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Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high-spatial-resolution [sup 40]Ar/[sup 39]Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S[sub 2], S[sub 3], and S[sub 4]) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S[sub 3] and S[sub 4] are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative [sup 40]Ar/[sup 39]Ar geochronology. Mean [sup 40]Ar/[sup 39]Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S[sub 3] and 345.5 ± 1.7 Myr for S[sub 4]. Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S[sub 3] and S[sub 4], these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region. [ABSTRACT FROM AUTHOR]