UPDATE ON DETRITAL ZIRCON U/PB GEOCHRONOLOGY FROM PROTEROZOIC AND EARLY PALEOZOIC ROCKS OF THE PENOBSCOT BAY INLIER, MAINE

The pre-Silurian Penobscot Bay inlier, located between the Fredericton Trough and pre-Acadian Coastal Arc, hosts a remarkably complete Proterozoic–Ordovician Ganderian metasedimentary succession, including the Ellsworth terrane which is postulated to document Middle Cambrian departure of Ganderia from the Columbian segment of Amazonia. We report new LA-ICPMS U/Pb data from detrital zircon samples of the St. Croix and Islesboro belts, located respectively west and east of the Turtle Head Fault. Sample WPB-13, collected from a quartzite clast in a shear zone near the base of the Beauchamp Point Formation, has a maximum depositional age (MDA) of 1750 Ma, with a large peak at 2070 Ma and minor 1750-1850 Ma and late Archean grains. Sample IL-20-04, collected from the Seven Hundred Acre Island Formation of northern Lasell Island south of Islesboro, has a prominent peak at ca. 650 Ma (MDA), as well as a large peak at 2050 Ma and a few late Archean grains. Sample WPB-15, collected from the uppermost Ingraham grits of the Benner Hill Formation (Benner Hill Sequence) has a single large peak at ca. 480 Ma with smaller age populations between ca. 950 and 2050 Ma. The Beauchamp Point Formation U/Pb age spectrum matches those of samples considered to have West African provenance from the Islesboro belt, indicating that the Turtle Head Fault may not be a terrane boundary. The ca. 650 Ma grains in sample IL-20-04 may be locally derived from (deformed) pegmatite in the Seven Hundred Acre Island Formation or they may reflect delivery from similar age plutonic rocks elsewhere in Ganderia (e.g., Brookville terrane) or Gondwana. The U/Pb age spectrum from sample WPB-15 of the Benner Hill Formation supports both a fossil-based Late Ordovician depositional age and proximal source for late Cambrian–Early Ordovician zircon, likely the Penobscot arc. Continued work in progress from this region expects to flesh out an extraordinary geologic record, now well-exposed on Maine’s Atlantic margin, of Pan-African orogenesis during Gondwana assembly, followed by the elimination of Paleozoic oceans to form Pangea.