The geochemical character of late Cadomian extensional magmatism in Jersey, Channel Islands

Abstract

Abstract An episode of magmatism, which may represent the last igneous phase attributable to the Cadomian orogenic cycle, is found in southern Jersey, Channel Islands. A zone, some 3 km wide, of intensive dyke injection, the Jersey Main Dyke Swarm, cuts across post-orogenic calc-alkaline plutons, though showing a close temporal link with their emplacement. The nature of the dyke swarm magmatism is dominantly but not exclusively basic. A clear acid—basic bimodality is present with very little intermediate material. Although subjected to varying degrees of alteration under low-grade metamorphic conditions, the primary nature of the rocks is often preserved. All those having a basaltic composition are olivine-normative, but show a high-K calc-alkaline character. Fairly high LREE enrichments are present. MORB-normalized spidergrams confirm that the magmatism was of a mature calc-alkaline type, similar to that generated at active continental margins such as the Andes. The presence of the swarm indicates a period of late post-orogenic crustal extension sub-parallel to the NE-SW strike of the proposed Cadomian active continental margin in the Armorican Massif. It could represent injection of the last remnants of magma from an already decayed subduction zone system into a newly cratonized environment.