Timing of Cadomian and Variscan tectonothermal activity, La Hague and Alderney, North Armorican Massif: Evidence from 40Ar/39Ar mineral ages

Abstract

AbstractThe La Hague region of northwest France exposes Palaeo‐Proterozoic Icartian gneisses which were reworked and intruded by calc‐alkaline plutonic rocks during the Cadomian Orogeny (about 700–500 Ma). 40Ar/39Ar mineral cooling ages have been determined to clarify the timing of the regional metamorphism of orthogneisses and the emplacement of quartz diorite plutons in this region. Metamorphic amphiboles within Icartian gneisses display discordant 40Ar/39Ar apparent age spectra interpreted to result from limited Variscan (about 350–300 Ma) overprinting of intracrystalline argon systems which initially cooled through post‐metamorphic hornblende closure temperatures during the Cadomian at about 600 Ma. Igneous hornblendes from the weakly foliated Jardeheu and Moulinet quartz diorites record isotope correlation ages of 599 ± 2 and 561 ± 2 Ma, respectively. Igneous hornblende and biotite from foliated quartz diorite on the nearby Channel Island of Alderney record isotope correlation ages of about 560 Ma. The results imply that metamorphic and plutonic events in the La Hague‐Alderney region were approximately contemporaneous with those recorded on Guernsey and Sark, which are thus likely to have formed part of the same tectonic block during the Cadomian Orogeny.