MONAZITE GEOCHRONOLOGY RECORDS EARLY ACADIAN AND NEOACADIAN TECTONOMETAMORPHIC EVENTS IN MIDCOAST MAINE

Bedrock exposures in midcoast Maine preserve evidence of multiple tectonometamorphic events associated with the construction of the northern Appalachians. Regional mapping of the Casco Bay Group along the Small Point peninsula has identified four phases of deformation and/or metamorphism during the Silurian through Permian (Eusden et al., 2016), the timing of which has been inferred from regional stratigraphic and structural relationships. In this study, we use in situ monazite geochronology and geochemistry collected via laser ablation split stream (LASS) to provide isotopic constraints on the deformation and metamorphic history recorded by three garnet+andalusite+staurolite±sillimanite schist members of the Cape Elizabeth Formation. The samples record sil+and+str M₂ and and+str M₃ metamorphism, often with textural zoning where inclusion-rich porphyroblast cores give way to inclusion-free rims. Structurally, the three samples lie along a gradient from D₃ hinge parallel extension and open folding (sample # 07296), to D₃ tighter folding transitioning into D₄ dextral shear (sample # 08026), to dextral shear dominated D₄ deformation (sample # 08025). The pervasive, early S₂ schistosity is observed throughout and formed during D₂ isoclinal folding in the early Acadian. Compositional maps of monazite grains were collected prior to LASS analysis to characterize zoning patterns and guide analysis locations. The cores exhibit patchy Th-rich domains, some of which are oriented parallel to the long axis of the grain. These domains vary broadly in Th/U and are interpreted as relicts of original growth c. 400 to 380 Ma. Thin overgrowth domains are commonly aligned with the fabric; these domains yield lower Th/U and younger dates (c. 375 to 360 Ma). We interpret the preserved cores (400 to 380 Ma) as growth during the M₂ stage, in part associated with the development of a second penetrative schistose fabric (S₂) and isoclinal folding (F₂) during the D₂ event in the early Acadian and later affected by D₃ hinge parallel extension and open folding. The thin overgrowth domains are interpreted to reflect recrystallization and/or modification during M₃ and are associated with the onset of D₄ ductile dextral shearing during the NeoAcadian Orogeny.