AN ALLEGHANIAN FAULT ZONE IN SOUTHEASTERN VERMONT: 40AR/39AR EVIDENCE FROM WHITE MICA IN RECRYSTALLIZED PHYLLONITES

New ⁴⁰Ar/³⁹Ar data show that a fault zone within the Vermont sequence of the Connecticut Valley Synclinorium (CVS) was active in the early Permian. Quadrangle-scale mapping in the Townshend and Bellows Falls areas (Armstrong, 1995, 1997) identifies the Westminster West fault zone by pervasive phyllonites with kinematic indicators suggesting left-lateral, top to the northwest vergence. These phyllonites are the product of retrograde shearing of higher grade schists present on both sides of the fault zone. Detailed optical and SEM-petrography of the phyllonites shows that the white mica and chlorite defining younger cleavages are recrystallized via a pressure-solution reprecipitation mechanism. Thus at this low metamorphic grade (chlorite-grade), ⁴⁰Ar/³⁹Ar dating of these white micas should yield crystallization ages rather than cooling ages. To test this hypothesis, we applied the ⁴⁰Ar/³⁹Ar dating method to white mica concentrates extracted from chlorite-grade phyllites and phyllonites in the fault zone.

Results indicate that west of the fault zone, muscovite cooling ages cluster at ~360 Ma, suggesting cooling from Acadian metamorphism. To the east, preliminary data suggest early Pennsylvanian cooling through muscovite closure (350^oC). In contrast, white mica ages from the fault rocks themselves are late Pennsylvanian. The fine-grained intergrowths of chlorite with white mica produce recoil in all of the age spectra. After eliminating the steps affected by recoil, age steps from samples on the margins of the fault zone climb from minima of ~310 Ma to > 330 Ma, reflecting a mixture of phyllonitic micas < 310 Ma with relict Acadian micas with cooling ages > 330 Ma. A sample from the core of the fault zone containing no optically visible relict Acadian mica produces an age spectrum that climbs from ~300 to 310 Ma. Thus the phyllonitic mica must be < 300 Ma, consistent with the recent U/Pb dating of monazites east and west of the fault zone (Spear and others, 2003). These results are significant because they show that Alleghanian deformation, so well documented in coastal New England, was transmitted across the relatively strong Central Maine crustal block to its boundary with the Laurentian crust (underlying the CVS) where strain was localized in fault zones along the eastern flank of the Chester and Athens domes.