Upper Proterozoic Basaltic Volcanism in the Northern Massif Armoricain, France

Abstract

S U M M A RY:A 2 km thick sequence of subaqueous basaltic rocks--the Hillion-Erquy Volcanic Formation (HEVF)--forms the lower part of the late Proterozoic Brioverian supracrustal sequence around the Baie de St. Brieuc, N Brittany. Basic dykes cutting the adjacent Pre-Brioverian crystalline basement (the Pentevrian) are interpreted as feeders to the volcanic sequence. As a result of the end Precambrian-early Palaeozoic Cadomian orogenic event, the basic volcanics and feeder dykes experienced a regional metamorphism varying from prehnite-pumpellyite facies in the east to upper greenschist (or amphibolite) in the west of the Baie de St. Brieuc region.The compositions of the basic rocks are evolved, ranging from basalts to basaltic andesites. The suite is subalkaline and tholeiitic in character. Both major and trace element discriminants indicate the suite to be of continental flood basalt affinity. Incompatible element diagrams show evidence for a certain degree of crustal contamination during the evolution of the suite.It is proposed that the HEVF was erupted in a rapidly subsiding ensialic basin, formed during a phase of crustal extension. Volcanism ceased abruptly and the basin rapidly filled with sediment.A brief review is given of several other broadly similar basic sequences occurring in other parts of N Brittany. Their chemistry is compared with that of the HEVF and the implications for a geotectonic reconstruction of the region in late Proterozoic times discussed.

Tectonic evolution of the Cadomian belt in north Brittany

“…Known as the Spilites de Paimpol (Barrois 1908;Auvray 1979), these rocks are here referred to as the Paimpol Volcanic Formation (Fig. 2) (Lees et al 1987). These are overlain to the south by the Brioverian at University of Auckland on May 31, 2015 http://sp.lyellcollection.org/ Downloaded from metasediments of the Minard Formation, which is a series of turbiditic greywacke sandstones, siltstones and mudstones.…”

“…2), both of which are coincident with ductile shear zones represented by steeply-dipping mylonite belts. The Brioverian rocks of the Roselier block comprise a thick sequence of metabasalts with metasedimentary interbeds, the Hillion Volcanic Formation (HVF) (Lees et al 1987), overlain by the epiclastic metasediments of the L6gu6 Formation (Fig. 2).…”

“…The geology of the Plurien block has been described in detail elsewhere (e.g. Jeannette 1971;Cogn6 et al 1980;Shufflebotham & Roach 1985;Lees et al 1987;Shufflebotham 1990) and is only briefly outlined here for the purposes of comparison with the Roselier block.…”

Neoproterozoic evolution of an active plate margin: North Armorican Massif, France

“…Lowermost parts of local Brioverian successions are represented by thick sequences of subalkaline to tholeiitic, submarine volcanics (Cabanis et al, 1987;Lees et al, 1987). These are exposed in the south of the Baie de Saint-Brieuc (Lanvollon Formation, Rabu et al, 1982Rabu et al, , 1983, along the southern margin of the North Trégor batholith (Spilites de Paimpol, Auvray, 1979) and north of Coutances (Dupret et al, 1985) (Fig.…”

“…Generally undeformed metadolerites (Dahouet dikes) cut the older orthogneisses on the west side of the Penthièvre complex. Lees et al (1987) interpreted these to have been feeders to the Brioverian volcanics. Regional folding of the Brioverian rocks was accompanied by a prehnite-pumpellyite facies to low amphibolite facies metamorphism (Ryan, 1973;Hebert and Ballèvre, 1993).…”