STRUCTURAL EVOLUTION AND TIMING OF ACADIAN TECTONISM IN EAST-CENTRAL MAINE: CONSTRAINTS FROM MICROSTRUCTURAL ANALYSIS AND IN-SITU MONAZITE GEOCHRONOLOGY

Recent studies suggest that deformation associated with the Siluro-Devonian Acadian orogeny migrated from SE to NW across northern New England and adjacent Canada. However, the nature of thermal migration and relationships between deformation and metamorphism during this orogenesis remain unclear and are the subject of our recent investigations. Early Silurian metapelitic rocks of the Appleton Ridge Formation in east-central Maine are well positioned to record the earliest stages of Acadian orogenesis in New England. In addition, these rocks record the effects of both high-angle convergence associated with the early stages of Acadian orogenesis, and later oblique dextral convergence associated with the waning stages of the orogeny. Detailed structural and geochronological studies of these rocks thus have the potential to reveal the timing of this important transition.

Staurolite porphyroblasts in oriented samples from the Appleton Ridge Fm. are a part of the peak metamorphic assemblage, and show evidence for multiple episodes of growth. Inclusion trains within the different growth zones of the staurolites suggest a complex history of foliation development and syntectonic porphyroblast growth. Asymmetric pressure shadows around many of the porphyroblasts suggest dextral shear deformation outlasted peak metamorphic mineral growth.

In an effort to obtain absolute age information on the timing of deformation and metamorphism in this region, in-situ electron microprobe analysis of compositionally zoned monazite grains was conducted at the University of Massachusetts, Amherst. Monazite grains were analyzed from within the various staurolite growth zones as well as from within the layer silicate dominated matrix. Quantitative compositional data from different age domains within the monazites were obtained and preliminary results show older Silurian cores (420-400 Ma) and younger Devonian rims (380-400 Ma) from all localities. The presence of 380-400 Ma rims on monazites within staurolite porphyroblasts suggests that peak metamorphism and all staurolite growth occurred in the Middle Devonian. The older monazite cores, which appear to be ubiquitous in these samples, are either detrital, or grew during an episode of lower-grade Silurian metamorphism.