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.