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 units in the Mira (Avalon) terrane of southeastern Cape Breton Island indicate that volcanic suites were erupted over a span of at least 100 Ma. The oldest dated rock is a quartz-feldspar rhyodacitic porphyry from the unit that hosts the Mindamar Zn-Pb-Cu-Ag-Au deposit in the Stirling belt, which has an age of 681(-2)+6 Ma. The most widespread volcanism and plutonism occurred at ca. 620 Ma in the East Bay Hills and Coxheath Hills belts, and probably the Sporting Mountain belt, as indicated 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 constraints. 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 dated at 574 +/- 3 Ma. A rhyolite flow from the contiguous Main-a-Dieu sequence yields a maximum age of ca. 563 Ma, and a minimum age for this sequence is indicated by overlying latest Precambrian to Cambrian fossiliferous sedimentary rocks. Middle Devonian plutonism in the Mira terrane 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 plutonic rocks in Mira terrane is similar to that in other parts of Avalon terrane in eastern Newfoundland and southern New Brunswick. Many of the dated rocks contain xenocrystic zircons of Middle Proterozoic ages which suggest a South American source.