TURNER MOUNTAIN SYENITE IN THE NORUMBEGA FAULT SYSTEM, EAST-CENTRAL MAINE: A POST-CARBONIFEROUS INTRUSION?

The Turner Mountain syenite, the only syenite in central-eastern and eastern Maine, crops out within the Norumbega fault system (NFS) in association with Carboniferous (?) redbeds and, like the nearby Lucerne pluton, has experienced extensive shearing. It has been assumed by previous workers to be a small member of the Silurian-Devonian Coastal Maine Magmatic Province, but recent mapping and petrographic study have revised the relations between the syenite and its surrounding rocks and suggest a different timing for its emplacement.

The elliptical syenite pluton, with its long axis parallel to the strike of the NFS, is surrounded on the north, west, and southwest, by the tilted redbeds and on the east and southeast by mylonite and ultramylonite derived from the Lucerne granitic pluton during the early phase of Norumbega ductile shearing. A brittle fault separates the granitic mylonite and ultramylonite zone from the syenite and redbeds; however, the syenite is now recognized as intruding the redbeds, with only minor modification of the contact by brittle faulting. A contact metamorphic aureole ranging from 5-10 m in width was found in the redbeds. Red siltstones within the aureole become pale green biotite hornfels due to conversion of hematite into biotite by increasing heat most likely from the syenite magma. The intrusive contact relation with the redbeds demonstrates a hitherto unsuspected post-redbed age for the syenite.

The syenite intrusion has long been mapped as a fault sliver within the NFS, but no ductile deformation is observed within the pluton, suggesting that it might have intruded the NFS between the early phase of ductile shearing and onset of very late-stage brittle deformation. The brittle reactivation of the Norumbega and the transtensional regime for the development of the pull-apart redbed basin may have facilitated the emplacement of the Turner Mountain syenite. Radiometric dating is needed to confirm the age of the syenite and refine the timing of events along the NFS.