AR-40 AR-39 MINERAL AGES FROM THE SOUTHERN CAPE-BRETON HIGHLANDS AND CREIGNISH HILLS, CAPE-BRETON ISLAND, CANADA - EVIDENCE FOR A POLYPHASETECTONOTHERMAL EVOLUTION/

Ar-40/Ar-39 ages for hornblende and muscovite from plutons and metamor phic country rocks in the southern Cape Breton Highlands and Creignish Hills, together with previously published U-Pb, Rb-Sr and Ar-40/Ar-39 ages, document a complex tectonothermal history that included (1) epi sodes of plutonic activity at ca. 600, 565-560, 555, 500-490-(?470), a nd 410-395 Ma (all of which cooled through 500-400-degrees-C within ca . 20 m.y.); and (2) deformation in the late Precambrian (> 550 Ma) and the Late Silurian-Middle Devonian (415-380 Ma). The latter was associ ated with development of a positive flower-like structure during maint enance of medium-pressure, greenschist-amphibolite facies, regional me tamorphic conditions that may be traced northward along the length of the Cape Breton Highlands. it may be correlated with the Kingston Comp lex and associated mylonite zone exposed in the Avalon Composite Terra ne of southern New Brunswick. Northward it merges with a major high-gr ade, polydeformed belt within the Gander Zone of Newfoundland. This re gionally significant tectonothermal event probably developed as a resu lt of accretion of the Avalon Composite Terrane to Laurentia and is in ferred to have occurred synchronously with collision of eastern North America and Gondwana (South America).