U-PB GEOCHRONOLOGICAL CONSTRAINTS ON THE VOLCANIC EVOLUTION OF THE MIRA (AVALON) TERRANE, SOUTHEASTERN CAPE-BRETON ISLAND, NOVA-SCOTIA

New U-Pb ages for late Precambrian volcanic and associated plutonic un its in the Mira (Avalon) terrane of southeastern Cape Breton Island in dicate that volcanic suites were erupted over a span of at least 100 M a. The oldest dated rock is a quartz-feldspar rhyodacitic porphyry fro m the unit that hosts the Mindamar Zn-Pb-Cu-Ag-Au deposit in the Stirl ing belt, which has an age of 681(-2)+6 Ma. The most widespread volcan ism and plutonism occurred at ca. 620 Ma in the East Bay Hills and Cox heath Hills belts, and probably the Sporting Mountain belt, as indicat ed by U - Pb ages and U - Pb maximum ages for rhyolite flows and U-Pb and Ar-Ar ages of crosscutting plutons, as well as stratigraphic const raints. Younger volcanic rocks occur in the Coastal belt, from which a rhyodacitic crystal tuff is dated at 575(-1)+4 Ma and a pluton is dat ed at 574 +/- 3 Ma. A rhyolite flow from the contiguous Main-a-Dieu se quence yields a maximum age of ca. 563 Ma, and a minimum age for this sequence is indicated by overlying latest Precambrian to Cambrian foss iliferous sedimentary rocks. Middle Devonian plutonism in the Mira ter rane is confirmed by an age of 378(-1)+5 Ma from the Lower St. Esprit granodiorite in the Coastal belt. The range of ages of volcanic and pl utonic rocks in Mira terrane is similar to that in other parts of Aval on terrane in eastern Newfoundland and southern New Brunswick. Many of the dated rocks contain xenocrystic zircons of Middle Proterozoic age s which suggest a South American source.