THE NATURE AND EXTENT OF THE EASTERN PART OF THE NORTHERN CONTACT ZONE OF THE SEBAGO PLUTON, SW MAINE

We conducted new mapping along an E-W corridor along the north contact of the Sebago pluon (NNE of Sebago Lake in southern Maine) as part of our lab’s ongoing work on understanding the relation of the pluton to its migmatite-granite complex country rocks (LaFleur et al., NEGSA08, Nyitrai et al., NEGSA09; Bohlen et al., NEGSA10). Prior to our group’s work, maps have the Sebago pluton lumped with its surrounding rocks (Sebago batholith). Our group’s work established a separate pluton central to the original map area, in contact with a migmatite-granite complex that comprises the remainder. Rocks in the pluton are typically medium-grained, and homogeneous 2-mica granite, whereas the complex outside the pluton is plastically deformed, strongly heterogeneous migmatites, and granites with varying solid-state fabrics. The two units are simple to distinguish at map scale. The nature of the contact of the Sebago pluton was unknown, and extent projected from large-scale mapping. Similarly, the pluton’s relation to the migmatite-granite complex was not known. Subsequent work has focused there. One outcome has been the mapping of a distinct, 6-8 km-wide contact zone at the ENE contact (Bohlen et al. NEGSA10). The present study is a continuation of that study to include the adjacent E part of the previously larger-scale-mapped north contact (W of New Gloucester, Maine).

Results of new mapping confirms the contact’s general E-W trend, and the general location of the contact where previous work at larger scale had it placed, but with about 50-100 meters deviation north or south along its length. Outcrop density is low in comparison with adjacent field areas, and the contact is not exposed, but can be inferred within 100 meters. Taking this into account, the shape of the contact is mapped as more complex than previous, larger-scale mapping permitted. Our smaller-scale mapping shows the contact ‘snaking’ N and S along the E-W trend. In addition, no contact zone is found in contrast to the one reported along the ENE contact of the pluton (Bohlen et al. NEGSA10). Rocks proximal to the contact are distinctly either deformed migmatites and granites (north, part of the migmatite-granite complex), or homogenous granites of the Sebago pluton (south). However, granite at the contact has recorded some solid-state strain locally as seen in thin section only.