PRELIMINARY GEOCHRONOLOGICAL DATA FROM ST -PIERRE AND MIQUELON ISLANDS

The islands of St. Pierre and Miquelon comprise part of the Avalon Com posite Terrane of the Appalachian orogen of North America. They are co mposed of several units including a plutonic metamorphic group (Cap de Miquelon Group) and a mainly ignimbritic volcanic group (St. Pierre G roup). These two units have been dated by U-Pb and Pb-Pb methods on zi rcon and are clearly temporally distinct. The Cap de Miquelon Group co nsists of a clastic sedimentary sequence that was intruded and contact metamorphosed by granitoid plutons dated at 615 +/- 14 Ma. The St. Pi erre Group is dominated by rhyolitic flows and ignimbrites; rhyolotic flows are dated at 581 +/- 19 and 584 +/- 12 Ma. The age data indicate that the Cap de Miquelon Group should be considered part of the ca. 6 30-550 Ma Avalonian orogenic cycle, rather than its crystalline baseme nt. Although the Cap de Miquelon plutonic rocks were produced prior to the St. Pierre volcanic rocks, they are coeval with well-knwon Avalon ian volcanic sequences of Marystown and Harbour Main Groups of souther n Newfoundland. The St. Pierre Group may be the temporal equivalent of a ca. 580 Ma dykes that intrude the Harbour Main Group volcanic rocks . Similar-aged magmatic events with similar characteristics have also been described in Nova Scotia (south coast of Cape Breton Island) and in southern New-Brunswick, suggesting that St. Pierre and Miquelon mag matism may represent part of making of regional tectonothermal events within the Avalon Composite Terrane.