The Cadomian Orogeny was a tectonic

Plate tectonics

Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

 event or series of events in the late Neoproterozoic

Neoproterozoic

The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...

, about 650-550 Ma, which probably included the formation of mountains. This occurred on the margin of the Gondwana

Gondwana

In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

 continent, involving one or more collisions of island arc

Island arc

An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....

s and accretion of other material at a subduction zone. The precise events, and geographical position, are uncertain, but are thought to involve the terrane

Terrane

A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...

s of Avalonia

Avalonia

Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, and the eastern coast of North America. It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States...

, Armorica

Armorican terrane

The Armorican terrane, Armorican terrane assemblage, or simply Armorica, refers to a microcontinent or group of continental fragments that rifted away from Gondwana towards the end of the Silurian and collided with Laurussia towards the end of the Carboniferous during the Variscan orogeny...

 and Iberia. Rocks deformed in the orogeny are found in several areas of Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, including northern France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

, the English Midlands

English Midlands

The Midlands, or the English Midlands, is the traditional name for the area comprising central England that broadly corresponds to the early medieval Kingdom of Mercia. It borders Southern England, Northern England, East Anglia and Wales. Its largest city is Birmingham, and it was an important...

, southern Germany

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, Bohemia

Bohemia

Bohemia is a historical region in central Europe, occupying the western two-thirds of the traditional Czech Lands. It is located in the contemporary Czech Republic with its capital in Prague...

, southern Poland

Poland

Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...

 and the southwest Iberian Peninsula

Iberian Peninsula

The Iberian Peninsula , sometimes called Iberia, is located in the extreme southwest of Europe and includes the modern-day sovereign states of Spain, Portugal and Andorra, as well as the British Overseas Territory of Gibraltar...

. The name comes from Cadomus, the Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 name for Caen

Caen

Caen is a commune in northwestern France. It is the prefecture of the Calvados department and the capital of the Basse-Normandie region. It is located inland from the English Channel....

, northern France. L Bertrand gave the orogeny its name in 1921, naming it after Cadomus the Gaulish name for Caen

Caen

Caen is a commune in northwestern France. It is the prefecture of the Calvados department and the capital of the Basse-Normandie region. It is located inland from the English Channel....

 in Normandy

Normandy

Normandy is a geographical region corresponding to the former Duchy of Normandy. It is in France.The continental territory covers 30,627 km² and forms the preponderant part of Normandy and roughly 5% of the territory of France. It is divided for administrative purposes into two régions:...

. 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

Andes

The Andes is the world's longest continental mountain range. It is a continual range of highlands along the western coast of South America. This range is about long, about to wide , and of an average height of about .Along its length, the Andes is split into several ranges, which are separated...

. Sediments deposited on the continental margin

Continental margin

The continental margin is the zone of the ocean floor that separates the thin oceanic crust from thick continental crust. Continental margins constitute about 28% of the oceanic area....

 were pushed up onto the continent, at the same time as intrusions of calc-alkaline

Calc-alkaline

The calc-alkaline magma series is one of two main magma series in igneous rocks, the other magma series being the tholeiitic. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it...

 magmas occurred.

Basement

The pre Cadomian basement rocks consist of Orosirian

Orosirian

The Orosirian is the third geologic period in the Paleoproterozoic Era and lasted from 2050 Mya to 1800 Mya . Instead of being based on stratigraphy, these dates are defined chronometrically....

 or Paleoproterozoic

Paleoproterozoic

The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...

 Icart Gneiss

Gneiss

Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...

 dated at close to . These are exposed on Guernsey

Geology of Guernsey

Guernsey has a geological history stretching further back into the past than most of Europe. The southern part is constructed of Icart Gneiss. The Icart Gneiss is an augen gneiss of granitic composition containing potassium feldspar. This was formed from a granite dated at using U-Pb dating on...

 and at Cape La Hague in Brittany

Brittany

Brittany is a cultural and administrative region in the north-west of France. Previously a kingdom and then a duchy, Brittany was united to the Kingdom of France in 1532 as a province. Brittany has also been referred to as Less, Lesser or Little Britain...

. This is termed the Icartian succession. Near Tregor

Trégor

Trégor is one of the nine provinces of Brittany, in its northwestern area. It comprises the western part of the department of Côtes-d'Armor and a small part of the northeast of Finistère, as far as the river Morlaix...

 the gneiss in the Cadomian Perros-Guirec complex has been U-Pb dated to . Sark

Sark

Sark is a small island in the Channel Islands in southwestern English Channel, off the French coast of Normandy. It is a royal fief, geographically located in the Channel Islands in the Bailiwick of Guernsey, with its own set of laws based on Norman law and its own parliament. It has a population...

 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.

Brioverian

Brioverian sequence was deposited during or before the Ediacaran

Ediacaran

The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...

 period . Most of these sediments are mudstone

Mudstone

Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...

, siltstone

Siltstone

Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...

 or sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

, but there are also some conglomerate

Conglomerate (geology)

A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...

, phtanites (quartzite

Quartzite

Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...

 cemented with lime) and contaminated limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

. 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 . Foliated quartz diorite

Diorite

Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...

 occur at Baie de St Brieuc, at Coutances

Coutances

Coutances is a commune in the Manche department in Normandy in north-western France.-History:Capital of the Unelli, a Gaulish tribe, the town took the name of Constantia in 298 during the reign of Roman emperor Constantius Chlorus...

, La Hague, Guernsey

Guernsey

Guernsey, officially the Bailiwick of Guernsey is a British Crown dependency in the English Channel off the coast of Normandy.The Bailiwick, as a governing entity, embraces not only all 10 parishes on the Island of Guernsey, but also the islands of Herm, Jethou, Burhou, and Lihou and their islet...

, Alderney

Alderney

Alderney is the most northerly of the Channel Islands. It is part of the Bailiwick of Guernsey, a British Crown dependency. It is long and wide. The area is , making it the third-largest island of the Channel Islands, and the second largest in the Bailiwick...

 and Sark

Sark

Sark is a small island in the Channel Islands in southwestern English Channel, off the French coast of Normandy. It is a royal fief, geographically located in the Channel Islands in the Bailiwick of Guernsey, with its own set of laws based on Norman law and its own parliament. It has a population...

. These are only roughly dated . The last magmatism identified is the Jersey Dyke Swarm

Geology of Jersey

The geology of Jersey is characterised by the Late Proterozoic Brioverian volcanics, the Cadomian Orogeny, and only small signs of later deposits from the Cambrian and Quaternary periods...

 from .

Partial melting has resulted in migmatite

Migmatite

Migmatite is a rock at the frontier between igneous and metamorphic rocks. They can also be known as diatexite.Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally...

s. These form belts named after St Malo, Dinan

Dinan

Dinan is a walled Breton town and a commune in the Côtes-d'Armor department in northwestern France.-Geography:Its geographical setting is exceptional. Instead of nestling on the valley floor like Morlaix, most urban development has been on the hillside, overlooking the river Rance...

, and St Cast in north east Brittany. The Rance valley has a metamorphic sequence ranging from phyllite

Phyllite

Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the...

s, amphibolite

Amphibolite

Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...

, metatexites, diatexites which have melted into granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

. Leucogranite

Leucogranite

In geology, leucogranites are amongst the youngest intrusions related to anatexis of continental crust anywhere in the world. Leucogranites are commonly found in deformed metapelitic/metagraywacke sequences that have been thrusted over basements during crustal thickening associated with continental...

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

Manche

Manche is a French department in Normandy named after La Manche , which is the French name for the English Channel.- History :Manche is one of the original 83 departments created during the French Revolution on March 4, 1790...

 and Tregor

Trégor

Trégor is one of the nine provinces of Brittany, in its northwestern area. It comprises the western part of the department of Côtes-d'Armor and a small part of the northeast of Finistère, as far as the river Morlaix...

 region there are pillow lava

Pillow lava

Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in...

s, basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

 erupted under water. A later stage of acid volcanics in the form of andesite

Andesite

Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...

 and rhyolite

Rhyolite

This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...

 have been erupted on Brioverian sediments on Jersey at , and at St Germain-le-Gaillard in Lower Normandy. In the Tregor region there are units called Tuffs de Treguier

Tréguier

Tréguier is a port town in the Côtes-d'Armor department in Brittany in northwestern France. It is the capital of the province of Trégor.-Geography:Tréguier is located 36 m. N.W. of Saint-Brieuc by road. The port is situated about 5½ m...

and Ignimbrites de Lezardrieux

Lézardrieux

Lézardrieux is a commune in the Côtes-d'Armor department in Bretagne in northwestern France.The village is situated near the mouth of the estuary of the Trieux river - the suspension bridge across the river at this point is a French national monument...

.

The southern edge of the Gulf of St Malo between Tregor and Cancale

Cancale

Cancale is a commune in the Ille-et-Vilaine department in Brittany in north-western France. It is known as the birthplace of Saint Jeanne Jugan.-Demographics:Inhabitants of Cancale are called Cancalais....

 shows the deformational structures of the Cadomian Orogeny. These are East-west to North East trending upright folding. Schist

Schist

The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...

osity is developed parallel to the axial plane of the folds. Prehnite-pumpellyite facies

Prehnite-pumpellyite facies

The prehnite-pumpellyite facies is a metamorphic facies typical of subseafloor alteration of the oceanic crust around mid-ocean ridge spreading centres....

 to mid-amphibolite facies have been produced by metamorphism

Metamorphism

Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...

. 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.

Post orogeny

After the orogeny, continental sediments were washed on top of the deformed Cadomian rocks. They include conglomerate

Conglomerate (geology)

A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...

 and sandstone in Brittany, Jersey and Alderney. The Cap Frehel red beds are dated at . 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 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 , 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.