40Ar/39Ar dating of the long range dikes,Newfoundland

Stepwise outgassing experiments were performed on eleven whole rock samples from a sequence of diabase dikes from the Great Northern Peninsula. Gas released at high temperatures (> ? 925–950°C) was consistently characterized by anomalously high apparent ages. These are attributed to the release of excess radiogenic argon, gas which probably resides in the mineral augite. At temperatures below ? 700°C, a wide range in apparent age was observed. On the other hand, the medium temperature region (? 700–950°C) is characterized by a relatively small range in apparent age. Data points corresponding to this region are rather well correlated in an isochron-type diagram; the slope of the best-fitting straight line corresponds to an aget = 605 ± 10m.y. We suggest that the outgassing of unaltered plagioclase is dominating the age spectra in this temperature interval. Consequently, we take 605 ± 10 m.y. as the time of emplacement of the Long Range dike swarm. This event may have marked the opening of a proto-Atlantic Ocean. The Acadian Orogeny (perhaps the ocean-closing event ? 380 m.y. ago) appears to be recorded in the low-medium temperature regions in the cases of two rather extensively altered samples.