Cadomian magmatism in the North Armorican Massif

Abstract

This review concerns intrusive magmatism related to the Cadomian orogeny in the northeastern part of the Armorican Massif of France. The Cadomian orogeny is interpreted to represent tectono-thermal activity in a continental margin above a subduction zone. The North Armorican Shear Zone (NASZ) separates two major Cadomian terranes, the North Armorican Composite Terrane (NACT) and the Central Armorican Terrane (CAT). The amalgamation of three terranes within the NACT (the St Brieuc Terrane (SBT), the St Malo Terrane (SMT) and the Mancellian Terrane (MT)) around 540 Ma by sinistral transpression is a key element in the Cadomian orogeny. Cadomian plutonic complexes intrude thc c. 2000 Ma Icartian Gneisses and form the local basement to, and are emplaced into the Brioverian succession. Cadomian magmatism spans approximately 275 Ma, from c. 700 Ma to c. 425 Ma.Early Cadomian foliated plutonic complexes occur within the Guernsey-La Hague structural block of the SBT; they represent volcanic arc granites derived in a subduction zone environment. Late Cadomian post-tectonic plutonic complexes within the GuernseyLa Hague structural block were emplaced c. 500 Ma, exhibit a variety of 'mixed magma' features and have a normal to mature continental arc geochemistry. To the southeast, the Jersey structural block within the SBT reprcsents a higher structural level as well as a greater distance away from the probable trench site. Here, thc late Cadomian posttectonic plutonic complexes were emplaced during Cambrian and Ordovician-Silurian times and represent mature continental arc activity, probably associated with the decay of the Cadomian subduction zonc systcm aftcr transprcssional terrane accretion.The St Malo Terrane, to thc southcast, represents an inverted intra-or behind-arc basin. The migmatite belts within this terranc developcd by crustal anatexis. Emplacement of homogeneous diatexites and leucogranites synkinematically with sinistral strike-slip movement along shear belts gencrated by the transpressive terrane accretion allows this cvent to be dated at c. 540 Ma.The Mancellian Terrane, further to the southeast, comprises low-grade Brioverian metasediments into which has been emplaced the Mancellian Batholith, a late Cadomian post-tectonic series of predominantly granite complexes. These complexes have the geochemical features of volcanic arc granites.Overall, Cadomian magmatism rcpresents c. 275 Ma of plutonic activity broadly related to subduction beneath a continental margin. The early part of the Cadomian orogenic cycle includes volcanic-arc plutonic suites and basaltic volcanic rocks with associated feeder dykes. The peak of the Cadomian orogeny involves intra-or behind-arc basin inversion with concomitant mid-crustal anatexis and sinistrally transpressive terrane accretion c. 540 Ma. The late evolution begins with andesite-rhyolite volcanism and calcalkalinc plutonism occurs over a period of c. 100 Ma during thc decay of the Cadomian subduction zone system.

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

“…Hornblende and biotite from a sample of the Fort Tourgis Quartz Diorite of Alderney record mutually similar cooling ages of about 560 Ma. Although the complex textural features of this rock have been interpreted by Power et al (1990) to record metamorphic recrystallization, we consider the mineralogy and textures of the dated sample to result from igneous processes (magma-mixing and flow) and thus believe the age to record post-magmatic cooling. The age is similar to that recorded diorites are significantly younger than the about 605 Ma ages recorded from foliated quartz diorites of Guernsey and Sark (Dallmeyer et al 1991a).…”

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

“…An elongated belt of Cadomian calc-alkaline plutonic igneous complexes extends in an ENE-WSW direction from the Cherbourg Peninsula (la Hague complex), through the Channel Islands, to the Tregor region of the Cotes-du-Nord, (and perhaps even further to the Gneiss de Brest complex in West Finistere), constituting a major part of the St Brieuc Terrane in the geotectonic model of Strachan et al, (1989) (see also Brown et al 1990). The petrological and chemical characters of many of these complexes have been reviewed by Brown et al (1990) and indicate an origin primarily by mantle differentiation and lower crustal melting, rather than by intra-crustal melting of preexisting sedimentary material or much older continental crust, evidence for the existence of which is fragmentary (cf.…”

“…Those in the NW and SE contain a significant component of material of basic composition, though this is absent in the SW complex. These plutonic complexes are among the youngest dated Cadomian plutons in the Armorican Massif (see below, and Brown et al 1990). …”

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

“…Many of the plutonic units typically have calc-alkaline affinities and are considered to have been intruded in a supra-subduction zone setting (Brown et al 1990;Gibbons & Moreno 2002). The various formations that make up the Brioverian Supergroup have been argued to be back-arc and marginal basin deposits (Rabu et al 1990;Egal et al 1996;Ballèvre et al 2001;Chantraine et al 2001;Miller et al 2001).…”