THE POSITION AND NATURE OF THE GANDER-AVALON BOUNDARY, SOUTHERN NEW-BRUNSWICK, BASED ON GEOCHEMICAL AND ISOTOPIC DATA FROM GRANITOID ROCKS

In southern New Brunswick, the Gander-Avalon boundary is obscured by b oundary-parallel faults and various cover sequences. Siluro-Devonian g ranites, which intrude unequivocal Gander or Avalon rocks, display exc lusively negative (-1.9 +/- 1.0) and positive (+1.9 +/- 0.7) epsilon(N d)(T) signatures, respectively. Such contrasting Nd isotopic signature s, combined with other geochemical differences between plutons, are po tentially valuable tools for terrane analysis. Nine small Devonian plu tons intruding the boundary zone fall into contrasting geochemical gro ups with (La/Lu)(N) <4 and >4. The former are topaz-bearing granites, while the latter are volcanic-are-type granites. Except for one pluton , with an epsilon(Nd)(T) signature of -2.0, epsilon(Nd)(T) values rang e from -0.4 to +0.7, spanning the gap between ''type'' Avalon and ''ty pe'' Gander plutons. These results suggest the plutons sampled either (i) stratigraphically overlapping or tectonically interleaved Gander a nd Avalon basement rocks, or (ii) a distinct basement source beneath t he boundary zone. Our results demonstrate that the Gander-Avalon bound ary in southern New Brunswick is nor a simple throughgoing crustal fau lt, and that the Gander and Avalon zones are underlain by different co ntinental basement blocks. Comparison with results from Newfoundland a nd Nova Scotia suggests that these basement blocks are continuous thro ughout the Canadian Appalachians.