Résumés
Abstract
New U-Pb LA-ICP-MS ages and geochemical data from volcanic rocks in the southwestern part of the Miramichi terrane in Maine, USA, and the Eel River area, New Brunswick, Canada, indicate that calc-alkaline continental arc volcanism began in the earliest Ordovician (ca. 480 Ma) and continued into the Middle Ordovician (at least as late as ca. 463 Ma). The overlap of volcanic rock ages from the Greenfield (ca. 478–463 Ma) and Danforth, Maine (ca. 467–475 Ma), and Eel River, New Brunswick (ca. 480–468 Ma), segments of the terrane confirm previously uncertain correlations of the Olamon Stream (Greenfield), Stetson Mountain (Danforth), and Porten Road and Eel River (Eel River) formations in these areas. The youngest Middle Ordovician ages overlap those of the oldest units of the Tetagouche Group in northern New Brunswick which are attributed to crustal extension leading to opening of the Tetagouche back-arc basin.
Silicic and mafic Tetagouche Group rocks plot in within-plate fields on tectonic discrimination diagrams, distinctly different from the volcanic arc fields for the coeval silicic and mafic rocks in the two Maine segments and the Eel River segment in New Brunswick. Continental arc volcanism in the southwestern Miramichi terrane did not cease when arc extension/rifting leading to formation of the Tetagouche back-arc basin began at ca 470 Ma in New Brunswick, as proposed in a previous model that invoked a single migrating arc. Instead, southwestern Miramichi volcanism had extended from at least ca. 480 to 463 Ma—incompatible with short-lived Meductic-phase arc volcanism (ca. 476–472 Ma) proposed in that model.
Résumé
De nouvelles datations U-Pb par LA-ICP-MS et des données géochimiques provenant de roches volcaniques de la partie sud-ouest du terrane de Miramichi dans le Maine, aux États-Unis, et dans le secteur de la rivière Eel, au Nouveau-Brunswick, Canada, révèlent que le volcanisme d'arc continental calcoalcalin a commencé au tout début de l'Ordovicien (il y a environ 480 Ma) et s'est poursuivi jusqu'à l'Ordovicien moyen (au moins jusqu'à environ 463 Ma). Le chevauchement des âges des roches volcaniques des segments de Greenfield (environ 478 à 463 Ma) et Danforth, Maine, (environ 467 à 475 Ma) et de la rivière Eel, Nouveau-Brunswick, (environ 480 à 468 Ma) du terrane confirme des corrélations auparavant incertaines entre les formations d'Olamon Stream (Greenfield), de Stetson Mountain (Danforth), de Porten Road et d’Eel River dans ces secteurs. Les âges les plus récents de l'Ordovicien moyen chevauchent ceux des unités les plus anciennes du Groupe de Tetagouche dans le nord du Nouveau-Brunswick, attribuées à une extension crustale ayant conduit à l'ouverture du bassin arrière-arc de Tetagouche.
Les roches siliceuses et mafiques du Groupe de Tetagouche se situent sur les schémas de discrimination tectonique dans des champs intraplaques nettement différents des champs d'arc volcanique des roches siliceuses et mafiques contemporaines des deux segments du Maine et du segment de la rivière Eel au Nouveau-Brunswick. Le volcanisme d'arc continental dans le terrane sud-ouest de Miramichi ne s'est pas interrompu au début de l’extension / du rifting de l'arc ayant abouti à la formation du bassin arrière-arc de Tetagouche, vers 470 Ma, au Nouveau-Brunswick, comme l’avançait un modèle précédent invoquant un arc migratoire unique. Le volcanisme dans le sud-ouest de Miramichi s'est plutôt prolongé d'au moins 480 à 463 Ma, ce qui est incompatible avec le volcanisme d’arc de la phase de courte durée de Meductic (environ 476 à 472 Ma) avancé dans ce modèle.
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