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| Avalon terrane | |
|---|---|
| Name | Avalon terrane |
| Region | Newfoundland, Nova Scotia, New Brunswick, Quebec, New England, British Isles |
| Type | Terrane |
| Period | Neoproterozoic–Paleozoic |
| Lithology | sandstone, siltstone, shale, volcanic rocks, conglomerate, carbonate |
| Notable structures | Hope Fault, Red River Fault, Cobequid–Chedabucto Fault |
| Notable fossils | Ediacaran biota, trilobites, archaeocyaths |
Avalon terrane is a composite Neoproterozoic to early Paleozoic microcontinent and accretionary terrane that crops out across eastern North America and parts of the British Isles. It is a key element in reconstructions of Gondwana, Avalonia (historically used for the terrane), and the assembly of Laurentia and Baltica, implicated in major orogenic events such as the Caledonian orogeny and the Acadian orogeny. Studies of the terrane integrate data from Newfoundland, Nova Scotia, New Brunswick, Massachusetts, Maine, England, Wales, and Ireland.
The terrane is defined as a coherent package of Neoproterozoic to Cambrian strata and associated igneous rocks separated by faults from surrounding bodies such as Laurentia and Gondwana-derived blocks. Principal exposures occur in the Avalon Zone (Newfoundland) and the Meguma terrane adjacency, bounded by major structures including the Hope Fault, Cobequid Fault System, and the inferred suture with Laurentian crust along the Red Indian Line. Mapping and geochronology from institutions like the Geological Survey of Canada and the United States Geological Survey refine its limits against adjacent units such as the Gander Zone, Meguma Zone, and the Central Maine terranes.
Interpretations invoke rifting from Rodinia during the Neoproterozoic, drift as an independent microcontinent within the Iapetus Ocean, and subsequent accretion to peri-Laurentian margins during the Paleozoic. Radiometric ages from U–Pb zircon studies, paleomagnetic data, and faunal provinciality link Avalon to peri-Gondwanan margins like Cadomian terrane fragments tied to the Armorican terrane and West African craton. Collisional events include interactions with the Ganderian terrane and docking during stages of the Caledonide belt formation and the later Variscan orogeny in correlated terranes of Europe.
Stratigraphic sequences feature Neoproterozoic siliciclastic successions such as the Conception Group, St. John's Group, and equivalents, with intercalated volcanic assemblages comparable to the Long Harbour Group and Brigus Formation. Overlying Cambrian–Ordovician shallow-marine carbonates and siliciclastics include trilobite-bearing units analogous to the Hicks Group and the Musquodoboit Group. Lithologies range from arkosic sandstone, siltstone, and shale to mafic to felsic volcanic rocks, with local carbonate buildups equivalent to Ediacaran–Cambrian platform deposits observed in Burgess Shale-age comparisons for facies, and correlations to the Stromatolite-bearing horizons in coeval terranes.
Biostratigraphic control derives from Ediacaran biota occurrences, early Cambrian trilobites, and small shelly fossils that constrain depositional ages. Notable fossil assemblages include Ediacaran taxa comparable to those in Mistaken Point, early Cambrian trilobites similar to taxa from Avalon province localities in England and Wales, and shelly fossils used to correlate with Siberian and Baltic successions. Geochronologic constraints rely on U–Pb zircon ages from tuffs and intrusive units, supplemented by isotope work (e.g., Hf isotopes, Nd isotopes) that tie magmatism and sedimentation to global Neoproterozoic events such as the Sturtian glaciation and the Gaskiers glaciation.
Metamorphic grades across the terrane vary from unmetamorphosed to low- to medium-grade greenschist and amphibolite facies assemblages recording burial and heating during docking and orogenesis. Structural fabrics include pervasive foliation, regional folds, thrust systems, and strike-slip faults such as the Cobequid–Chedabucto Fault System, recording deformational phases correlated with the Taconic orogeny, Acadian orogeny, and later tectonothermal events. Metamorphic mineral assemblages include chlorite, biotite, garnet, and staurolite in higher-grade lenses, and geochronologic resets have been documented by researchers at Memorial University of Newfoundland and Dalhousie University.
Boundary delineation uses fault mapping, detrital zircon provenance, paleomagnetic data, and faunal evidence to correlate Avalon exposures from Newfoundland through the Maritimes Basin into the New England Appalachians and across the Irish Sea to parts of Southwest England and southern Ireland. Correlative links are drawn to the Cadomian orogen assemblages of Brittany and the Armorican terrane, supporting models of peri-Gondwanan affinity prior to Iapetus opening. Debates persist about precise sutures with Ganderia and the timing of docking to Laurentia, with competing models advocated in literature from Harvard University, University of New Brunswick, and the British Geological Survey.
The terrane hosts mineralization styles including orogenic gold, volcanogenic massive sulfide (VMS) deposits, and associated base-metal occurrences, with historical and modern mining in locales such as Buchans, Gander, and the Meguma mining district. Placer and lode gold occurrences exploit structural traps along shear zones like the Cobequid Fault, while VMS prospects relate to Neoproterozoic volcanic belts analogous to deposits in Lisheen and Massive Sulfide districts of Ireland. Hydrocarbon potential in synrift and passive-margin sequences has been evaluated in the Maritimes Basin and offshore basins by companies including BHP and ExxonMobil in collaboration with national agencies.
Category:Terranes