TIMING OF CADOMIAN AND VARISCAN TECTONOTHERMAL ACTIVITY, LA-HAGUE ANDALDERNEY, NORTH ARMORICAN MASSIF - EVIDENCE FROM AR-40 AR-39 MINERAL AGES/

The La Hague region of northwest France exposes Palaeo-Proterozoic Ica rtian gneisses which were reworked and intruded by calc-alkaline pluto nic rocks during the Cadomian Orogeny (about 700-500 Ma). Ar-40/Ar-39 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 Ar-40/Ar-39 apparent age spectra interpre ted to result from limited Variscan (about 350-300 Ma) overprinting of intracrystalline argon systems which initially cooled through post-me tamorphic hornblende closure temperatures during the Cadomian at about 600 Ma. Igneous hornblendes from the weakly foliated Jardeheu and Mou linet quartz diorites record isotope correlation ages of 599 +/- 2 and 561 +/- 2 Ma, respectively. Igneous hornblende and biotite from folia ted quartz diorite on the nearby Channel Island of Alderney record iso tope correlation ages of about 560 Ma. The results imply that metamorp hic and plutonic events in the La Hague-Alderney region were approxima tely contemporaneous with those recorded on Guernsey and Sark, which a re thus likely to have formed part of the same tectonic block during t he Cadomian Orogeny.