Cadomian geodynamic evolution of the northeastern Armorican Massif (Normandy and Maine)

Abstract

The oldest Precambrian rocks of the northeastern Armorican Massif are represented by the migmatites and gneisses of La Hague which make up the Lower Proterozoic Pentevrian basement. This is overlain by the low-grade Upper Proterozoic rocks of the Brioverian succession which were affected by Cadomian deformation and plutonism. The Lower Brioverian Group consists of four tholeiitic volcanic formations overlain by terrigenous units which contain some black chert horizons. In the Coutances area this assemblage was affected by a local tectonometamorphic event which led to the formation of the Constantian arc (585 Ma). The uplift and erosion of this arc yielded the Upper Brioverian flysch which includes spilitic rocks at Vassy and sulphide deposits at Rouez. A plate tectonic model is proposed for the Cadomian orogeny in the northeastern Armorican Massif. Major and trace element geochemical features of the Brioverian igneous rocks allow us to assign it to a ‘Pacific’ tectonic setting. An island-arc system was established on Pentevrian crust remnants and led to magmatic arcs separated by marginal basins floored by MORB. Strain analysis shows that Cadomian thrusts and folds verge northwestwards, synthetic to southeasterly-directed subduction. A second southeastward-dipping subduction zone developed and was responsible for the closure of the back-arc basin and the generation of the Mancellian Batholith. The folded Upper Brioverian rocks were then intruded by these late-tectonic granodiorites at c. 540 Ma. The Brioverian rocks and Cadomian plutons were then uplifted, eroded and unconformably overlain by the Lower Cambrian cover. In the northeastern Armorican Massif, this Cadomian evolution corresponds to a major period of crustal growth and represents progressive cratonisation of the Brioverian basin and arc system.

Basement isotopic signatures and Neoproterozoic paleogeography of Avalonian-Cadomian and related terranes in the Circum-North Atlantic

“…600-550 Ma. Subduction polarity follows the consensus summarized by Dupret et al (1990), Rabu et al (1990), and Strachan et al (this volume) for Cadomia and that proposed by Gibbons and Horak (this volume) for East Avalonia. By Cambrian times, a similar position for the Meguma terrane is suggested by detrital zircon data from the Meguma Group ) that, based on its Nd signature (Clarke and Halliday, 1985;Clarke et al, 1993), must have had a provenance with very similar isotopic characteristics to that of Cadomia (Fig.…”

“…1) are defined by their Neoproterozoic evolution on an active continental margin, and by the peri-Gondwanan "Acado-Baltic" fauna of their shallow-marine Cambro-Ordovician overstep sequences (e.g., Rast and Skehan, 1983;Murphy and Nance, 1989;Nance et al, 1991). They include (1) the Avalon composite terrane of New England and Atlantic Canada (e.g., O'Brien et al, 1983;Nance, 1986;, referred to as "West Avalonia"; (2) the predrift continuation of this terrane in southern Britain (e.g., Thorpe et al, 1984;Pharoah et al, 1987;Pauley, 1990a), referred to as "East Avalonia"; and (3) the North Armorican composite terrane in the Cadomian orogenic belt of northwestern France and Bohemia (e.g., Chaloupsky, 1990;Dupret et al, 1990;Rabu et al, 1990;Egal et al, this volume;Strachan et al, this volume), referred to as "Cadomia. "…”

The structural evolution of the English Channel area

“…It crops out to the west of the area in Cornwall and the Armorican massif and involves (i) Icartian gneisses, (ii) Cadomian metasediments, volcanics and granites (Dupret et al, 1990), and (iii) Variscan metasediments and granites (Le Corre et al, 1991). The Precambrian rocks are divided into two units: (i) the older orthogneisses and migmatites pertain to a composite basement of Icartian (2500 Ma) and Pentevrian (750 Ma) age that is well exposed on Alderney, Guernsey and in North Brittany (Chantraine et al, 2001); (ii) the younger Brioverian unit (620 to 540 Ma) was deposited unconformably on the composite basement and comprises turbidites (mudstone, siltstone and greywacke) and volcanics.…”

“…Plate tectonic models for the Cadomian orogeny (Dupret et al, 1990;Chantraine et al, 2001) involve: (i) an island-arc system developed on Pentevrian crust remnants; (ii) Cadomian continental collision, basin closure and structural compartmentalisation marked by the development of NE-SW crustal faults; (iii) crustal melting and peraluminous granite generation at ca. 540 Ma (Jonin 202 Dupret et al, 1990;Chantraine et al, 1996): OAFZ, Ouessant Alderney Fault Zone; CCFZ, Central Cotentin fault zone; A and B, locations of cross-sections in Fig. 4 and Vidal, 1975); (iv) late tectonic uplift and erosion.…”

“…For example, the Ouessant-Alderney fault zone (OAFZ) (Fig. 1), is expected to be close to a major Cadomian fault zone (Dupret et al, 1990). It corresponds to the northern, nearsurface limit of the Armorican platform, to the north of which thick Mesozoic and Tertiary strata were deposited.…”