IMPLICATIONS OF SILURIAN U-PB AGES OF STRATIFIED ROCKS ON VINALHAVEN AND NORTH HAVEN ISLANDS, PENOBSCOT BAY, MAINE

Volcanic rocks intruded by the Vinalhaven pluton are interbedded with the fossiliferous strata of the Ames Knob Formation and the Seal Cove Formation. The Ames Knob Formation contains a diverse faunal assemblage interpreted as Wenlockian to Lockhovian in age (Berry & Boucot, 1970; Gates, 2001), whereas the stratigraphically higher Seal Cove Formation contains poorly-preserved ostracodes, corals and brachiopods interpreted to be Lockhovian in age (Brookins et al., 1974). However, a new U-Pb age for the Vinalhaven pluton (420.3 ± 0.5 Ma) apparently conflicts with the interpreted fossil ages. This apparent conflict is important to resolve because the fossil age of the Ames Knob Formation has been used to correlate volcanic units throughout the Coastal Volcanic Belt (CVB) of Maine and New Brunswick.

To investigate this temporal discrepancy, we determined U-Pb zircon ages for silicic volcanic units that stratigraphically bracket the Ames Knob Formation and the overlying Seal Cove Formation. The oldest silicic volcanic rock in the sequence occurs in the Polly Cove Formation, which is correlative with the Ames Knob Formation (Gates, 2001). This lithic-poor welded tuff yields a U-Pb zircon date of 426 ± 1 Ma, indicating the Ames Knob fossil assemblages are of Wenlockian age. A lithic-poor welded tuff from the Vinalhaven rhyolite (stratigraphically higher than the Seal Cove Formation) yielded zircon crystals with complex growth histories, and our initial attempts to date the rock have produced a preliminary magmatic age. The zircon crystals appear to be binary mixtures of two growth domains: early Cambrian (ca. 540 Ma) cores and 420 ± 2 Ma magmatic mantles. The age of both the Ames Knob Formation and the Seal Cove Formation are no younger than the Pridolian.

We suggest three first-order implications. First, the range of the brachiopod genus Quadrifarius, described from a number of localities in the CVB (Berry & Boucot, 1970), should be extended to Ludlovian time. Second, the strata on Vinalhaven are contemporaneous with the Cranberry Island volcanic series exposed on and around Mt. Desert Island, Maine (Seaman et al., 1995) and the Eastport volcanics (Gates & Moench, 1981). Finally, because the volcanic strata were folded prior to intrusion of the pluton, the timing of deformation in this area is tightly constrained to about 420 Ma.