New detrital zircon ages from multiple accreted terranes in south-central Maine: Evidence for varying source regions in pre-Silurian time

The metasedimentary rocks of the Central Maine, Casco Bay, and Fredericton 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 (i.e., Benner Hill and Jam Brook formations) in south-central Maine.

Samples targeted for detrital zircon separation were collected across a ~40 km long, perpendicular-to-strike transect west of Penobscot Bay, Maine. Along this transect, metasedimentary rocks were collected 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, and (5) the Ordovician Benner Hill sequence (Hart Neck Fm.).

Crystallization ages were determined for detrital zircon grain populations from seven samples using UPb isotope ratios gathered by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) at Middlebury College (n = 62–164). Results vary considerably amongst the samples with some showing strong peri-Gondwanan source region signatures (i.e., ~600 Ma peaks), while others do not. The youngest detrital zircon ages from each sample provide maximum depositional ages for the respective units ranging from Neoproterozoic to Late Ordovician.

The considerable variability in detrital zircon age distributions from the different analyzed terranes strongly suggests a complex pre-Acadian tectonic evolution in and around the Iapetus and Rheic Ocean basins. Additionally, though the sampled terranes are currently juxtaposed in close proximity, variable contributions of inferred West African craton, peri-Gondwanan, and Cambro-Ordovician source regions indicate they originally formed in very different settings.