Magmatic History of the Avalon Terrane of Southern New Brunswick, Canada, Based on U-Pb (Zircon) Geochronology

U-Pb (zircon) dates from volcanic and plutonic units confirm that igneous activity throughout the Caledonian Highlands of southern New Brunswick occurred in two main episodes: ca. 620 Ma and ca. 560-550 Ma. The ca. 620 Ma episode is represented by mainly metatuffaceous rocks of the Broad River Group and associated dioritic to granitic plutons that form most of the eastern highlands. Igneous rocks of similar age and type have been documented in Avalonian areas throughout the northern Appalachian Orogen and are interpreted generally to have formed during subduction at a continental margin. In contrast, the ca. 560-550 Ma igneous episode may be unique to the Caledonian Highlands. It is represented by tuffs and bimodal flows of the Coldbrook Group and co-genetic gabbroic and granitic plutons that occur mainly in the western highlands. This voluminous magmatism may have taken place in a volcanic arc and intra-arc extensional setting, perhaps like that represented by older but lithologically similar rocks of the ca. 575 Ma Fourchu and Main-à-Dieu groups in southeastern Cape Breton Island. It contrasts in petrological character with igneous activity of similar age in eastern Newfoundland, which produced alkaline to peralkaline rocks. Younger volcanic rocks of minor extent also occur in the Caledonian Highlands: (1) rhyodacitic flows with a maximum age of 479 ± 8 Ma forming a small dome in the Coldbrook Group in the west-central highlands, and (2) small areas in the southwestern highlands of flow-banded rhyolite with a maximum age of ca. 367 Ma. Although Paleozoic igneous rocks have been reported in other parts of the Avalon terrane, this is the first confirmation of their presence in the Caledonian Highlands.