TECTONIC SETTING OF AVALONIAN VOLCANIC AND PLUTONIC ROCKS IN THE CALEDONIAN HIGHLANDS, SOUTHERN NEW-BRUNSWICK, CANADA

The Caledonian Highlands of southern New Brunswick consist of Late Pro terozoic to Cambrian rocks generally considered typical of the Avalon terrane of the northern Appalachian Orogen. Mainly tuffaceous volcanic and sedimentary rocks of the Broad River Group and cogenetic dioritic to granitic plutons with ages ca. 620 Ma form most of the eastern Cal edonian Highlands. They have petrological features indicative of origi n in a continental margin subduction zone. Significantly younger ca. 5 60-550 Ma dacitic to rhyolitic lapilli tuffs and flows, laminated tuff aceous siltstone, basaltic and rhyolitic flows, and elastic sedimentar y rocks of the Coldbrook Group form most of the western highlands, and occur locally throughout the highlands. The mainly tuffaceous lower p art of the group has been intruded by gabbroic and syenogranitic pluto ns that are interpreted to be cogenetic with basaltic and rhyolitic fl ows in the upper part of the group. This voluminous subaerial magmatis m may have formed during postorogenic extension in the earlier ca. 620 Ma subduction zone complex represented by the Broad River Group and a ssociated plutons. This tectono-magmatic model differs from other inte rpretations that related most of the igneous units to ca. 630-600 Ma s ubduction, and did not recognize the importance of ca. 560-550 Ma magm atism. The ca 620 Ma subduction-related volcanic and plutonic rocks of the Caledonian Highlands are comparable to units in other parts of th e Avalon terrane, but voluminous ca. 560-550 Ma igneous activity like that represented by the Coldbrook Group and related plutons has not be en documented yet in other Avalonian areas.