THE GEOLOGY OF THE DEBLOIS PLUTON COMPLEX AND SURROUNDING ROCKS, EASTERN MAINE

Field and petrologic studies of the Deblois Pluton Complex and surrounding rocks of the Fredericton Trough and St. Croix terrane are in progress to better understand the geologic evolution of the Northern Appalachians in eastern Maine. Mapping confirms a fault contact between rocks of the peri-Gondwanan St. Croix terrane and those of the Fredericton Trough (Merrimack-Harpswell terrane). The early-late Ordovician Cookson Group of the St. Croix terrane was subjected to a phase of deformation that produced tight isoclinal folds prior to deposition of the sediments in the Fredericton Trough in the early Silurian. This suggests that the Salinic/Acadian orogeny began in the late Ordovician or earliest Silurian in coastal Maine. This is slightly earlier than thought by most workers, but appears to be consistent with published paleomagnetic data.

The Deblois Pluton Complex (DPC) is the second largest pluton in Maine (>1600 km²) and intrudes rocks of the St. Croix and Merrimack-Harpswell terranes. Granitoids in the DPC range from feldspar-megacrystic (up to 6 cm), seriate, biotite-hornblende varieties with rapakivi textures to medium-coarse grained two-mica varieties with traces of garnet and no rapakivi textures. Contacts with the country rock are sharp and discordant. Feldspar megacrysts up to 3 cm occur at the contact suggesting growth prior to emplacement at the present level of exposure. Chemical data indicate an overall range of composition from granodiorite to granite (65-75%SiO₂), and comparison of normative compositions for medium-grained samples with phase relations in the system Q-Ab-Or indicates emplacement at ~2 kb. A well defined but narrow (~1 km) contact metamorphic aureole in which sillimanite and staurolite are developed is superimposed on the regional greenschist-facies metamorphism.

Field relations demonstrate that the DPC intruded after terrane amalgamation in the Salinic/Acadian orogeny in coastal Maine, in agreement with published age data. Post intrusion movement along the Norumbega Fault Zone has sheared parts of the DPC.