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 igne ous activity throughout the Caledonian Highlands of southern New Bruns wick 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 t he Broad River Group and associated dioritic to granitic plutons that form most of the eastern highlands. Igneous rocks of similar age and t ype have been documented in Avalonian areas throughout the northern Ap palachian Orogen and are interpreted generally to have formed during s ubduction at a continental margin. In contrast, the ca. 560-550 Ma ign eous episode may be unique to the Caledonian Highlands. It is represen ted by tuffs and bimodal flows of the Coldbrook Group and co-genetic g abbroic and granitic plutons that occur mainly in the western highland s. This voluminous magmatism may have taken place in a volcanic arc an d intra-arc extensional setting, perhaps like that represented by olde r but lithologically similar rocks of the ca. 575 Ma Fourchu and Main- a-Dieu groups in southeastern Cape Breton Island. It contrasts in petr ological character with igneous activity of similar age in eastern New foundland, which produced alkaline to peralkaline rocks. Younger volca nic rocks of minor extent also occur in the Caledonian Highlands: (1) rhyodacitic flows with a maximum age of 479 +/- 8 Ma forming a small d ome in the Coldbrook Group in the west-central highlands, and (2) smal l areas in the southwestern highlands of flow-banded rhyolite with a m aximum age of ca. 367 Ma. Although Paleozoic igneous rocks have been r eported in other parts of the Avalon terrane, this is the first confir mation of their presence in the Caledonian Highlands.