U-PB AND 40AR 39AR DATING IN THE AVALON TERRANE AT CAPE-MIQUELON (ST-PIERRE AND MIQUELON ARCHIPELAGO, FRANCE)

The islands of St. Pierre and Miquelon are part of the Avalon Zone, located in the northern Appalachian Orogen of North America. The geology of St. Pierre and Miquelon is similar to that of the Burin peninsula in Newfoundland; however the archipelago also includes an amphibolite facies metamorphic unit at Cape Miquelon. U-Pb and 40Ar/39Ar dating on plutonic units intruding the foliated metamorphic unit establishes the existence of a ca. 610-620 Ma magmatic event, significantly older than the ca. 580 Ma of the volcanic St. Pierre Group. The Cape Blanc granite and a related granite dyke yielded U-Pb zircon ages of ca. 617(613-622) and 616+5/-4 Ma respectively. A similar U-Pb zircon age of 620 +/- 3 Ma has been obtained on equivalent granitic pipes within the metasedimentary sequence at Anse de Miquelon. A dioritic intrusion at Anse a la Vierge has yielded a comparable 40Ar/39Ar hornblende cooling age (plateau) of 611+/-6 Ma and is interpreted to belong to the same plutonic suite, also represented in the Avalon Zone of Newfoundland and Maritime Canada. These data also indicate that the tectonometamorphic event at Cape Miquelon occurred, prior to or contemporaneous with, the 610-620 Ma plutonic episode.