ELECTRON MICROPROBE AGE DATING OF MONAZITE FROM THE ACADIAN OROGENY, PRESIDENTIAL RANGE, NEW HAMPSHIRE

This study used a unique method of monazite age dating to constrain the timing of metamorphic and igneous activity in the Presidential Range during the Acadian orogeny. In situ electron microprobe analysis was conducted to obtain quantitative compositional information for multiple age domains within compositionally zoned crystals of monazite. Three thin sections were dated, a two-mica granite from the Bickford pluton, and a staurolite schist and migmatitic gneiss from the Devonian Littleton Formation.

A total of seven analyses on three grains from the Bickford two-mica granite gave a late Devonian/early Carboniferous age (353 - 375 Ma) with one analysis showing an older component (403 Ma). The younger ages are likely crystallization ages and the older age may be a relict component.

Eight analyses each on two grains from the staurolite schist show two distinct age domains. An inherited Silurian core domain (416 - 420 Ma) is surrounded by Eifelian to Emsian monazite growth (388 - 398 Ma). The older domain is interpreted to be a relict detrital component possibly reflecting the source age of the Littleton Fm. The younger domain is interpreted to represent a phase (s?) of metamorphism in the region and is approximately 10 m.y. younger than other reported ages.

Eight analyses on one grain of the gneiss revealed Silurian (418 Ma.) and Carboniferous (341 Ma.) domains. Eight analyses on another grain showed three domains; 1) 466 – 425 Ma. in the core; 2) 404 – 396 Ma. in the rim; and 3) 370 Ma between the rim and core. Taken together these ages are possibly related to relict Silurian to Ordovician detrital components, Eifelian to Emsian metamorphism (396 – 404 Ma, and migmatization (370 - 341 Ma) associated with intrusion of the Bickford granite.

Electron microprobe age dating is rapid and promising, yet given the uncertainty of this technique (approx. 10 m.y.) and the range and complexity of ages reported here, more work is needed to fully evaluate this method and the significance of the ages.