Geochronology of the Baie Verte Peninsula, Newfoundland: Implications for the Tectonic Evolution of the Humber and Dunnage Zones of the Appalachian Orogen

Abstract

U-Pb analyses of zircon from the Burlington Granodiorite suggest intrusion at c. 460-465 Ma. Hornblende and biotite from northern border phases of the pluton record similar plateau dates suggesting rapid post-magmatic cooling. Hornblende and biotite from central portions of the pluton record markedly younger plateau dates (410-420 Ma) which are interpreted to date contact metamorphic effects associated with the widespread emplacement of Silurian-Devonian igneous suites. Northern portions of the Burlington Granodiorite are polydeformed and regionally metamorphosed. Hornblende and biotite from this terrane yield plateau ages of 345-350 Ma. U-Pb analyses of zircon from the Dunamagon Granite indicate emplacement at c. 440-460 Ma, thereby providing an upper limit for tectonic juxtapositioning of the Humber and Dunnage Zones along the Baie Verte Line. plateau ages of 345-350 Ma are recorded by biotite from the polydeformed and variably metamorphosed stock. Similar ages are also recorded by hornblende and biotite throughout northerly portions of the Mings Bight (Humber Zone) and Pacquet Harbour (Dunnage Zone) Groups. These results indicate that the tectonic evolution of the Baie Verte Line was poly genetic, and involved: (1) regionally significant tectonothermal activity prior to the Middle Ordovician, likely a result of the amalgamation of the Humber and Dunnage Zones following the obduction and westward transport of oceanic crust and mantle, which was previously rooted along the Baie Verte Line; and (2) Middle to Late Paleozoic (dissipating by 345-355 Ma) tectonothermal activity centered along easterly segments of the Baie Verte Line. The origins of the later tectonism are uncertain, but could, in part, have resulted from initiation of either a large-scale Variscan megashear system or through processes of lithospheric detachment and crustal shortening. The regional metamorphism associated with this orogenic activity altered primary U-Pb and Rb-Sr isotopic systems within various igneous suites exposed in northeastern portions of the Burlington Peninsula, which may explain some inconsistent geochronological results previously obtained.