A LATE DEVONIAN AGE FOR THE CHESTER SHEAR ZONE, CENTRAL MAINE: EVIDENCE FROM 40AR/39AR AGE SPECTRA

Argon-argon geochronology of minerals in the matrix of phyllites from the Silurian Smyrna Mills Formation in the Chester Shear Zone, Lincoln, Maine, constrains the age of the slaty cleavage development to Late Devonian or younger. These metasedimentary rocks contain muscovite in three textural populations: 1) detrital muscovite up to 75 µm long distributed among fine to medium silt-size quartz and albite grains; 2) authigenic muscovite finely intergrown with chlorite to form stacks up to 100 µm in diameter; and 3) cleavage-forming muscovite + chlorite intergrowths up to 50 µm long. Backscattered-electron (BSE) petrography shows that the micas in the youngest fabric truncate both the detrital and authigenic micas. Step heating experiments from seven samples all yield sigmoidal age spectra. These age spectra have a hump-shape in low-temperature steps, indicating ³⁹Ar recoil, consistent with the fine grain size and intergrown nature of the matrix. Following this hump, in steps interpreted to be unaffected by recoil, apparent ages climb steadily from minimum apparent ages as young as 385 Ma to maximum ages as old as 480 Ma. The samples with the lowest minimum apparent age steps are those in which the cleavage-forming mica population dominates. On the other hand, the oldest apparent age steps are from samples that have the highest modal abundance of detrital micas as observed from BSE petrography. We interpret the Middle to Late Devonian age to be the maximum crystallization age of the cleavage-forming micas. The age of the detrital micas appears to be at least Early Ordovician and most probably reflects a Cambrian or older cooling age in an unidentified provenance area. We speculate that the fabric in Chester Shear Zone was produced by inboard strain localization during the Late Devonian activity of the Norumbega Fault Zone.