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

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.

The groundwater invertebrate fauna of the Channel Islands

“…Intrusive igneous rocks, principally pink granites, gabbros, diorites and granodiorites, considered to be of early Paleozoic age, outcrop at the south west, north west and south east corners of the island. Minor intrusions by many later dolerite, lamprophyre and quartz-feldspar porphyry dykes are common in all of the bedrock strata (Lees 1990). …”

Cadomian magmatism in the North Armorican Massif

“…The field relations, petrography, isotopic age data and geochemistry of these complexes are considered by Power et al (1990a); here we summarize their gross characteristics. Early Cadomian foliated plutonic rocks which form part of the Penthi6vre Complex located on the east side of the Baie de St Brieuc are described in Shufflebotham (1990) and geochemical data for these rocks are given in Shufflebotham (1989 Table 2.…”

“…Cogn6 1972). This tectonostratigraphy has proved ambiguous iil respect of the unconformable relationship between supposed pre-Cadomian basement and the lateProterozoic Brioverian succession cover in the type locality (Guerrot & Peucat 1990;Shufflebotham 1990). This follows the recognition that the local basement to the Brioverian succession at Jospinet, on the east side of the Baie de St Brieuc, is in fact an early part of the Cadomian orogenic cycle (Guerrot & Peucat 1990;Strachan et al 1989), rather than basement from an earlier orogenic cycle.…”

“…Basaltic rocks form a significant part of the Brioverian sequence, especially in its lower part, and an understanding of the geochemistry of these rocks has contributed to an interpretation of the overall geotectonic framework of the Cadomian orogeny (Cabanis et al 1987;Lees et al 1987;Dupret et al 1990; and references therein). Intermediate and acid volcanics occur within the upper parts of the Brioverian sequences (Roach et al 1986;Dupret et al 1990; and references therein). However, plutonic complexes form an important component of the Armorican Massif, which north of the SASZ, are related to either the Cadomian or the Variscan orogenies.…”

“…2). It stretches from the Channel Islands Region (Bland et al 1986), southeast to the Mancellian Batholith (Jonin 1981) and west across the Baie de St Brieuc (Shufflebotham 1990;Barrat 1986) to the Tregor (Auvray 1979; Graviou & Auvray 1990). The range of apparent ages of plutonic rocks considered in this review is from late Precambrian to Ordovician-Silurian, from c. 700-650 Ma down to c. 425 Ma.…”