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

Journal of the Geological Society, Mar 2005 by Inglis, J D, Samson, S D, D'Lemos, R S, Miller, B V

Abstract:

The La Hague peninsula, Normandy, NW France, contains the most northerly components of the Late Neoproterozoic Cadomian orogeny. U-Pb dates for single-grain and small multiple-grain zircon separates from carefully selected plutons provide geochronological constraints on the timing of deformation and magmatism within the region. The syntectonic Jardeheu quartz diorite and Omonville granodiorite were emplaced and contemporaneously deformed during the development of a greenschist-facies fabric, pervasive across the peninsula. U-Pb zircon dates of 610 ± 1 Ma for the Jardeheu quartz diorite and 608 ± 2 Ma for the Omonville granodiorite are similar to a previously reported sensitive high-resolution ion microprobe U-Pb date for metamorphism of the basement, and indicate that a regionally significant period of metamorphism and magmatism occurred between 620 and 608 Ma. An emplacement age of 573.9 1.3/-0.9 Ma for the syntectonic Thiébot granodiorite places limits on the timing of deformation along the Thiébot Shear Zone. Zircons from the undeformed St Martin monzonite give a precise U-Pb age of 581.3 ± 1.1 Ma. The new dates point to reactivation of magmatism and localized greenschist-facies deformation between 582 and 573 Ma.

Keywords: La Hague, NW France, Cadomian orogeny, U-Pb, absolute age.

The Cadomian belt of the North Armorican Massif (NW France and British Channel Islands) contains fragments of c. 2 Ga basement rocks intruded by, and/or in tectonic contact with, Neoproterozoic plutons and a Neoproterozoic supracrustal volcanic and sedimentary sequence termed the Brioverian Supergroup (Rabu et al. 1990; Egal et al. 1996; Strachan et al. 1996; Samson & D'Lemos 1998, 1999; Chantraine et al. 2001). 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). Deformation, plutonism and metamorphism within the Cadomian belt is believed to have occurred at different times in different parts of the belt between c. 615 Ma and c. 540 Ma. Several workers have highlighted contrasting lithological associations and structural histories in different parts of the belt that have been termed domains, terranes or segments (Strachan et al. 1989, 1996; Egal et al. 1996; Samson et al. 2003). These structural segments, which most workers consider to be dismembered parts of the same supra-subduction margin, were amalgamated and juxtaposed to approximately their current relative positions prior to deposition of a Cambro-Ordovician overstep sequence marking the development of a passive margin. During Variscan orogenesis (c. 350-258 Ma) further deformation and magmatism affected the North Armorican Massif. However, within the Proterozoic units the Variscan overprint is typically non-penetrative, being localized to brittle fault systems or adjacent to intrusions. Hence, although the geological evolution is complex, many of the Neoproterozoic relationships survive intact.

Several recent workers have attempted to elucidate further the nature and evolution of the c. 2 Ga Palaeoproterozoic basement and the timing of Neoproterozoic deformational and magmatic events within the northern part of the North Armorican Massif (Miller et al. 1999; Samson & D'Lemos 1999: D'Lemos et al. 2001). In the Channel Islands it has been shown that a suite of foliated syntectonic quartz diorite-tonalite intrusions were emplaced at c. 615-608 Ma (Miller et al. 1999). Structural analyses indicate that the main structural grain of the basement components (dominantly granite orthogneiss) also developed during this time (Tribe & D'Lemos 1996). However, no robust radiometric dates or detailed analyses of the P- T conditions of this metamorphic event currently exist. Others working in the Channel Islands region have also demonstrated a second period of 'post-tectonic' magmatism at c. 580-570 Ma, and it has been suggested that the recorded periods of magmatism represent two distinct pulses of magmatic activity in the supra-subduction zone (D'Lemos et al. 2001; Miller etui. 2001).

The La Hague region of Normandy, France (Fig. 1), has an extensive coastline and high tidal range exposing both orthogneiss and paragneiss, traditionally believed to form part of the pre-Cadomian basement, as well as numerous foliated to undeformed Cadomian intrusions. The region thus offers the potential to derive information on the conditions and timing of deformation and metamorphism of the basement, as well as constrain a record of Cadomian plutonism and deformation in this sector of the orogenic belt. A programme of detailed field mapping, petrographic and microstructural analysis tied to U-Pb zircon dating has been undertaken in the La Hague region. The aims of this paper are to summarize the results of those investigations and provide a detailed geological history of the La Hague region.