A REVISED INTERPRETATION OF CAMBRIAN AND ORDOVICIAN ROCKS IN THE BOURINOT BELT OF CENTRAL CAPE-BRETON ISLAND, NOVA-SCOTIA

New U-Pb (zircon) data from the northern Boisdale Hills show that rhyo lite on Long Island and syenogranite of the nearby Mount Cameron pluto n have similar ages of 505 +/- 3 Ma and 509 +/- 2 Ma, respectively. Th ese ages are similar to those known or inferred from some felsic pluto nic units elsewhere in central Cape Breton Island and suggest that lat est Cambrian to earliest Ordovician igneous activity was widespread in the Bras d'Or terrane. The new dates also suggest that stratigraphic and structural relationships in the Boisdale Hills are more complex th an previously interpreted. The dated rhyolite and other volcanic and s edimentary rocks in the northern Bourinot belt in the Boisdale Hills w ere previously included in undivided Middle Cambrian Bourinot Group. I n the southern Bourinot belt, the Bourinot Group was subdivided into t he Eskasoni, Dugald, and Gregwa formations. The Eskasoni Formation is dominantly a bimodal volcanic suite with petrological characteristics indicative of origin in a continental within-plate tectonic setting. I ts present contacts, both with adjacent older metamorphic and plutonic rocks and with the apparently overlying fossiliferous Dugald and volc anogenic Gregwa formations, are faulted. Our new mapping demonstrates that the Eskasoni, Dugald and Gregwa formations can be extended into t he central Bourinot belt where they were previously undivided. However , continuity cannot be demonstrated between these Middle Cambrian unit s and the dated volcanic and associated sedimentary units in the north ern Bourinot belt. Hence, assuming that both U-Pb and fossil ages are correct, our interpretation is that the Upper Cambrian Lower Ordovicia n Northern Boisdale Hills volcanic unit is younger than the Bourinot G roup, although petrochemical data suggest that it formed in a similar tectonic regime. The presence in the Bourinot belt of fauna characteri stic of the Acado-Baltic faunal province appears to tie the Bras d'Or terrane to other Avalonian (peri-Gondwanan) terranes. However, the Bra s d'Or terrane differs from the adjacent Mira terrane which includes L ower as well as Middle and Upper Cambrian units and lacks volcanic and plutonic rocks of this age.