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

• Published in: Swiss Journal of geosciences 2007-06, Vol.100 (1), p.125-135
• Main Authors: Monjoie, Philippe, Bussy, François, Schaltegger, Urs, Mulch, Andreas, Lapierre, Henriette, Pfeifer, Hans-Ruedi
• Format: Article
• Language: English
• Subjects: Age; Amphiboles; Argon; Asthenosphere; Clouds; Contamination; Continental crust; Depletion; Earth mantle; Earth Sciences; Embankments; Gabbro; Gabbros; Geology; Isotopes; Lava; Lead; Magma; Massifs; Oceanic crust; Permian; Radiometric dating; Rock intrusions; Rocks; Sciences of the Universe; Shear zone; Silicates; Titanium; Zircon
• ISSN: 1661-8726; 1661-8734
• DOI: 10.1007/s00015-007-1210-8

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.