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Two whole-rock samples of the Jurassic North Mountain Basalt have been dated using the 40Ar/39Ar method. A single sample of fresh, medium-grained, holocrystalline basalt from the lower ow unit of this thick (i.e., 400 m) sequence gave coincident plateau and isochron correlation ages of 201 ± 2.5 Ma, in agreement with a zircon U-Pb age of 202 ± 1 Ma for this same ow unit. This 40Ar/39Ar age contrasts with earlier conventional K-Ar whole-rock ages of ca. 192 Ma for the North Mountain Basalt, which are similar to other K-Ar and Ar/Ar ages for correlative basalts of eastern North America. The second dated sample is a zeolite-bearing basalt from the middle ow unit of the North Mountain Basalt. This sample gave a discordant age spectrum with excess argon, but the isochron correlation age of 206 Ma, albeit with a large error (i.e., 56 Ma), is similar to that for the fresh sample. The data indicate that reliable whole-rock 40Ar/39Ar ages for the basalts are attainable, but that fresh samples must be used. In light of this, it is suggested that the younger 192 Ma ages may reect a widespread thermal event related to zeolite formation. RESUME Deux echantillons de roche totale de basalte jurassique du mont North ont ete dates au moyen de la methode 40Ar/39Ar. Un echantillon unitaire de basalte holocristallin a grain moyen frais de l'unite d'ecoulement inferieure de cette sequence epaisse (c.-a-d. 400 m) a fourni des âges concordants par correlation tabulaire et isochrone de 201 ± 2,5 Ma, ce qui correspond a la datation au U-Pb a partir de zircon situant la meme unite d'ecoulement a 202 ± 1 Ma. Cette datation 40Ar/39Ar fait contraste avec les datations de la roche totale conventionnelles anterieures au K-Ar situant a environ 192 Ma le basalte du mont North, soit des âges semblables a d'autres datations au K-Ar et Ar/Ar de basaltes correlatifs de l'Est de l'Amerique du Nord. Le deuxieme echantillon date est un basalte renfermant de la zeolite et provenant du milieu de l'unite d'ecoulement du basalte du mont North. Cet echantillon a fourni un spectre d'âges discordant presentant un exces d'argon, mais la datation par correlation isochrone le situant a 206 Ma, en depit d'une erreur importante (c.-a-d. 56 Ma), fournit un âge semblable a celui de l'echantillon frais. Les donnees revelent qu'il est possible d'obtenir des dates de roche totale ables au moyen de la methode 40Ar/39Ar, mais qu'il faut utiliser des echantillons frais. Compte tenu de ce fait, on suppose que les âges plus recents de 192 Ma pourraient correspondre a un phenomene thermique repandu apparente a la formation de zeolite.

40Ar/39Ar age of the Jurassic North Mountain Basalt, southwestern Nova Scotia