- Cadomian Orogeny
The Cadomian Orogeny was a tectonic event or series of events in the late
Neoproterozoic , about 650-550 Ma, which probably included the formation of mountains. This occurred on the margin of theGondwana continent, involving one or more collisions ofisland arc s and accretion of other material at asubduction zone . The precise events, and geographical position, are uncertain, but are thought to involve theterrane s ofAvalonia , Armorica and Iberia. Rocks deformed in the orogeny are found in several areas ofEurope , including northernFrance , the EnglishMidlands , southernGermany ,Bohemia , southernPoland and the southwestIberian Peninsula . The name comes from "Cadomus", theLatin name forCaen , northern France. L Bertrand gave the orogeny its name in 1921, naming it after Cadomus theGaulish name forCaen inNormandy . [L Bertrand: "Les anciennes mers de la France et leur depots". 1921 Flammarion, Paris not consulted for this] He defined the end as being marked by Lower Palaeozoic red beds.The interpretation is that the belt was formed as oceanic crust subducted below the Armorica land mass in a similar way to the
Andes . Sediments deposited on thecontinental margin were pushed up onto the continent, at the same time as intrusions ofcalc-alkaline magmas occurred.Basement
The pre Cadomian basement rocks consist of
Orosirian orPaleoproterozoic IcartGneiss dated at close to Ma|2018. These are exposed on Guernsey and atCape La Hague inBrittany . This is termed the Icartian succession. NearTregor the gneiss in the Cadomian Perros-Guirec complex has been U-Pb dated to Ma|1790.Sark also has a gneiss basement. Vidal et al studied Strontium isotope ratios and concluded that the basement is not widely present below the Cadmonian 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. [R. S. D’Lemos, R. A. Strachan & C. G. Topley: [http://sp.lyellcollection.org/cgi/reprint/51/1/3 "The Cadomian orogeny in the North Armorican Massif: a brief review"] Geological Society, London, Special Publications; 1990; volume 51; page 3-12]Brioverian
Brioverian sequence was deposited during or before the
Ediacaran period Ma|750|540. 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 Briverian 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 Ma|690|500. Foliated quartz
diorite occur at Baie de St Brieuc, atCoutances , La Hague,Guernsey ,Alderney andSark . These are only roughly dated Ma|700|580. The last magmatism identified is the Jersey Dyke Swarm from Ma|425.Partial melting has resulted in
migmatite s. These form belts named afterSt Malo ,Dinan , andSt Cast in north east Brittany. The Rance valley has a metamorphic sequence ranging fromphyllite s,amphibolite ,metatexite s,diatexite s which have melted intogranite .Leucogranite s 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 andTregor region there arepillow lava s,basalt erupted under water. A later stage of acid volcanics in the form ofandesite andrhyolite have been erupted on Brioverian sediments on Jersey at Ma|533, and at St Germain-le-Gaillard in Lower Normandy. In the Tregor region there are units called "Tuffs deTreguier " and "Ignimbrites deLezardrieux ".The southern edge of the
Gulf of St Malo between Tregor andCancale shows the deformational structures of the Cadomian Orogeny. These are East-west to North East tending upright folding.Schist osity 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 atSt 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.
Post orogeny
After the orogeny, continental sediments were washed on top of the deformed Cadomian rocks. They include
conglomerate and sandstone in Brittany, Jersey and Alderney. The Cap Frehelred bed s are dated at Ma|472. These post orogeny deposits were dumped from braided stream channels. The basins for these sediments followed the structural grain in the Cadomian Orogeny.Models
There are two overall models for the formation of the Cadomian Orogeny.
Firstly by P. Bale & J. P. Brun that compression to the south west of a volcanic arc and back arc basin onto the edge of the continent at the St Malo and Mancellian regions Ma|590 formed the orogeny. Crustal thickening took place.
Secondly P. J. Treloar considers that the North Armorican Massif was put together from a series of terranes Ma|540, their joins are the shear zones. The terranes are known as St Brieuc, St Malo and Mancellian terranes. Deformation in this model is caused by oblique subduction and is strike-slip, but not crustal thickening. The migmatite belts are explained by high heat flow in marginal basins.
References
* [http://www.gsf.fi/publ/foregsatlas/article.php?id=4 "The Geological and Tectonic Framework of Europe"] , by J.A. Plant, A. Whittaker, A. Demetriades, B. De Vivo, and J. Lexa.
* [http://www.globalchange.umich.edu/Ben/Publications/gsab01.pdf Avalonia and the Cadomian belt]
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