A SILURIAN AGE FOR THE PASSAMAQUODDY BAY VOLCANIC SEQUENCE IN SOUTHWESTERN NEW BRUNSWICK: IMPLICATIONS FOR REGIONAL CORRELATIONS

Siluro-Devonian bimodal volcanism is a common feature of the northern Appalachians comprising three main belts: the Coastal Volcanic Belt (CVB) of southern New Brunswick, Maine, and Massachusetts; and the Tobique and Piscataquis belts of northern New Brunswick and Maine, respectively. The CVB is a laterally extensive (500+ km exposed), and thick (4 km+ in the Passamaquoddy Bay area of southwestern New Brunswick) belt of bimodal volcanic rocks erupted in an extensional tectonic setting. The Passamaquoddy Bay (PB) sequence is at the northern most part of the CVB. The age and internal correlation of formations of the CVB has been the subject of controversy for over 30 years. However, the most researchers agreed that the CVB represents Late Silurian – Early Devonian volcanism, and correlated the PB sequence with the Eastport Formation in Maine. We have determined U-Pb ziron ages by the laser ablation ICPMS (LAM-ICPMS) technique for felsic tuffs and rhyolite flows that represent most of the stratigraphy of the PB sequence and part of the Eastport in Maine. These isotopic ages range from 419 +/- 6 Ma to 436 +/- 6 Ma, suggesting that volcanism of the PB sequence was ongoing for a minimum of about 10 Ma. The isotopic ages of the PB sequence are similar to other recent isotopic ages determined for the CVB such as the Cranberry Island Series (424 +/- 1 Ma), and a pluton of the South Penobscot Intrusive Suite (424 +/- 6 Ma), both in Maine. These isotopic ages for the PB sequences are older than fossil ages previously determined for the Eastport. Emerging isotopic ages for the CVB indicate that it is similar in age to the Tobique and Piscataquis belts. Together these belts represent voluminous volcanism related to continental extension during the Silurian development of the northern Appalachians.