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

Abstract

The 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). "Ad9Ar 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. Mstamorphic amphiboles within Icartian gneisses display discordant "Ar/"Ar 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 f 2 and 561 k 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.

Timing of Cadomian deformation and magmatism within La Hague, NW France

“…However, 40 Ar/ 39 Ar thermochronological dating of amphibolite-facies hornblende from metabasites within the metamorphic basement in La Hague did not yield Palaeoproterozoic ages for metamorphism (Dallmeyer et al 1994). The 40 Ar/ 39 Ar data were interpreted to result from amphibolite-grade metamorphism and subsequent cooling below c. 5008 C during a c. 600 Ma event (Dallmeyer et al 1994). Such a suggestion does not preclude an earlier metamorphic event, as the 40 Ar/ 39 Ar age could have been significantly reset by Cadomian events.…”

“…Indeed, Power et al (1990b) used geochemical similarities between the Jardeheu quartz diorite in the north and the Moulinet quartz diorite in the Thiébot Shear Zone to argue that they had a cogenetic, and therefore temporal, relationship. However, subsequent 40 Ar/ 39 Ar thermochronological dates (Dallmeyer et al 1994) proved this relationship incorrect. We argue that, on the basis of their relative timing of emplacement, the foliated intrusions may be placed into two broad suites.…”

“…An upper intercept age of discordant multigrain zircon analyses of 2018 AE 15 Ma for Icart gneiss at Soldiers Bay, Guernsey (Calvez & Vidal 1978), and a similar U-Pb zircon fraction age of c. 2100-2000 Ma reported by Dupret et al (1990) from the orthogneisses of the Thiébot Shear Zone in southern La Hague, were taken to support this theory of an Icartian metamorphic event (Power & Gibbons 1996). However, 40 Ar/ 39 Ar thermochronological dating of amphibolite-facies hornblende from metabasites within the metamorphic basement in La Hague did not yield Palaeoproterozoic ages for metamorphism (Dallmeyer et al 1994). The 40 Ar/ 39 Ar data were interpreted to result from amphibolite-grade metamorphism and subsequent cooling below c. 5008 C during a c. 600 Ma event (Dallmeyer et al 1994).…”

“…There has been considerable debate on the age of the protolith and age of metamorphism of the basement exposed within La Hague and other parts of northern Cadomia (Roach 1966;Dallmeyer et al 1994;Power & Gibbons 1996;Samson & D'Lemos 1998Inglis et al 2004). Adams (1967) and Leutwein et al (1973) were the first to postulate that Palaeoproterozoic basement existed within the La Hague-Channel Islands region.…”

“…A sample of mylonitic megacrystic granite from a large xenolith within the Moulinet quartz diorite yielded a U-Pb zircon date of 645 AE 12 Ma, whereas the Moulinet quartz diorite yielded a U-Pb zircon date of 585 þ13/ À12 Ma (Guerrot & Peucat 1990). Dallmeyer et al (1994) provided more precise igneous hornblende 40 Ar/ 39 Ar dates from both the Jardeheu quartz diorite (599 AE 2 Ma) and the Moulinet quartz diorite (561 AE 2 Ma), both of which were taken to record post-magmatic cooling.…”