Silurian-Early Devonian mafic rocks of the Piscataquis volcanic belt in northern Maine
DOI :
https://doi.org/10.4138/1858Résumé
The Silurian-Early Devonian eruptive rocks at selected sites in the Piscataquis volcanic belt in northern Maine are composed of mafic and felsic rocks with rare intermediate types. The mafic rocks are mainly basalts that are commonly strongly fractionated and have the characteristics of intraplate continental tholeiites. The mantle-normalized incompatible trace element patterns of the rocks display negative Nb-Ta anomalies that were probably inherited from sub-continental lithosphere modified by pre-Silurian subduction processes. However, they do not have Ti depletion typical of subduction-related magmas. On the other hand, the youngest (late Early Devonian) Edmunds Hill Formation is composed of calc-alkali andesites that exhibit features typical of arc-related magmas. The Silurian-Early Devonian volcanic rocks were primarily erupted in a within-plate extensional environment that terminated with limited subduction. This extensional environment is inferred to be associated with the sinistral accretion of the Avalon Composite Terrane that produced an oblique pull-apart rift in adjacent Laurentia. R᳧SUM᳧ Les roches rutives du Silurien et du Dvonien prcoce provenant de sites choisis dans la ceinture volcanique de Piscataquis du Maine septentrional sont composes de roches mafiques et felsiques avec de rares termes intermdiates. Les roches mafiques sont principalement des basaltes qui sont communment diffrencs et qui prsentent les caractristiques des tholiites continentales intraplaques. Les patrons d'lments traces normaliss par le manteau pour les roches mafiques montrent des anomalies ngatives en Nb et Ta qui furent probablement hrites de la lithosphere sous-continentale modifie par des processus de subduction pr-siluriens. Cependant, elles ne preentent pas d'appauvrissement en Ti typique des magmas relis ᳲ la subduction. Par ailleurs, la Formation d'Edmunds Hill, qui est la plus jeune (fin du Dvonien prcoce), est compose d'andsites calco-alcalines prsentant des caractristiques typiques des magmas relis aux arcs. Les roches volcaniques du Silurien et du Dvonien prcoce firent ruption dans un environnement intraplaque extensionnel qui se termina par une composante mineure de subduction. On deduit que cet environnement extensionnel est associ ᳲ l'accrtion senestre du terrain composite d'Avalon qui produisit un rift de dcrochcment oblique dans le terrain de Laurentia adjacent. [Traduit par le journal]Téléchargements
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