Abstracts
Abstract
Although small in volume, Cambrian volcanic rocks associated with Acado-Baltlc sedimentary rocks In Newfoundland, New Brunswick, and Nova Scotia, Norway and Poland display characteristics indicating formation in a tensional tectonic setting. They generally form bimodal (basalt-rhyolite) suites. Mafic rocks are of both alkaline and tholeiitic affinities but all have withln-plate major element and trace element characteristics. The small volume and low frequency of eruption suggests that in most cases the volcanic rocks resulted from only small amounts of lithospherlc extension, and that rupturing and separation of plates may not have taken place. Volcanism over the Acado-Baltic province appears to have been less common in the Late Cambrian than in the Early and Middle Cambrian. This suggests that tension persisted throughout the Early and Middle Cambrian, but waned in the Upper Cambrian, possibly in direct or indirect response to processes which led to closing of the lapetus Ocean.
RESUME
Malgre un volume restraint, les volcanltes cambrlennes associees aux roches sedimentaires acado-baltlques a Terre-Neuve, au Nouveau-Brunswick ainsi qu'en Nouvelle-Ecosse, Norvege et Pologne montrent des caracteristiques qui suggerent leur formation en regime tectonlque de tension. Elles foment generalement des suites bimodales (basalte-rhyolite). Les roches mafiques ont des affinites tant alcalines que tholeiitiques bien qu*elles revetent un caractere intraplaque au plan des elements majeurs et en traces. Du volume restreint et de la faible frequence eruptive, on retire 1*impression que, pour la plupart, ces volcanltes ne rsultent que d'une extension lithospherlque de faible valeur et qu'aucune rupture ou separation des plaques n'a eu lieu. La province acado-baltique aurait connu un volcanisme mollis prononce a l'eo-Cambrlen et au Cambrlen moyen qu'au tardi-Cambrlen. Ceci semble indiquer que la tension a perdure de l'eo-Cambrlen au Cambrlen moyen et s'est. par la suite, attenuee au Cambrlen superieur, possiblement en reponse dlrecte ou indlrecte aux processus qui ont suscite la fermeture de 1*ocean lapetus.
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