Cadomian orogeny

Cadomian orogeny
Name	Cadomian orogeny
Period	Neoproterozoic–Cambrian
Region	Armorican Massif; Anglo-Paris Basin; Iberian Massif; Brabant Massif
Countries	France; United Kingdom; Spain; Portugal; Belgium; Germany
Coordinates	48°N 2°W
Orogeny type	Collisional and accretionary orogen
Orogenic belt	Variscan foreland precursor
Age	~650–540 Ma
Named for	Cadomian (Cadomian refers to Cadomian crustal domain)

Cadomian orogeny The Cadomian orogeny was a late Neoproterozoic to early Cambrian mountain-building event that shaped a series of terranes now exposed across western Europe, producing basement units in regions such as the Armorican Massif, Avalonia, and parts of the Iberian Peninsula. It involved subduction, arc accretion, continental collision, and widespread magmatism that preconditioned the lithosphere for later Paleozoic events including the Variscan orogeny and influenced sedimentary basins like the Rheic Ocean margins. Studies of its rocks integrate data from field mapping, geochronology, and isotopic systems developed in laboratories associated with institutions such as the British Geological Survey and the CNRS.

Overview and Geologic Setting

The orogeny affected crustal blocks now correlated with the Armorican Block, Brabant Massif, and terranes of Avalonia and the Moldanubian Zone, creating sutures adjacent to the former Rheic Ocean and proto‑Atlantic margins near the Iapetus Ocean. Regional exposure includes the Massif Central, Cornubian Batholith periphery, and the Galician‑Trás-os-Montes Zone; relationships are constrained by paleomagnetic studies from teams at the University of Cambridge and the Université de Rennes. Geological mapping by the Geological Survey of Finland and the Geological Survey of Portugal has refined the distribution of Cadomian terranes and their juxtaposition against the Avalon Zone and the Armorican Terrane Assemblage.

Tectonic Evolution and Phases

Tectonic models invoke successive stages: intraoceanic arc formation, arc‑continent collision, terrane accretion, and late extensional collapse. Early arc formation is linked to subduction zones inferred from remnant ophiolites mapped by researchers at the Institute of Geological Sciences, UCL and from field work coordinated with the Geological Survey of Spain. Collision episodes correlate with isotopic ages from laboratories at the ETH Zurich and the University of Barcelona, while post‑orogenic extension and basin development are tied to rifting events preceding the opening of the Iapetus Ocean and involve comparisons to rifted margins studied by the Geological Survey of Canada.

Stratigraphy and Lithologies

Cadomian sequences include metasedimentary successions, volcanic‑arc assemblages, and syn‑orogenic granitoids preserved in units such as the Brioverian and equivalents across the Armorican Massif and the Minho Complex. Typical lithologies are turbiditic slates, greywackes, metabasalts, tonalites, and leucogranites documented by mapping projects from the British Geological Survey and the Instituto Geológico y Minero de España. Stratigraphic correlations use detrital zircon populations analyzed at facilities like the NERC Isotope Geoscience Laboratory and the Max Planck Institute for Chemistry to link units across the North Sea Basin and the Paris Basin.

Metamorphism and Magmatism

Regional metamorphism ranges from greenschist to amphibolite facies with localized high‑pressure assemblages; metamorphic gradients have been characterized by studies from the University of Lisbon and the Vrije Universiteit Brussel. Magmatism produced voluminous calc‑alkaline plutons, porphyry bodies, and syn‑kinematic granites with U‑Pb ages produced at the GEOTOP lab and the Lamont‑Doherty Earth Observatory. Isotopic signatures (Sr‑Nd‑Pb) link magmatism to both mantle‑derived sources and reworked Mesoproterozoic basement tied to crustal domains studied by the Geological Survey of Norway.

Structural Features and Deformation

Structures include major thrust systems, fold nappes, and steep shear zones that record transport directions and exhumation paths; key examples are preserved in the Armorican Massif and the Central Iberian Zone. Mapping and structural synthesis by teams from the University of Oxford and the Université de Lorraine emphasize imbricate thrust stacks, upright and overturned folds, and transcurrent shear zones tying Cadomian fabrics to later Variscan overprinting observed in the Bohemian Massif and the Massif des Central.

Timing, Paleogeography, and Correlation

High‑precision geochronology (LA‑ICP‑MS, SHRIMP, ID‑TIMS) yields ages clustering between ~650 Ma and ~540 Ma, allowing correlations with coeval events such as the Pan‑African orogeny and continental reorganizations involving Laurentia and Gondwana. Paleogeographic reconstructions by groups at the University of Edinburgh and the Potsdam Institute for Climate Impact Research place Cadomian terranes along the northern edge of Gondwana before dispersal and accretion to peri‑Gondwanan margins, with implications for sediment routing into basins like the Avalon Basin.

Economic Significance and Mineralization

Cadomian belts host mineralization including tin‑tungsten greisen systems, base‑metal veins, gold mineralization, and skarn deposits exploited in regions such as Cornwall, the Galicia region, and parts of the Iberian Pyrite Belt. Exploration projects by companies and surveys including the British Geological Survey and mining firms active in Portugal and Spain target greisenized leucogranites and VMS‑type targets; provenance studies using detrital zircon and heavy minerals from the University of Salamanca inform resource models.

Category:Geology