IS THERE A RELATION BETWEEN TRANSPRESSIVE DEFORMATION AND PLUTON EMPLACEMENT IN SOUTHERN MAINE?

Structural, geochemical and geochronological data from transpressive orogenic belts show regional deformation and the emplacement of plutons to be coeval processes. Therefore, it is important to consider the relation between these processes in order to understand what must be fundamentally connected orogenic processes.

In southern Maine, the crystallization of the Sebago granite (c. 293 Ma) is coeval with strain accommodation within the dextral-transpressive Norumbega shear zone system (NSZS) located immediately SE. Mapping in the SE contact zone of the batholith and surrounding areas shows no simple cross-cutting relation between the pluton and the NSZS. Instead, subconcordant granite sheets, migmatites and metamorphic rocks are interlayered within the NW boundary zone of the NSZS (similar to recent maps). We separate the former Sebago batholith into a granite pluton restricted to the southern portion (the Sebago pluton, sensu stricto, corresponding to petrochemical 'group 1' granites of Tomascak et al., 1996; Contrib. Mineral. Petrol.), and a migmatite domain that underlies the remaining area mapped previously as the batholith (the Sebago migmatite domain). Structures in the migmatites SE of the pluton parallel the local fabric of the NSZS (NE-SW striking and steeply dipping). Traverses across strike to the SE into the NSZS show a progressive change in planar structures from NNE-striking, moderately SE-dipping to NE-SW-striking and steeply SE-dipping, with subhorizonal lineations. These structural changes are concurrent with a progressive increase in outcrop of plastically deformed concordant granite sheets and schlieric granites, and an increase in fabric intensity. We suggest that these observations, in concert with geochemical and geochronological data, illustrate granite ascent via structural control, but that deformation outlasted granite crystallization as the ascent conduit closed. We continue to investigate the relation and timing between the migmatites and the granite, and deformation in the NSZS, using microstructural analysis relative to mapping, and isotope geochemistry and geochronology of granites.