U-PB ZIRCON GEOCHRONOLOGY OF THE LAMOINE GRANITE, PENOBSCOT BAY INLIER, COASTAL MAINE, USA

In coastal Maine, USA, the pre-Silurian Penobscot Bay inlier hosts a well-exposed and relatively complete section of the peri-Gondwanan tectonic element known as Ganderia. The inlier, bounded northward by the Fredericton Trough, an intra-Ganderian Silurian suture, and southward by the pre-Acadian Coastal Arc, includes in its eastern part the mid-Cambrian Ellsworth terrane interpreted to record Ganderia’s departure from the Columbia segment of Amazonia prior to 500 Ma. The deformed Lamoine Granite, a thin south-dipping sill, shares the same northwest-vergent fabric as the enclosing Ellsworth Schist. It crops out along strike from the Goose Cove Rhyolite; similar bimodal volcanic rocks in the Ellsworth terrane display isoclinal, northwest-vergent folds and other structures related to crustal thickening in a major subhorizontal shear zone, locally steepened on Acadian folds. In order to constrain the ages of magmatism and deformation, we used ID-TIMS on zircons to obtain a crystallization age of 492 ± 1.7 Ma. Likely, the Lamoine correlates with Penobscot arc volcanic rocks in the St. Croix and Annidale belts, a subtle westward age progression suggesting rollback of an east-dipping slab. The main fabric clearly post-dates 492 Ma. It remains unclear whether 1) an early S₁ metamorphic fabric correlates with that observed in schist pebbles supposedly at the base of the 504 Ma Castine Volcanics; and 2) whether, in the absence of a local Penobscottian stitching pluton, the main deformation is pre-Middle Ordovician as in New Brunswick and Newfoundland, or post-Sandbian as in the northwesterly St. Croix part of the inlier.