U-PB ZIRCON GEOCHRONOLOGY AND EBSD ANALYSIS OF ROCKS IN THE PASSAGASSAWAKEAG GNEISS, MID-COAST MAINE

Rocks along mid-coast Maine preserve a record of >100 million years of terrane accretion and tectonic activity. Despite its location between two of the most extensive terranes – the Casco Bay and Fredericton belts – the highly metamorphosed and deformed Passagassawakeag Gneiss has remained relatively enigmatic. Uncertainties in the age of the protoliths and the timing of overprinting deformation and metamorphism within the unit has hindered understanding of the role of the Passagassawakeag Gneiss during the tectonic history of the northern Appalachians. Here we present new U-Pb zircon geochronology that constrain the protolith ages of seven samples along the 75-kilometer length of the Passagassawakeag Gneiss and electron backscatter diffraction (EBSD) results that characterize the conditions of deformation recorded by five of these samples.

U-Pb zircon dates measured from three orthogneiss samples indicate multiple episodes of magmatism in the belt. One sample yields an intrusive age of 383 ± 1 Ma (1σ), consistent with crystallization during the Devonian Acadian orogeny. Two other orthogneiss samples, from the southern part of the belt, yield dates of 452 ± 1 (1σ) and 453 ± 1 Ma (1σ). U-Pb detrital zircon spectra were generated from four meta-sedimentary rocks within the belt. Two samples from the southern portion of the belt yield peaks at ca. 500-600 Ma, 1000 Ma, 1100 Ma, and 1200 Ma, with minor peak at 1550 and 1900 Ma; these results are broadly consistent with previously published results from the adjacent Casco Bay belt and are consistent with a peri-Gondwanan source. Spectra from the other two samples yield peaks at 450 Ma and 1050 Ma, with smaller peaks at 1200 Ma, 1300 Ma and 1750 Ma. These results are consistent with new U-Pb results from the adjacent Fredericton belt.

EBSD analysis documents significant variability in the recorded strain history with higher-T deformation conditions recorded in the southern end of the belt and lower-T deformation towards the north. Incomplete overprinting complicates the deformation history of these rocks. In summary, this ongoing work demonstrates that the Passagassawakeag Gneiss belt is a composite belt that includes a range of meta-sedimentary and meta-igneous protoliths with complex deformation histories that reflect a protracted history of tectonic activity.