INTRODUCTION One of the most challenging aspects that arises from recent geochronological studies on the tim- ing of deformation and metamorphism along the Laurentian margin (Humber zone) of the north- ern Appalachians is to reconcile and interpret age differences of tectonic events along the strike of the New England–Newfoundland segment of the orogen. In the western New England Appalachians, metamorphism attributed to Taconian orogeny occurred during the Middle to Late Ordovician (470 to 445 Ma; Laird et al., 1984; Sutter et al., 1985) and is found in rocks that crop out west of the Cameron’s line (Armstrong et al., 1992). Taconian metamorphic rocks of the Laurentian margin are locally overprinted by a metamorphic and tectonic event dated as Middle to Late Devo- nian (386 to 355 Ma) that is attributed to the Aca- dian orogeny (Laird et al., 1984; Sutter et al., 1985; Armstrong et al., 1992). The time period between the two orogenic episodes has been clas- sically interpreted as a quiet, nonmetamorphic period characterized by erosion of the Taconian wedge and sedimentation within the contempo- raneously forming Connecticut Valley–Gaspé trough (Late Silurian to late Early Devonian; Os- berg et al., 1989). In western New England, sparse Silurian metamorphic cooling ages have been interpreted as recording prolonged cooling following the peak Taconian metamorphism (Laird et al., 1984) or as the result of partial argon loss during the Acadian metamorphic overprint (Spear and Harrison, 1989). The Taconian orog- eny of western New England and adjacent Que- bec has been related to the attempted subduction of the Laurentian continental margin along an east-facing subduction zone, which was accom- panied by the emplacement of ophiolitic nappes onto the Laurentian margin (Stanley and Rat- cliffe, 1985; Pinet and Tremblay, 1995). In Newfoundland, regional metamorphism and deformation of the Laurentian margin have classically been interpreted, as in the contiguous northern Appalachians, as being Middle Ordovi- cian (Williams, 1979; Colman-Sadd, 1982). However, recent 40 Ar/ 39 Ar and U/Pb geochrono- logical studies from the Newfoundland internal Humber zone have outlined mostly Early Sil- urian ages (Dunning et al., 1990; Cawood et al., 1994), which are interpreted as the result of a ma- jor continent-continent collision (the Salinian orogeny). In that region, the Taconian orogeny has been attributed to a minor thermal event caused by the westward emplacement of thin lay- ers of ophiolites and allochthons at high crustal levels (Cawood et al., 1994, 1995). The Quebec Appalachians represent a critical transition zone between the New England and Newfoundland Appalachians, but age constraints on metamorphism and deformation of the inter- nal Humber zone have been sparse and were rarely placed into a regional geodynamic context. The scope of this paper is to present new laser step-heating 40 Ar/ 39 Ar age constraints from the southern Quebec Appalachians internal Humber zone that show that both Ordovician and Silurian metamorphism and deformation are recorded. GEOLOGY OF THE SOUTHERN QUEBEC APPALACHIANS The southern Quebec Appalachians encom- pass two major tectonostratigraphic zones of pre- Silurian rocks (Fig. 1; Williams, 1979): the Hum- ber zone, which represents Cambrian-Ordovician rocks belonging to the Laurentian continental margin, and the Dunnage zone, which is made of ophiolitic complexes, volcanic rocks, melanges, and synorogenic flysch deposits. The two zones are in tectonic contact along the Baie Verte– Brompton line (Fig. 1; Williams and St.-Julien, 1982). Southeast of the Baie Verte–Brompton line, Silurian and Devonian rocks of the Gaspé belt (Bourque et al., 1995) unconformably and structurally overlie rock units of the Dunnage zone (Pinet and Tremblay, 1995). The Humber zone is subdivided into an exter- nal and internal zone (St.-Julien and Hubert, 1975). The external Humber zone is character- ized by the imbrication of northwest-directed thrust nappes (Fig. 1). Rock units are weakly de- formed and attain the prehnite-pumpellyite to lower greenschist facies of regional metamor- phism. The internal Humber zone is made up of lower to upper greenschist metamorphic rocks Geology; July 1997; v. 25; no. 7; p. 583–586; 3 figures. 583 Ordovician and Silurian metamorphic cooling ages along the Laurentian margin of the Quebec Appalachians: Bridging the gap between New England and Newfoundland Sébastien CastonguayMMInstitut National de la Recherche Scientifique-Géoressources, C.P. 7500, Sainte-Foy, Alain Tremblay Québec G1V 4C7, Canada Gilles Ruffet UMR-Géosciences Azur, Centre National de Recherche Scientifique–Université de Nice– Gilbert FéraudMMSophia Antipolis, 06108, Nice, Cedex 02, France Nicolas PinetMConsultant Geologist, 1067 avenue Des Érables, Québec, Québec G1R 2N3, Canada Marc SossonMUMR-Géosciences Azur, Centre National de Recherche Scientifique–Université de Nice–Sophia Antipolis, 06560, Valbonne, France Data Repository item 9735 contains additional material related to this article. ABSTRACT Geochronological studies on the timing of deformation and metamorphism along the Laurentian margin have shown that the ages of metamorphism events change along the strike of the Newfoundland–New England segment of the Appalachian orogen. The Quebec Ap- palachians represent a critical transition zone, where new single-grain laser 40 Ar/ 39 Ar plateau ages show that both Middle Ordovician and Silurian deformation and metamorphism are recorded. Parts of the analyzed samples give 40 Ar/ 39 Ar muscovite ages ranging from 468 to 461 Ma. These samples were taken within structural windows of the eastern internal Humber zone that are overlain by ophiolitic and melange rocks of the Dunnage zone. Other analyzed samples yielded Silurian 40 Ar/ 39 Ar muscovite ages clustering from 425 to 421 Ma. These sam- ples were taken across the Notre Dame Mountains anticlinorium, which represents a struc- tural and metamorphic dome within the internal Humber zone. The Middle Ordovician ages of southern Quebec are coeval with data from western New England and are interpreted as representing cooling shortly after the peak Taconian regional metamorphism. The Silurian ages, which have never been previously documented in the Quebec Appalachians, are compa- rable to those described in Newfoundland; however, they are not attributed to tectonism that shows characteristics of an orogen-scale continent-continent collision (Salinian orogeny), but rather those of late Taconian backthrusting and normal faulting or a period of crustal exten- sion that followed the main Taconian crustal thickening.