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TheCadomian orogeny occurred in the LateNeoproterozoic-EarlyCambrian, 700-425 Ma.[1] There was a continuum between its post-orogenic processes and the opening of theRheic Ocean and the concomitant rifting ofAvalonia.
In 1921 L. Bertrand named this orogeny afterCadomus, theLatin version of theGaulish name forCaen inNormandy.[2] He defined it as the latePrecambrian orogeny which in the NorthernArmorican Massif resulted in the folding and uplift of the Brioveriansuccession (see below) prior to the deposition of the EarlyPalaeozoicred bedsequences.[1]
This orogeny occurred as a result of one or more collision between variousterranes and the northern margin ofGondwana. These terranes have since drifted from Gondwana and have beenaccreted toBaltica (the proto Europeantectonic plate). Therefore, their palaeogeographical position is uncertain. The investigations of this orogeny started with examinations of its remnants in various part ofEurope. More recently there also have been studies of this orogeny in locations in North Africa and also theArabian Peninsula,Jordan,Israel and thePersian Gulf as these areas were also affected by it.[3]
This collection of terranes is collectively referred to as peri-Gondwanan terranes because they were along the northern margin of Gondwana. Those which were accreted to Baltica in theVariscan orogeny (LateDevonian-Carboniferous) whenAfrica was approachingLaurussia are: Franconia and Thuringia (now inGermany) in theSaxothuringian Zone; Bohemia, which forms the Teplá-Barrandian and Brunovistulian zones of theBohemian Massif (Czech Republic, northernAustria, easternGermany, southern andPoland); Armorica (inFrance); theOssa-Morena and Central-Iberian zones of theIberian Massif in theIberian Peninsula.[4]
The Cadomian orogeny started with a southwardoblique subduction beneath the northern margin of Gondwana that began ca. 760 Ma. It involved the development of an Andean-typevolcanic arc ca. 650-600 Ma and the formation of aback-arc basin behind this arc which opened ca. 590 to 560 Ma and was closed ca. 545-540 Ma. A short-lived retro-arc basin formed c. 543 Ma. Theslab of the subducting plate broke off.Rift basins formed in the Early to MiddleCambrian (c. 530–500 Ma). This continued until the opening of the opening of the Rheic Ocean.[5][6]
The collision was anisland arc-continent type and was a two-phase one due to the subduction being oblique. It first started in the eastern part of Peri-Gondwana at c. 560–570 Ma. There was a continuum between the opening of the rift basins and the opening of the Rheic Ocean. This was a two-stage process, too. It started from the west of Peri-Gondwana and expanded eastward.[6]
TheArmorican Massif pre Cadomian basement rocks consist ofOrosirian orPaleoproterozoic Icartgneiss dated at close to2,018 million years ago. These are exposed onGuernsey and atCape La Hague inNormandy. This is termed the Icartian succession. NearTrégor the gneiss in the Cadomian Perros-Guirec complex has been U-Pb dated to1,790 million years ago.Sark also has a gneiss basement. Vidal et al. studied Strontium isotope ratios and concluded that the basement is not widely present below the Cadomian orogeny, and Rabu et al. argued that the existing surface gneiss are fragments detached from the African craton. However it is possible that a basement of gneiss exists at depth below Normandy and Brittany. This gneiss was formerly known as Pentevrian. However it is now known that the Pentevrian type rocks do not predate the Cadomian orogeny, and this name is no longer appropriate for the Icart Gneiss.[7]
The Brioverianseries is a thicksiliciclasticsuccession deposited during the Cadomian cycle found in the Armorican Massif area inBrittany andNormandy (NWFrance) and in theChannel Islands.
The Brioverian sequence of rocks was deposited during or before theEdiacaran period750 to 540 million years ago. Most of these sediments aremudstone,siltstone orsandstone, but there are also someconglomerate, phtanites (quartzite cemented with lime) and contaminatedlimestone. These seem to be deposited in asubmarine 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 as690 to 500 million years ago. Foliated quartzdiorite occur at Baie de St Brieuc, atCoutances, La Hague,Guernsey,Alderney andSark. These are only roughly dated700 to 580 million years ago. The last magmatism identified is theJersey Dyke Swarm from425 million years ago.
Partial melting has resulted inmigmatites. These form belts named afterSaint-Malo,Dinan, and St Cast in northeast Brittany. The Rance valley has a metamorphic sequence includingphyllites,amphibolite, metatexites, anddiatexites which have melted intogranite.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 inManche andTrégor region there arepillow lavas,basalt erupted under water. A later stage of acid volcanics in the form ofandesite andrhyolite have been erupted on Brioverian sediments on Jersey at533 million years ago, and at St Germain-le-Gaillard in Lower Normandy. In the Tregor region there are units calledTuffs deTréguier andIgnimbrites deLézardrieux.
The southern edge of theGulf of Saint-Malo between Tregor andCancale 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 bymetamorphism. Movement was concentrated in shear belts such as at St Cast. The movement on the belts was sinistral and horizontal.
During the orogeny themigmatite belts of north east Brittany were formed. Baie de St Brieuc and St Malo migmatites were deformed with the typical structure.
After the orogeny, continental sediments were washed on top of the deformed Cadomian rocks. They includeconglomerate and sandstone in Brittany, Jersey and Alderney. The Cap Frehelred beds are dated at472 million years ago. These post orogeny deposits were dumped from braided stream channels. The basins for these sediments followed the structural grain in the Cadomian Orogeny.
There are two overall models for the features of the Cadomian Orogeny in the Armorican Massif.
In the first one Brun & Bale (1990) propose obliquethrusting SW-ward along an E-NE-trending plate boundary. Three main tectonic units, the Trégor volcanic and plutonic belt, the Guingamp-St Malo high temperature belt and the Baie de St Brieuc metamorphic-thrust were thrust over each other SW-ward 590–580 Ma.[8]
In the second one, Treloar and Strachan (1990) consider that the North Armorican Massif was put together from a series of terranes540 million years ago. Theirsutures are theshear zones. The terranes are the St Brieuc, St Malo and Mancellian terranes. Deformation in this model was s caused by oblique subduction and isstrike-slip. There was no crustal thickening. Themigmatite belts would then be explained by high heat flow in marginal basins.[9]
