Age studies on slates: Applicability of the 40Ar/ 39Ar stepwise outgassing method

Abstract

Results of 40Ar/ 39Ar stepwise outgassing experiments are reported for six slates, one hornfels and one metasilt-stone from the Lower Paleozoic Meguma Group of Nova Scotia, Canada. The age spectrum of the hornfels and of one slate collected from the vicinity of a granite contact have both been completely reset by the thermal effects of the intrusion. The latter, however, has not produced any recognizable mineralogical modification of the slate. Two samples in the collection contain potassium feldspar, possibly of detrital origin, and both have yielded discordant age spectra which cannot be unambiguously interpreted. Three apparently “clean” slates collected from localities well removed from the effects of the granite batholith have yielded quite well-defined age plateaus at ⋍415 Ma (revised decay and isotopic constants). The mean total gas 40Ar/ 39Ar age or, equivalently, the mean K-Ar age is ⋍400 Ma. From these data we suggest a minimum value ⋍400–415 Ma for the time of initiation of the Acadian Orogeny in Nova Scotia which culminated with post-deformational “granitic” intrusion ⋍370–380 Ma ago.