Contrasting magma types and timing of intrusion in the Permian layered mafic complex of Mont Collon (Western Alps, Valais, Switzerland): evidence from U/Pb zircon and 40Ar/39Ar amphibole dating

Philippe Montjoie (1, 2) , F. Bussy (2) , U. Schaltegger (3) , A. Mulch (2, 4) , Henriette Lapierre (1) , H.-R. Pfeifer (2)

1 LGCA - Laboratoire de Géodynamique des Chaines Alpines
2 Institute of Mineralogy and Geochemistry, IMG-ANTHROPOLE
3 Department of Mineralogy
4 GS - Department of Geological Sciences [Stanford]

We have selected and dated three contrasting rock-types representative of the magmatic activity within the Permian layered mafic complex of Mont Collon, Austroalpine Dent Blanche nappe, Western Alps. A pegmatitic gabbro associated to the main cumulus sequence yields a concordant U/Pb zircon age of 284.2 ± 0.6 Ma, whereas a pegmatitic granite dike crosscutting the latter yields a concordant age of 282.9 ± 0.6 Ma. A Fe-Ti-rich ultrabasic lamprophyre, crosscutting all other lithologies of the complex, yields an 40Ar/39Ar plateau age of 260.2 ± 0.7 Ma on a kaersutite concentrate. All ages are interpreted as magmatic. Sub-contemporaneous felsic dikes within the Mont Collon complex are ascribed to anatectic back-veining from the country-rock, related to the emplacement of the main gabbroic body in the continental crust, which is in accordance with new isotopic data. The lamprophyres have isotopic compositions typical of a depleted mantle, in contrast to those of the cumulate gabbros, close to values of the Bulk Silicate Earth. This indicates either contrasting sources for the two magma pulses – the subcontinental lithospheric mantle for the gabbros and the underlying asthenosphere for the lamprophyres – or a single depleted lithospheric source with variable degrees of crustal contamination of the gabbroic melts during their emplacement in the continental crust. The Mont Collon complex belongs to a series of Early Permian mafic massifs, which emplaced in a short time span about 285-280 Ma ago, in a limited sector of the post-Variscan continental crust now corresponding to the Austroalpine/Southern Alpine domains and Corsica. This magmatic activity was controlled in space and time by crustal-scale transtensional shear zones.

Mots clés: gabbro; mantle; Permian; Western Alps; geochronology

Domaines: Planète et Univers [physics]; Sciences de la Terre

Philippe Montjoie, F. Bussy, U. Schaltegger, A. Mulch, Henriette Lapierre, et al.. Contrasting magma types and timing of intrusion in the Permian layered mafic complex of Mont Collon (Western Alps, Valais, Switzerland): evidence from U/Pb zircon and 40Ar/39Ar amphibole dating. Swiss Journal of Geosciences, 2007, 100 (1), pp.125 à 135. ⟨10.1007/s00015-007-1210-8⟩. ⟨insu-00353661⟩