STRAIN PARTITIONING AND THE TIMING OF HIGH-GRADE ACADIAN METAMORPHISM IN SOUTH-CENTRAL MAINE

While much progress has been made in the understanding of mid-Paleozoic collisional tectonism in the northern Appalachians, most metamorphic and structural age relationships are still based on indirect information gleaned from post-metamorphic cooling ages and inferred relationships to plutonic rock bodies of known age. This approach in south-central Maine has revealed a prolonged period (mid-Silurian to Early Carboniferous) of complex tectonic activity that can only be resolved through the direct dating of metamorphic mineral growth and deformational fabrics. Here we report new U-Pb (SHRIMP) ages from metamorphic zircon and monazite that provide information on the timing of deformation and metamorphism in this region.

Rocks along the eastern margin of the high-grade portion of the Acadian orogen in Maine can be divided into 4 general lithotectonic belts. From east to west these belts include: (1) the Cam.-Ord. St. Croix belt, (2) the Late Ord. (?)-Early Sil. Fredericton belt, (3) the Ord. Liberty-Orrington belt, and (4) the Late Ord. (?)-Early Dev. central Maine belt. Metamorphic monazite in the Megunticook Fm of the St. Croix belt is 419 ± 4 Ma. This age confirms that high-grade metamorphism in this belt occurred in the Late Silurian and that Devonian regional metamorphism and penetrative deformation was negligible in this area. In contrast, rocks of the Liberty-Orrington belt, which were folded, metamorphosed to low grades and intruded by plutons in the Late Silurian, were strongly affected by Devonian tectonism. Metamorphic zircon and monazite from different localities within the Liberty-Orrington belt are 371 ± 4 Ma and 407 ± 5 Ma, respectively and provide evidence of multiple high-grade metamorphic events in the belt. In addition, an initial crystallization age of 385 ± 5 Ma has been determined for an orthogneiss along the western margin of the Liberty-Orrington belt (Cubley & West, this volume).

Collectively, the new data indicates multiple episodes of Devonian high-grade metamorphism affected rocks of the Liberty-Orrington belt and that Middle to Late Devonian deformation was more intense in this belt than rocks in the adjacent central Maine belt. It is speculated that Devonian dextral transpression focused deformation on the Liberty-Orrington belt which resulted in differential uplift and the juxtapositioning of terrains with different metamorphic histories.