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| Cadomian | |
|---|---|
| Name | Cadomian |
| Type | Orogeny |
| Period | Neoproterozoic–Ediacaran |
| Region | Northwestern Europe |
| Area | Armorican Massif, Normandy, Brittany, Iberian Massif |
Cadomian The Cadomian denotes a Neoproterozoic–Ediacaran orogenic and related sedimentary-magmatic episode affecting parts of northwestern Europe including the Armorican Massif, Iberian Massif, and Channel region. It records a complex interplay among terrane accretion, rifting, basin formation and closure, and polyphase metamorphism that links with regional events such as the Pan-African orogeny, Avalonian terrane accretion, and the opening of the Rheic Ocean.
The name derives from coastal toponymy associated with Caen and the historic province of Normandy as used in early 20th-century regional geology by researchers from institutions such as the British Geological Survey, the Centre national de la recherche scientifique, and the University of Rennes who synthesized mapping across the Armorican Massif, Brittany, and the Cotentin Peninsula. Subsequent usage in stratigraphic compilations by the International Commission on Stratigraphy, the Geological Society of London, and regional syntheses in the Geological Society of France stabilized the term in references discussing the Neoproterozoic evolution of the Avalon Zone, the Iberian Peninsula, and adjacent microcontinents.
Cadomian deformation and magmatism span broadly from about 650 to 540 million years ago, overlapping the Ediacaran and latest Cryogenian intervals recognized by the International Commission on Stratigraphy and constrained by radiometric dates from zircon U-Pb studies undertaken at institutions such as the Geological Survey of Norway and laboratories at ETH Zurich and the University of Cambridge. The orogen affected terranes interpreted as parts of the Avalonian-Cadomian realm between the margins of the Laurentia-adjacent microcontinents and the proto-Gondwana margins, with links to rift and back-arc processes documented in comparisons with the Variscan and Pan-African belts.
Cadomian successions include volcaniclastic sequences, turbidites, arkosic sandstones, and intraformational conglomerates intercalated with felsic to intermediate volcanic units; these are exposed in classic sections in the Armorican Massif, the Mise-en-scène of Brittany exposures, and the Lizard Complex equivalents on the Cornwall margin. Lithostratigraphic units correlate with regionally recognized formations such as the Schistes de la Hague-type slates, arkoses comparable to the Bray Group and volcanic suites akin to the Rousay Formation in the Orkney Islands when interpreted in paleogeographic syntheses. Sedimentary facies analyses and provenance studies employing heavy mineral and detrital zircon populations link those deposits to hinterland sources analogous to the Avalon Composite Terrane and the Armorican Massif basement exposures.
The tectonic evolution is reconstructed as an accretionary-collisional cycle involving subduction, arc formation, and terrane docking that ties into models of Avalonian terrane assembly and subsequent interaction with the proto-Gondwana margin; this has been discussed alongside plate reconstructions from groups at the Paleomap Project, the University of Toronto paleogeography group, and researchers publishing in journals affiliated with the Geological Society of America. Proposed geodynamic scenarios invoke continental rifting, back-arc extension, and eventual closure associated with collision-related shortening and basement exhumation, with regional comparisons to orogenic processes in the Caledonian and Variscan belts used to constrain timing and kinematics.
Cadomian magmatism comprises calc-alkaline to peraluminous granitoids, rhyolites, and associated mafic enclaves documented by geochemical and isotopic work from laboratories at Uppsala University, Université de Bretagne Occidentale, and RWTH Aachen University; these suites record arc to post-collisional affinities and have been characterized using whole-rock geochemistry, Sr-Nd-Hf isotopes, and U-Pb zircon geochronology. Metamorphism ranges from low- to medium-grade greenschist and amphibolite facies metamorphism with localized granulite-facies conditions in exhumed basement slices, featuring mineral assemblages and P-T paths reported in regional metamorphic studies coordinated through the European Geosciences Union and the International Geoscience Programme.
Fossil evidence within Cadomian successions is sparse but includes Ediacaran-type macrofossils and trace fossils comparable to assemblages reported from the Ediacara Hills, the White Sea region, and the Avalon Peninsula, as well as acritarchs and microfossils studied by micropaleontologists at the Natural History Museum, London and the Muséum national d'Histoire naturelle. Biostratigraphic correlations rely heavily on detrital zircon populations, chemostratigraphic signals such as carbon isotope excursions documented in comparative studies with sections in the Anti-Atlas and Sardinia, and regional paleontological constraints from Ediacaran-bearing sequences.
Cadomian-related mineralization includes hydrothermal base-metal and vein-hosted tin–tungsten–molybdenum occurrences associated with late- to post-orogenic granites exposed in Brittany and Cornwall, exploration targets historically investigated by entities such as the British Geological Survey and modern exploration companies regulated under the European Union mineral policy frameworks. The orogen also controls placer and lode occurrences of tin in the Cornubian Batholith-adjacent terrains and localized base-metal sulfide mineralization investigated in economic geology studies by universities including Aberystwyth University and Université de Rennes 1.
Category:Orogenies Category:Neoproterozoic geology