A PETROLOGIC AND MICROSTRUCTURAL STUDY OF THE SCARBORO FORMATION (CASCO BAY SEQUENCE) IN SOUTH-CENTRAL MAINE

Ordovician rocks of the Liberty-Orrington belt in south-central Maine are flanked by younger rocks to the southeast (Fredericton belt) and northwest (central Maine belt) and thus occupy an important structural position in the region. The tectonic setting of this belt of older rocks remains unclear, although several models have been proposed to explain it (e.g., allochthonous thrust sheet or transpressional uplift of basement rocks). This study focuses on Middle Ordovician metapelitic rocks of the Scarboro Formation (upper part of the Casco Bay sequence) exposed within the Liberty-Orrington Belt. Detailed petrologic and microstructural analyses of these previously unstudied rocks will provide information on the metamorphic and deformational history of the region.

Rocks of the Scarboro Formation are light gray, fine to medium grained, porphyroblastic, mica schists. The porphyroblasts include dark gray andalusite, garnet, staurolite, and rarely cordierite. Discontinuous complexly folded quartz segregations (0.5 to 5cm thick) are ubiquitous. Compositional layering and the dominant foliation within the unit are generally parallel and trend N10-25°E and dip steeply (>60°) to the southeast. North trending asymmetric Z-folds are common and they deform compositional layering and the northeast trending foliation. Metamorphic mineral assemblages indicate low-pressure metamorphism at andalusite/staurolite to sillimanite grade conditions. Texturally zoned garnet porphyroblasts and partial pseudomorphs after andalusite suggest polymetamorphism.

Microstructural analysis of porphyroblast-matrix relationships are in progress and will provide information on the timing of metamorphic mineral growth relative to deformational fabrics in the rocks. In addition, mineral chemistry and major element X-ray compositional maps of porphyroblasts will provide detailed information on the conditions of metamorphism. These combined petrologic and microstructural studies of the Scarboro Formation should provide constraints on the tectonic setting of the Liberty-Orrington Belt in south-central Maine.