Cadomian Orogeny - Brioverian

Brioverian

Brioverian sequence was deposited during or before the Ediacaran period 750 to 540 million years ago. Most of these sediments are mudstone, siltstone or sandstone, but there are also some conglomerate, phtanites (quartzite cemented with lime) and contaminated limestone. These seem to be deposited in a submarine fan. There are also acid and basic volcanics interbedded. Some authors believe that there was an unconformity during the Brioverian succession, as there are phtanite stones in the upper parts believed to be eroded from the lower parts, but this is still inconclusive.

Cadomian magmatism has been dated in the intrusions and volcanics as 690 to 500 million years ago. Foliated quartz diorite occur at Baie de St Brieuc, at Coutances, La Hague, Guernsey, Alderney and Sark. These are only roughly dated 700 to 580 million years ago. The last magmatism identified is the Jersey Dyke Swarm from 425 million years ago.

Partial melting has resulted in migmatites. These form belts named after St Malo, Dinan, and St Cast in north east Brittany. The Rance valley has a metamorphic sequence ranging from phyllites, amphibolite, metatexites, diatexites which have melted into granite. Leucogranites and anatectic granites have been derived from sediments melting. East Brittany and Lower Normandy contains the Mancellian batholith which consists of intrusions of granodiorite and granite. These other plutons are derived from fractionally crystallized mantle melts.

At the Baie de St Brieuc and near Coutances in Manche and Tregor region there are pillow lavas, basalt erupted under water. A later stage of acid volcanics in the form of andesite and rhyolite have been erupted on Brioverian sediments on Jersey at 533 million years ago, and at St Germain-le-Gaillard in Lower Normandy. In the Tregor region there are units called Tuffs de Treguier and Ignimbrites de Lezardrieux.

The southern edge of the Gulf of Saint-Malo between Tregor and Cancale shows the deformational structures of the Cadomian Orogeny. These are East-west to North East trending upright folding. Schistosity is developed parallel to the axial plane of the folds. Prehnite-pumpellyite facies to mid-amphibolite facies have been produced by metamorphism. Movement was concentrated in shear belts such as at St Cast. The movement on the belts was sinistral and horizontal.

During the orogeny the migmatite belts of north east Brittany were formed. Baie de St Brieuc and St Malo migmatites were deformed with the typical structure.