SILURIAN AND EARLY DEVONIAN AGES FOR THE MIXER POND AND ASSOCIATED ORTHOGNEISS PROVIDE AGE CONSTRAINTS ON MAGMATIC ACTIVITY, MIGMATIZATION, AND DEXTRAL SHEAR EVENTS IN THE LIBERTY–ORRINGTON BELT OF SOUTH–CENTRAL MAINE

New U–Pb zircon and Lu–Hf garnet data reveal Silurian and Early Devonian crystallization ages on previously undated, elongate granitic orthogneiss bodies which intruded into an upper amphibolite facies migmatite complex within the Liberty–Orrington belt of south-central Maine. In situ sampling for geochronological analyses was conducted along a 20-km, SW to NE transect stretching across the Liberty–Morrill–Brooks West 7.5-minute quadrangles. Our youngest zircon age [~410 Ma] was obtained from the southwesternmost sampling area and corresponds in age to the Haskell Hill gneiss. Older zircon ages [ca. 421–431 Ma] from central and northeastern segments of the transect represent the main portion of the Mixer Pond orthogneiss and correlate with other gneissic bodies in the region [e.g. Blinn Hill, Lake St. George, and North Union]. Our single garnet age [~402 Ma] was determined from a separate and distinct outcrop ~50 meters away from rocks yielding an ~426 Ma zircon age. Assemblages with garnet + muscovite are uncommon and interpreted as early pegmatitic dikes which intruded the main body of Mixer Pond orthogneiss and predated regional dextral shear. When ca. 421 to 431 Ma zircon ages are combined with the ~410 Ma [zircon] and ~402 Ma [garnet] ages, our data support at least two and possibly three distinct magmatic pulses inside a zone of high-strain ductile deformation. These ages provide constraints on the age of migmatization, upper amphibolite facies metamorphism, and shear deformation. In addition, our data further substantiate a 400 Ma age reported by Tucker et al. [2001] from a small, mildly deformed graphic granitic body which intruded into the migmatite, thus constraining the age of migmatization while expanding upon the nature and range of Silurian magmatism in the Liberty–Orrington belt.