DETRITAL ZIRCON AGES FROM CONGLOMERATE OF THE DOUGLAS ISLANDS, NARRAGUAGUS BAY, MAINE: A NEWLY RECOGNIZED NEOPROTEROZOIC-CAMBRIAN BASEMENT UNIT

A thick poorly dated unit of rounded clast-supported conglomerates occur in the Douglas Islands in outer Narraguagus Bay in Down East Maine. The conglomerates are weakly deformed, and cut by two sets of basaltic dikes, and intruded by Devonian granite. The conglomerates are generally organized and clast-supported with bedding defined by grain size variations. Clasts consist of 50% felsic volcanics, 10% mafic volcanics, 20% granitic rocks and 20% sedimentary rocks including quartzite (Gates, 1989), and is compositionally similar to the poorly exposed conglomerates of Addison 15 km to the east. Depositional age is unknown, but has long thought to be Silurian due to correlation to rhyolitic and sedimentary strata on nearby Flint Island (Gates, 1989), and correlation to the Silurian Oak Bay Fm in New Brunswick. We dated detrital zircon from two samples, and the maximum depositional age (MDA) suggests deposition of the conglomerate of Douglas Islands is latest Neoproterozoic-Cambrian. Our samples have an abundance of young grains that transition to the first young peak age, and thus picking the youngest population to constrain maximum depositional age (MDA) is complicated. A weighted mean age of the ten youngest grains is ~548-550 Ma but the youngest three grains give ages of ~539-543 Ma. We cannot rule out Silurian deposition (Gates and Cary, 1982), but the lack of younger zircon dates would be odd given the abundance of rhyolitic volcanics in nearby Silurian strata. About 70% of the zircons define two major populations: P1 at ~560 Ma, but the dominant population, P2, occurs at ~630 Ma, which is the same time as peak magmatic activity on Avalon. About 30% of the grains form smaller pre-Neoproterozoic populations at 1.1-1.2 Ga, 1.55 Ga, 1.75 Ga, and 2.05 Ga. Our nascent data set on zircon crystallinity assessed by Raman spectrometry in the pre-Neoproterozoic grains suggest source rocks had two distinct thermal histories: one at ~0.6 Ga and one much older. Our data could be interpreted to indicate that the Conglomerate of Douglas Islands is Neoproterozoic-Cambrian and is basement to Silurian volcanic section. The main zircon population is similar to magmatic events in the Caledonian Highlands of New Brunswick, and it may be allied with the Avalon terrane.