| Mapping, structural, and petrologic studies, combined with U-Pb geochronology, show that Grand Manan Island is underlain by mainly fault-bounded Neoproterozoic to early Cambrian volcanic, sedimentary, and plutonic rock units which may correlate with those in the New River terrane on the mainland. The oldest rocks are black shale, siltstone, and quartzite (Thoroughfare Formation) and marble (Kent Island Formation), the latter represented only by blocks in the 611+/-3 Ma Three Islands Granite. The Thoroughfare Formation is in faulted with mafic to felsic flows and tuff, mudstone, and banded iron formation of the Ingalls Head Formation, from which rhyolitic samples yielded U-Pb ages of ca. 618 Ma. Intermediate to felsic volcanic rocks, siltstone, and arkose of the Long Island Bay Formation are interpreted on structural grounds to be younger than the Ingalls Head Formation; a minimum age is constrained by the ca. 547 Ma High Duck Island Granite. The Great Duck Island Conglomerate separates the Long Island Bay Formation from black shale, siltstone, quartzite, and minor conglomerate of the overlying Flagg Cove Formation, for which the ca. 535 Ma Stanley Brook Granite provides a minimum age. The mainly volcaniclastic Priest Cove Formation, from which a crystal tuff yielded a U-Pb age of 539+/-5 Ma, is interpreted to be a lateral facies equivalent of volcanic flows and breccia of the Ross Island Formation. Volcaniclastic rocks and wacke in southern Grand Manan Island and subaerial mafic to felsic volcanic and associated arkosic rocks on adjacent offshore islands may be younger than all of the above units, based on their less deformed character. Mafic and intermediate flows and tuff of the North Head Formation of uncertain age form the northern tip of Grand Manan Island. |
