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

Abstract.

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.