ABSTRACT In New England, the Acadian orogen is divided into Western and Eastern meta- morphic belts on the basis of contrasting pressure-temperature (P-T) paths, peak meta- morphism conditions, and ages. Along-strike correlations of structures indicate that rock units affected by Devonian metamorphism and deformation in southern Québec belong to the Western Acadian belt, and share the same tectonic evolution as in New England. Muscovite 40 Ar/ 39 Ar ages obtained from greenschist-grade, Ordovician volcanic and plutonic rocks of the Ascot Complex in south- ern Québec indicate that Acadian metamor- phism and deformation peaked ca. 380–375 Ma. Ordovician muscovite ages of ca. 462 Ma are locally well preserved in the granitic intru- sion of the Ascot Complex, and provide con- straints on the timing of island-arc plutonism in the Dunnage zone of southern Québec. Ordovician 40 Ar/ 39 Ar age spectra do not show important losses of radiogenic argon, indicat- ing that Acadian metamorphism has been low grade, with temperatures below the Ar iso- topic closure temperature proposed for mus- covite. Acadian peak metamorphism is signif- icantly younger, and P-T conditions of regional deformation have been lower in southern Québec than in amphibolite-grade rocks of the Western Acadian belt of New Eng- land. In the northern Appalachians, Acadian metamorphism and deformation are attrib- uted to Devonian age crustal overthickening related to plate convergence. The comparison of Devonian tectonic fabrics in New England and southern Québec suggests that progres- sion in the timing and P-T conditions of Aca- dian peak metamorphism in both areas are the result of the irregular geometry of the col- lision zone and of the northward migration of metamorphic and structural fronts along the orogen. INTRODUCTION In the northern Appalachians, Paleozoic meta- morphism is the result of three different orogenic events, the Taconian (Ordovician), the Acadian (Late Silurian to Middle Devonian), and the Alleghanian (Late Carboniferous to Early Per- mian). The existence of a Silurian orogeny (the Salinic orogeny; Dunning et al., 1990) has been proposed to account for metamorphism and plu- tonism of Early Silurian age in the Newfound- land Appalachians, but there are still uncertain- ties concerning the significance of the cooling ages and structures attributed to that event (Hib- bard et al., 1995; Castonguay et al., 1997; Trem- blay et al., 1997). From New England to New- foundland, there are substantial overlaps between structural and metamorphic features attributed to the Taconian and Acadian orogenies (Sutter et al., 1985; Armstrong et al., 1992; Tremblay and Pinet, 1994; Cawood et al., 1995), and Acadian metamorphism and plutonism occur in rock units varying from Cambrian to Lower Devonian. As for most orogenies, there is diachronism of the timing of collisional and thermal events along the Acadian orogen (Osberg et al., 1989; Stockmal et al., 1990; Rast and Skehan, 1993). In New England, Armstrong et al. (1992) sepa- rated the Acadian metamorphic realm into East- ern and Western belts on the basis of different pressure-temperature (P-T) path evolution, peak metamorphism conditions, and ages. The West- ern Acadian belt is characterized by high-temper- ature–high-pressure conditions (Barrovian-type metamorphism) related to crustal thickening, whereas the Eastern Acadian belt is characterized by high-temperature–low pressure conditions (Buchan-type metamorphism) associated with synkinematic plutonism (Armstrong et al., 1992). Isotopic dating indicates that Eastern Acadian synkinematic magmatism started as early as 425 Ma and ended by about 410 Ma. Peak meta- morphism and regional structures in the Western Acadian belt are dated as 395–385 Ma, and pegmatites and granites dated as 375–370 Ma and associated to dome-stage deformation and folding of metamorphic isograds characterize the end of the Acadian orogeny (Armstrong et al., 1992). In the Québec Appalachians, regional metamorphism in Ordovician oceanic rocks of the Dunnage zone (Williams, 1979) and over- lying Silurian and Devonian rocks of the Gaspé Belt (Bourque et al., 1995) has been attributed to the Acadian orogeny (Malo et al., 1995). Peak metamorphism is loosely dated as Lower-Middle Devonian because it postdates the deposition of Lower Devonian rocks, and predates the intru- sion of Middle Devonian granites (Simonetti and Doig, 1990; Malo et al., 1995). Granitic plutons of inferred Ordovician age occur locally in the Dunnage zone and have been interpreted as syn- volcanic intrusions (Tremblay et al., 1994). 136 Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40 Ar/ 39 Ar evidence for collision diachronism Alain Tremblay* Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Gilles Ruffet † UMR-Géosciences Azur, Centre National de Recherche Scientifique, Université de Nice–Sophia Antipolis, 06108 Nice Cedex 02, France Sébastien Castonguay § Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada GSA Bulletin; January 2000; v. 112; no. 1; p. 136–146; 9 figures. Data Repository item 20003 contains additional material related to this article. *E-mail: tremblay@gsc.nrcan.gc.ca. † E-mail: ruffet@naxos.unice.fr. § E-mail: scastong@x1.nrcan.gc.ca.