Magmatisme ordovicien dans les hautes terres d’Antigonish, Nouvelle-Écosse, Canada: modèle tectonique
DOI :
https://doi.org/10.4138/atlgeo.2025.001Résumé
Les hautes terres d’Antigonish font partie d’Avalonia dans la section continentale de la Nouvelle-Écosse et elles reposent principalement sur des roches volcaniques et sédimentaires d’arc du Néoprotérozoïque, à faible teneur, d’environ 620 à 600 MA ainsi que sur des plutons du même âge. Les hautes terres préservent par ailleurs des traces du magmatisme qui a duré la majeure partie de l’Ordovicien (environ 495 à 455 Ma), période pendant laquelle Avalonia s’est détaché de la marge septentrionale de Gondwana pour migrer sous les traits d’un microcontinent sur environ 2 000 km vers le nord avant d’entrer en collision avec Baltica et Laurentia au cours du Silurien jusqu’au Dévonien. La longévité du magmatisme de l’Ordovicien (environ 50 Ma) correspond à un environnement de subduction, un milieu compatible avec la majorité des reconstitutions paléogéographiques. L’affinité intraplaque tholéiitique-alcaline continentale des roches mafiques et de la signature de type A des roches felsiques est toutefois plus caractéristique d’un milieu d’arrière-arc que d’un arc typique. Les roches felsiques de type A proviennent en outre d’une croûte plus basse et plus sèche que celle caractéristique des magmas d’arc felsiques.
Les données isotopiques du Sm-Nd sur roche totale des compositions mafiques et felsiques se situent à l’intérieur des enveloppes strictement restreintes précédemment délimitées qui définissent, respectivement, l’évolution du manteau avalonien et lithosphérique subcontinental (MLS), ainsi que des sources crustales. Ces données laissent supposer que (i) la croûte est demeurée juxtaposée au MLS depuis le détachement d’Avalonia de Gondwana à son accrétion à Baltica au cours du Silurien, puis à Laurentia au cours du Dévonien précoce, et que (ii) les hautes terres d’Antigonish étaient éloignées de la ou des zones de subduction qui ont refermé l’océan Iapetus durant la migration vers le nord, de sorte qu’elles ont été peu affectées par les collisions consécutives.
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© J. Brendan Murphy, Donnelly B. Archibald, R. Damian Nance, John W.F. Waldron 2025

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