CONTRASTING STYLES OF DEVONIAN GRANITIC MAGMATISM IN COASTAL MAINE

We have studied two Devonian granitic complexes in coastal Maine. The Deblois Pluton Complex (DPC) consists of medium-grained to feldspar megacrystic two-mica, biotite-hornblende, and biotite granite/granodiorite with an overall range of composition from (ca. 65-75% SiO₂). Contacts with the country rock are sharp and discordant, and a well-defined metamorphic aureole is developed in metasediments from the Merrimack-Harpswell and St. Croix terranes. The DPC is tectonically undeformed, except in the vicinity of the Norumbega Fault Zone (NFZ) where a penetrative foliation is developed. The Waldoboro Pluton Complex (WPC) is also dominated by two-mica and biotite granite/granodiorite (ca. 62-75% SiO₂) but contacts with the country rock are often not well defined. The country rocks (Bucksport formation) along the western margin and inliers of country rock within the complex have been pervasively intruded by granitic magma along foliation planes and fold axes. New mineral chemical data clearly suggest the intrusive nature of layer-parallel granite veins and sheets within the country rock. Some of these veins and sheets are boudinaged. Parts of the WPC are foliated whereas other parts are massive, and mylonites are developed along the eastern margin where the WPC cross cuts the Sennebec Pond Fault. The latter separates the Merrimack-Harpswell and St. Croix terranes.

Published age data demonstrate that both the DPC (384 Ma) and WPC (368 Ma) intruded after deformation and metamorphism associated with terrane amalgamation during the Salinic/Acadian orogeny. The foliation and other features (eg. deformation bands, kink bands, microboudinage, grain rotation, myrmekite) that are locally present in the WPC are similar to those seen in synkinematic granitic intrusions. They may have developed in response to shear deformation associated with dextral strike-slip motion along the NFZ in the vicinity of the Casco Bay restraining bend. Formation of mylonites along the eastern margin of the WPC suggests reactivation of the Sennebec Pond Fault by movement along the NFZ.