GEOCHRONOLOGY AND "TERRANOLOGY": THE ON-GOING ROLE OF U-PB DATING IN UNRAVELLING TERRANES IN SOUTHERN NEW BRUNSWICK, CANADA

The traditional "Avalon zone" of southern New Brunswick (and elsewhere in the Appalachian orogen) is complicated by major faults and shear zones that juxtapose belts of rocks of contrasting character. The significance of these contrasts and hence the number of distinct tectonostratigraphic terranes in the area remain controversial. Detailed mapping and petrochemical studies have clarified the distribution and character of geological components within inferred terranes, but precise geochronology has been essential to place constraints on correlations of rock units, both within individual terranes and among terranes, and to interpret geological evolution of the region.

One of the most important constraints on terrane history in southern New Brunswick is the recognition that the "Kingston complex" of earlier workers is a Silurian (not Neoproterozoic) continental margin arc (not extensional) complex, now termed the Kingston terrane. It is separated from the adjacent Brookville terrane to the southeast by a Silurian high-grade metamorphic complex and mylonite zone. New U-Pb dates have clarified the extent of the Brookville terrane, and confirmed that its subduction-related plutonic units are Neoproterozoic to Cambrian and distinctly younger than those of the Caledonia and New River terranes, located southeast and northwest, respectively, of the Brookville and Kingston terranes. Both the Caledonia and New River terranes include rock units with ages and petrochemical features characteristic of the Avalon terrane sensu stricto. However they are separated from one another by the non-Avalonian Brookville and Kingston terranes. Hence Neoproterozoic to Silurian rocks of the New River terrane are not likely to be part of the Avalon terrane sensu stricto. U-Pb dates have shown that rocks with affinities to the Miramichi/Gander terrane occur northwest of the New River terrane, and hence the New River terrane may be part of the infrastructure of "Ganderia", rather than "Avalonia".

More geochronology is in progress to further unravel the complex tectonothermal evolution of southern New Brunswick, and interpret its relationship to the rest of the northern Appalachian orogen.