129 The position and nature of the Gander- Avalon boundary, southern New Brunswick, based on geochemical and isotopic data from granitoid Joseph B. Whalen, Leslie R. Fyffe, Frederick J. Longstaffe, and George A. Jenner Abstract: In southern New Brunswick, the Gander-Avalon boundary is obscured by boundary-parallel faults and various cover sequences. Siluro-Devonian granites, which intrude unequivocal Gander or Avalon rocks, display exclusively negative (- 1.9 f 1.O) and positive (+ 1.9 f 0.7) E,~(T) signatures, respectively. Such contrasting Nd isotopic signatures, combined with other geochemical differences between plutons, are potentially valuable tools for terrane analysis. Nine small Devonian plutons intruding the boundary zone fall into contrasting geochemical groups with (LaILu), < 4 and >4. The former are topaz-bearing granites, while the latter are volcanic-arc-type granites. Except for one pluton, with an rNd(T) signature of -2.0, rNd(T)values range from -0.4 to +0.7, spanning the gap between "type" Avalon and "type" Gander plutons. These results suggest the plutons sampled either (i) stratigraphically overlapping or tectonically interleaved Gander and Avalon basement rocks, or (ii) a distinct basement source beneath the boundary zone. Our results demonstrate that the Gander-Avalon boundary in southern New Brunswick is not a simple throughgoing crustal fault, and that the Gander and Avalon zones are underlain by different continental basement blocks. Comparison with results from Newfoundland and Nova Scotia suggests that these basement blocks are continuous throughout the Canadian Appalachians. Rkum6 : Dans la region mkridionale du Nouveau-Brunswick, le track de la limite Gander-Avalon est brouille par les failles bordibres parallkles et masquk par diverses skquences de couverture. Les granites d'lge Siluro-Dkvonien, recoupant sans ambigu~te les roches de Gander ou dlAvalon, ont fourni exclusivement, et respectivement, les signatures de rNd(T) nkgative (- 1,9 f 1,O) et positive (+ 1,9 f 0,7). Ces signatures contrastantes des isotopes de Nd, combinkes B d'autres donnks gkochimiques diffkrentes selon le pluton, sont des paramktres estimks valables pour l'analyse des terranes. Neuf petites intrusions dtvoniennes recoupant la zone de lirnite sont classkes dans deux groupes gkochimiques distingues par (LaILu), < 4 et >4. Le premier groupe inclut les granites B topaze, tandis que le dernier comprend les granites de type d'arc volcanique. A l'exception du pluton dont la signature de rNd(T) = -2,0, les valeurs de rNd(T) varient de -0,4 B +0,7, chevauchant le hiatus entre les plutons de a type * Avalon et de type - Gander. Ces rksultats suggbrent que ces plutons sont des exemples soit (i) de roches de socle de Gander et d'Avalon stratigraphiquement chevauchantes ou tectoniquement intercalks, ou (ii) de roches d'un socle source distinct sous la zone formant la limite. Nos rtsultats demontrent que la limite Gander-Avalon dans la rkgion mkridionale du Nouveau-Brunswick ne reprksente pas simplement une faille coupant toute la croiite, et que les zones de Gander et d'Avalon sont supportkes par diffkrents blocs de socle continentaux. La comparaison avec les rksultats obtenus pour Terre-Neuve et la ~ouvelle-~cosse suggkre que ces blocs de socle sont presents en continuitk au travers les Appalaches au Canada. [Traduit par la r&ction] J.B. Whalen.' Geological Survey of Canada, 601 Booth Street, Ottawa, ON KIA OE8, Canada. L.R. Fyffe. New Brunswick Department of Natural Resources and Energy, P. 0. Box 6000, Fredericton, NB E3B 5H1, Canada. F.J. Longstaffe. Department of Earth Sciences, The University of Western Ontario, London, ON N6A 5B7, Canada. G.A. Jenner. Department of Earth Sciences, Memorial University of Newfoundland, St. John's, NF A1B 3x5, Canada. Geological Survey of Canada Contribution 40694. Corresponding author (e-mail: jwhalenagsc. emr. ca). Introduction The boundary between the Gander and Avalon zones repre- sents one of the most fundamental geologic and tectonic boundaries in the Canadian Appalachian Orogen (see Williams 1979; van Staal et al. 1996). In southern New Brunswick, the actual location of this boundary is a subject of debate because of (i) the presence of numerous northeast-trending faults, most with complex movement histories extending into the Carboniferous (e.g . , Leger and Williams 1986; Nance 1987; Doig et al. 1990; St. Peter and Fyffe 1990), and (ii) numer- ous Siluro-Devonian and Carboniferous cover sequences that obscure relationships between rocks with unequivocal Gander or Avalon affinity (e.g., Fyffe and Fricker 1987; Durling and Marillier 1990; St. Peter 1993). Can. J. Earth Sci. 33: 129- 139 (1996). Printed in Canada / Imprimd au Canada Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Depository Services Program on 10/10/13 For personal use only.