ALLEGHANIAN OVERPRINT IN THE MERRIMACK TERRANE, EASTERN MASSACHUSETTS: TEXTURAL, MINERALOGICAL, AND 40AR/39AR ISOTOPIC EVIDENCE

Overprinting mineral assemblages and fabrics in phyllitic rocks from the Merrimack terrane in eastern Massachusetts reflect a two stage history of deformation and metamorphism. Phyllosilicates (muscovite > chlorite) that define earlier M1 continuous cleavages (S₁ and S₂) exist in a chlorite to andalusite grade metamorphic field gradient. These fabrics are truncated by phyllosilicates of a later M2 chlorite grade spaced cleavage (S₃) in which muscovite is >> chlorite. In biotite through andalusite grade M1 phyllites S₁ and S₂ are poorly preserved in the matrix, but occur in some porphyroblasts as inclusion trails. These porphyroblasts pre-date the overprinting chlorite-grade (M2) spaced cleavage (S₃). Where S₃ is more strongly developed, M1 porphyroblasts are more completely replaced. Thus, these fabrics are the products of two distinct orogenic events: M1 that produced continuous cleavages and a metamorphic field gradient, and M2, that produced a pervasive chlorite-grade spaced S₃ cleavage and retrograded earlier, higher grade porphyroblasts.

Exploratory ⁴⁰Ar/³⁹Ar geochronology strengthens the argument that these two fabric-forming events belong to separate orogenies. Two chlorite-grade samples (one each from the Worcester and the adjacent Merrimack formations) were selected for ⁴⁰Ar/³⁹Ar analysis because the overprinting S₃ spaced cleavage did not completely destroy earlier cleavage generations. Both samples show climbing ⁴⁰Ar/³⁹Ar muscovite age spectra, which reflect mixed populations of M1 and M2 micas. The age spectra from a Worcester Formation sample climbs from 263 to 355 Ma, while the sample from the Merrimack Formation has an age spectrum that climbs from 277 to 381 Ma. The low temperature, Permian ages represent the maximum age for the time of crystallization of S₃ micas that define the chlorite-grade spaced cleavages. In contrast, the late Devonian ages of the high temperature heating steps represent the minimum age for the time of crystallization of the S₁ and S₂ micas that form the earlier crenulated continuous cleavages. Thus, the earlier cleavages are likely to be middle Devonian or older, most probably Acadian, while the overprinting cleavage is younger than early Permian, undoubtedly Alleghanian.