EVIDENCE FOR ALLEGHANIAN TRANSPRESSIVE TELESCOPING OF AN ACADIAN METAMORPHIC FIELD GRADIENT, SOUTH-CENTRAL CONNECTICUT

New ⁴⁰Ar/³⁹Ar ages from the prograde metamorphic sequence of Ague (1994) in southwestern Connecticut between the East Derby Shear Zone and New Haven paralleling the Housatonic River are used to constrain the metamorphic history of the southeastern Connecticut Valley Synclinorium. Mapping along an 8-mile SE-NW traverse from chlorite to staurolite-kyanite grade revealed isograds at least three times closer than supported by regional thermobarometry; yet, smooth, linear trends in the field gradients (Ague, 2003) preclude the possibility that large faults collapsed these isograds.

Amphibole age spectra yielded two plateau ages at 375±1 Ma and 377±1 Ma with correlation ages of 377±1 Ma and 377±2 Ma. These data are interpreted to indicate retrograde cooling through about 500°C in the late Devonian after peak Acadian thermal conditions. Muscovite-bearing samples consistently contain two fabric-forming generations of white mica. An earlier (S_n-1) muscovite fabric is intergrown with biotite at biotite and higher grades so these muscovite should yield cooling ages. A younger metamorphic fabric (S_n) overprints all earlier fabrics in which muscovite is universally intergrown with chlorite suggesting that S_n muscovites grew below closure. Data from these samples are consistent with this and produce sigmoid-shaped age spectra indicating ³⁹Ar recoil within the intergrowths of muscovite and chlorite. In samples where S_n-1 dominates, age spectra climb to 360 Ma. This is interpreted as the minimum cooling age of muscovite from Acadian metamorphism. In samples where S_n dominates the age spectra, steps interpreted to have age significance fall to ages as low as 260Ma—an age confirmed by several samples yielding acceptable correlation ages. The pervasive nature of these late Alleghenian S_n fabrics suggest the whole belt was part of a dextral transpressive shear zone. Telescoping of this structural section by the strain associated with this fabric becomes a viable explanation for the apparent steep metamorphic field gradient preserved in these rocks.