MESOSCOPIC STRUCTURE OF THE MIXER POND GNEISS, SOUTH-CENTRAL MAINE

Polyphase structural mapping complemented by recent U–Pb zircon/Lu–Hf garnet dating in the Mixer Pond Gneiss [MPG] reveals late Silurian–Early Devonian silicic magmatism and coeval[?] contractional deformation along the Liberty–Orrington belt of south-central Maine. Mesoscopic structures recorded in fine-grained granitic orthogneiss [petrologically complex MPG unit] include a variably SE-dipping gneissic foliation [S_n~20–85° dip] and shallow NE-plunging crenulation lineation [<10° plunge] associated w/ NW-vergent antiform-synform pairs. Near our ~420 Ma zircon sample locality, asymmetric folds in S_n are steeply inclined [axial surfaces >65°] and possess ~40–65° interlimb angles defining a close geometry; fold hinges are subhorizontal [~5° NE plunge] and subparallel to crenulation lineations developed on S_n. In other areas [e.g., ~402 Ma garnet locality], orthogneiss bodies are characterized by shallow NW-dips [S_n~35°]; strings of subhedral, mm-scale porphyroblastic garnet are often observed along the traces of synmetamorphic foliation. Where S_n is steeply NW-dipping [~80°], asymmetric K-feldspar porphyroclasts [rare σ-structures viewed on S_n traces] suggest tops-to-the-NE/dextral shear. Consistent w/ the MPG, kinematic indicators in adjacent country rocks [e.g., Devonian–Ordovician Passagassawakeag Gneiss] record contractional and strike-slip components of deformation. Where schistose fabrics dip moderately SW [S_n~40°], cm-scale felsic boudins occasionally observed along S_n traces support a tops-to-the NE/reverse sense of shear; conversely, parasitic folds and imbricated mineral pods associated with steep NW-dipping penetrative fabrics [S_n >70°] indicate dextral shear. In contrast to MPG structures described above, open-to-isoclinal folds in high-strain exposures of the Passagassawakeag Gneiss and other stratified metasedimentary units [e.g., Silurian–Ordovician Bucksport Formation] consistently plunge SW [~5-25°] and often contain a subvertical axial-planar crenulation cleavage [S_n+1]. Extensive U–Pb zircon dating [Gray et al., 2017], detailed 1:24000-scale geologic mapping [Pollock, in progress], and structural details reported in this study suggest that intrusive rocks of the MPG shared in the transpressional deformation history recorded along coastal Maine.