Determining ages of multiple muscovite-bearing foliations in phyllonites using the 40Ar/39Ar step heating method: Applications to the Alleghanian orogeny in central New England

Abstract

New ^40^Ar/^39^Ar data from chlorite-grade phyllonites in central New England demonstrate the effectiveness of the ^40^Ar/^39^Ar step heating method to date bulk separates containing multiple age populations of white mica. Detailed optical and scanning electron microscope (SEM) petrography of phyllonites collected along the Westminster West fault zone separating the Connecticut Valley Gaspé trough from the Bronson Hill anticlinorium shows that white mica and chlorite defining younger cleavages are recrystallized. The phyllonites are the product of retrograde shearing of higher-grade phyllites west of the fault zone. Samples dominated by two age populations outside of the fault zone yield minimum cooling ages and maximum growth ages. However, at low metamorphic grade, ^40^Ar/^39^Ar dating of white micas from the fault zone yields crystallization ages rather than cooling ages, constraining the timing of the latest movement of the fault zone. ^40^Ar/^39^Ar analyses using the step heating method indicate that west of the fault zone, muscovite cooling ages cluster at ∼365 Ma, suggesting cooling from Devonian (Acadian) metamorphism. To the east, ^40^Ar/^39^Ar analysis yields a muscovite cooling age of ∼318 Ma, suggesting cooling from a younger metamorphic event. In contrast, age steps from samples on the margins of the fault zone climb from minima of ∼306 Ma to \>340 Ma, reflecting a mixture of phyllonitic micas ≤306 Ma with relict Acadian muscovite cooling ages. A sample from the core of the fault zone yields a white mica age ∼300 Ma. These results illustrate the usefulness of the ^40^Ar/^39^Ar step heating method on bulk mineral separates regardless of complications arising from multiple age populations. Moreover, the data suggest that the boundary between the Connecticut Valley Gaspé trough (CVGT) and the Bronson Hill anticlinorium was likely a site of significant strain localization in late Pennsylvanian to early Permian times. These results imply that Late Paleozoic (Alleghanian) deformation, so well documented in coastal and southern New England, was transmitted across the composite Avalon-Ganderia crustal block to the boundary with Laurentian crust (underlying the CVGT) as far north as southeastern Vermont.