termination; Figeac and Lacapelle-Marival basins along Argentat fault) the Stephanian volcanism exhibit homogeous geochemical features. This magmatism has shoshonitic affinities. Their chondrite-normalized Rare Earth Element (REE) spectra are enriched in light REE, but almost flat for heavy REE, with a marked negative Eu anomaly. Primitive mantle-normalized element spectra show negative Nb, Ta, P, Sm, Ti, and positive Th, U, Pb anomalies, respectively. εNd values are negative and also homogeneous (between -6 and -4). This volcanism share the same geochemical patterns with the late-orogenic Stephanian-Permian magmatism from the southern part of the Variscan belt (Pyrénées, Alps, Sardinia). We interpret these characteristics as resulting from the partial melt of a metasomatism mantle. We propose a new mechanism to explain this melt: horizontal displacement along the mainlate-orogenic strike-slip fault may bring into contact an 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?. 2007. ⟨hal-00138515v1⟩