SILURIAN U-PB ZIRCON DATES FROM THE VINALHAVEN INTRUSION AND ASSOCIATED VOLCANIC ROCKS, PENOBSCOT BAY, MAINE
The volcanic sequence in the Vinalhaven area is interbedded with fossiliferous sedimentary rocks interpreted as Late Silurian to Early Devonian in age. However, these rocks are intruded by the Vinalhaven pluton which yields a U-Pb zircon date of 420 ± 1 Ma, a Ludlovian age that is not consistent with ages interpreted from the fossiliferous units; specifically the Ames Knob Formation and the Seal Cove Formation. The Ames Knob Formation contains a diverse faunal assemblage interpreted as Wenlockian to Lockhovian (Berry & Boucot, 1970; Gates, 2001), whereas the stratigraphically higher Seal Cove Formation contains poorly-preserved ostracodes interpreted to be Lockhovian (Brookins et al., 1974).
To investigate this temporal discrepancy, we determined U-Pb zircon dates for silicic volcanic rocks from Vinalhaven that stratigraphically bracket the fossiliferous units. A lithic-poor vitrophyre collected near the base of the section in the Polly Cove Formation (correlative to the Ames Knob Formation) yields a preliminary U-Pb zircon date of 425 ± 2 Ma. A lithic-poor welded tuff from the Vinalhaven rhyolite (stratigraphically higher than the Seal Cove Formation) yields a preliminary U-Pb zircon date of 421 ± 2 Ma. These dates are consistent with both the local field relationships and with published U-Pb ages of the Cadillac Mountain granite and the Cranberry Island volcanic series exposed on and around Mt. Desert Island, Maine (Seaman et al., 1995). Our initial results indicate that the Vinalhaven/North Haven layered sequence from the basal Ames Knob Formation to the upper Vinalhaven rhyolite was deposited/erupted (and folded) prior to ca. 420 Ma. These rocks and the fossils they contain are Silurian. If reevaluation of the fossil assemblages does not resolve the age discrepancy, then perhaps the Silurian time scale requires further calibration.