Abstracts
Abstract
This field trip is an excursion through the exquisite, nearly pristine exposures of a Silurian, felsic-dominated bimodal volcanic and sedimentary sequence exposed in the Passamaquoddy Bay area of southwestern, New Brunswick (Eastport Formation). These rocks form the northwest extension of the Coastal Volcanic Belt that extends from southwestern New Brunswick to the southern coast of Maine. The sequence is significant because it is part of a large bimodal igneous province with evidence for supervolcano-scale eruptions that began to form during the close of the Salinic Orogeny (about 424 Ma), and continued into the Acadian Orogeny (421–400 Ma). The geochemical characteristic of the rocks can be explained by extension related volcanism but the specific drivers of the extension are uncertain. The Passamaquoddy Bay sequence is 4 km thick and comprises four cycles of basaltic-rhyolitic volcanism. Basaltic volcanism typically precedes rhyolitic volcanism in Cycles 1–3. Cycle 4 represents the waning stages of volcanism and is dominated by peritidal sediments and basaltic volcanics. A spectrum of eruptive and emplacement mechanisms is represented ranging from the Hawaiian and Strombolian-type volcanism of the basaltic flows and pyroclastic scoria deposits, to highly explosive sub-Plinian to Plinian rhyolitic pyroclastic eruptions forming pyroclastic density currents (PDC) and high grade rheomorphic ignimbrites. During this field trip we will examine key exposures illustrating this spectrum of eruptive and emplacement processes, and their diagnostic characteristics, along with evidence for the interaction between mafic and felsic magmas and a variety of peperitic breccias formed as a result of emplacement of flows on wet peritidal sediments. The constraints the depositional setting and voluminous bimodal volcanism places on tectonic models will also be considered.
Keywords:
- Bimodal volcanism,
- Coastal Volcanic Belt,
- Late Silurian,
- New Brunswick
Résumé
Cette sortie sur le terrain est une excursion à travers les magnifiques affleurements pratiquement non altérés d'une séquence volcanique et sédimentaire bimodale silurienne à dominance felsique exposée dans la région de la baie de Passamaquoddy, au sud-ouest du Nouveau-Brunswick (Formation d'Eastport). Ces roches forment le prolongement nord-ouest de la Ceinture volcanique côtière qui s'étend du sud-ouest du Nouveau-Brunswick à la côte sud du Maine. La séquence est importante car elle fait partie d'une grande province ignée bimodale comprenant des preuves de super éruptions volcaniques qui ont commencé à se former à la fin de l'orogenèse salinique (environ 424 Ma) et se sont poursuivies pendant l'orogenèse acadienne (421–400 Ma). La caractéristique géochimique des roches peut être expliquée par le volcanisme lié à l'extension, mais les facteurs spécifiques de l'extension sont incertains. La séquence de la baie de Passamaquoddy a une épaisseur de 4 km et comprend quatre cycles de volcanisme basaltique-rhyolitique. Le volcanisme basaltique précède généralement le volcanisme rhyolitique dans les cycles 1–3. Le cycle 4 représente les stades décroissants du volcanisme et est dominé par des sédiments péritidaux et des roches volcaniques basaltiques. Une variété de mécanismes éruptifs et de mises en place est représentée, allant du volcanisme de type hawaïen et strombolien des coulées basaltiques et des dépôts de scories pyroclastiques, aux éruptions pyroclastiques rhyolitiques hautement explosives sous-pliniennes à pliniennes formant des courants de densité pyroclastiques et des ignimbrites rhéomorphes à haute teneur. Au cours de cette visite sur le terrain, nous examinerons les affleurements clés illustrant cette gamme de processus éruptifs et de mises en place, et leurs caractéristiques diagnostiques, ainsi que les preuves de l'interaction entre les magmas mafiques et felsiques et une variété de brèches pépéritiques formées à la suite de la mise en place de coulées sur des sédiments péritidaux humides. Les contraintes que le contexte de dépôt et le vaste volcanisme bimodal imposent aux modèles tectoniques seront également examinées.
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Appendices
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