DETRITAL ZIRCON GEOCHRONOLOGY OF STRATA FROM MULTIPLE ACCRETED TERRANES IN SOUTH-CENTRAL MAINE

The metasedimentary rocks of the Central Maine, Casco Bay, Fredericton, and St. Croix terranes in south-central Maine were accreted onto the Laurentian continent during the Silurian-Devonian Acadian orogeny. Metamorphism and deformation associated with this event erased primary features in the sedimentary protoliths of these terranes, leading to uncertainty about their pre-Acadian history. Presented here are findings from U-Pb geochronology carried out on detrital zircon grains sampled from these and other sub-units of uncertain origin (e.g., Benner Hill, Jam Brook and Clarry Hill formations) in a study area west of Penobscot Bay, Maine.

Samples targeted for detrital zircon separation were collected across an approximately 40 km long perpendicular to strike transect in south-central Maine. This transect involved the collection of metasedimentary rocks from each of the following lithotectonic belts, described from northwest to southeast: (1) The eastern margin of the Late Ordovician(?)-Silurian Central Maine belt (Hutchins Corner Fm.), (2) the Middle to Late Ordovician Casco Bay belt (Cape Elizabeth Fm.), (3) the Silurian Fredericton belt (Appleton Ridge Fm. and Ghent Phyllite), (4) the Jam Brook sequence of uncertain protolith age (Late Proterozoic-Ordovician?), (5) the Late Cambrian-Early Ordovician St. Croix belt (Megunticook Fm.), (6) the Clarry Hill Fm. of uncertain age (Late Cambrian-Early Ordovician?), and (7) the Ordovician Benner Hill sequence (Hart Neck Fm.). Efforts were made to sample the most quartz-rich meta-sedimentary lithologies within each of the units included in the study, but considerable variability was encountered given the wide range of rock units collected.

Deposition of the sampled terranes spans Cambrian through Early Silurian time and includes a prolonged period of tectonic evolution in and around the Iapetus and Rheic Oceans. Detrital zircon age distributions from the nine samples collected are used to infer maximum depositional age, sediment provenance, and tectonic setting of each terrane in an effort to better understand the nature and evolution of depositional environments within the pre-Acadian Iapetus and Rheic Oceans.