Geochronology of the Channel Islands and adjacent French mainland

Abstract

Rb-Sr whole-rock dating of igneous and metamorphic rocks of the Channel Islands region has delineated the following major events: 1) Formation of the Pentevrian basement gneisses c. 2600 Ma ago in the northern Guernsey, Alderney and Cap de la Hague region. 2) Possible isotopic rejuvenation of these gneisses during a second metamorphic episode c. 1950 Ma ago. 3) Formation of Pentevrian gneisses in the southern, St Malo-St Brieuc region, 1000–1100 Ma ago. 4) Deposition of the Brioverian sedimentary series in the interval 650–1000 Ma. 5) Emplacement of igneous rocks 650–690 Ma ago during initial phases of the Cadomian Orogeny (late Precambrian) followed by 6) a main regional metamorphic event 620–650 Ma ago, and 7) a post-metamorphic igneous phase 560–610 Ma ago. 8) On Jersey a further post-tectonic granite phase, 480–520 Ma, can be recognized Within this framework, nearly 130 K–Ar and Rb–Sr mineral ages from the area are interpreted as reflecting initiation of cooling and uplift of the Cadomian orogen as a whole in late Precambrian or Cambrian times. Only a very few, young minor intrusions are connected with Variscan orogenic events. Some preliminary comparisons can be made with late Precambrian igneous and metamorphic rocks elsewhere in the Massif Armoricain, southern Britain, eastern Canada and U.S.A.