40AR/39AR AGE CONSTRAINTS FOR THE RYE COMPLEX ROCKS ALONG THE NORUMBEGA FAULT ZONE, SOUTHERN MAINE AND NORTHERN NEW HAMPSHIRE

The metamorphosed Rye Complex of southern Maine and northern New Hampshire evolved from gneissic through mylonitic to ultramylonitic bands indicative of progressive strain localization associated with deformation along the Norumbega fault zone (NFZ). Amphiboles from mafic rocks within the Rye Complex show compositional zoning and textural variability indicating that the NFZ deformed episodically in time and space during retrogression from upper amphibolite to greenschist facies conditions. Granitic pegmatites crosscut strongly foliated amphibolites and schists. Ductile fabric in the pegmatites shares the same orientation as the host schists, thus ductile deformation continued with the same kinematics after pegmatite emplacement. ⁴⁰Ar/³⁹Ar data for amphibole from an amphibolite boudin, and for muscovite and K-feldspar from deformed pegmatites constrain the timing of this deformation and retrogression.

⁴⁰Ar/³⁹Ar spectra for magnesio-hornblende grains from the interior of an amphibolite boudin show evidence of excess argon and a minimum age of ~475 Ma. We interpret ~475 Ma as a maximum age for the hornblende cooling through its ~500°C closure temperature. This suggests a minimum age of at least Ordovician for the amphibolite.

The overall ⁴⁰Ar/³⁹Ar spectra for muscovite climb from minima of ~240 Ma and flatten to maxima of ~340 Ma. Two inseparable generations and sizes of muscovite grains are present within the pegmatites. Kinked and undulose muscovite fish are interpreted to have cooled below muscovite closure at ~350°C at ~340 Ma. Fine-grained muscovite within the foliation grew at ~240 Ma. ⁴⁰Ar/³⁹Ar spectra for K-feldspar climb from ~190 Ma to a flat portion at ~280 Ma. These ages are interpreted to record cooling through an upper closure temperature of ~250°C at ~280 Ma and a lower closure temperature of ~150°C at ~190 Ma.

Amphibole, muscovite fish and K-spar ages suggest contemporaneous deformation and exhumation from the Ordovician through Mississippian. New mineral growth during later high strain events is supported by Triassic crystallization ages for fine-grained muscovite.