MIGMATITE-GRANITE RELATIONS AT THE CENTIMETER- TO METER-SCALE: DOCUMENTING STRUCTURAL AND PETROGRAPHIC VARIATIONS IN A 3-D EXPOSURE, EASTERN SEBAGO MIGMATITE DOMAIN, SOUTHERN MAINE

If magma flow through deforming rocks is recorded by structures in migmatites and associated granites, study of such rocks requires multiple scales of observation. As part of a larger multi-disciplinary study of rocks of the N Appalachian migmatite-granite belt, we have documented variations at the cm- to m-scale of mineral fabrics and the geometry of granitic bodies in a ~150 x 75 x 5 m exposure for a 3-D examination of this record.

The exposure is in the center of the eastern Sebago migmatite domain (SMD) in southern Maine that mantles the 400 km² Permian Sebago granite pluton on its W, N and E. The SMD is composed of mostly metapelitic stromatic migmatite and diatexite, with cm- to m-scale bodies of granite with compositions from medium-grained 2-mica granite to pegmatite, and fabrics from unfoliated to augen gneiss and schlieric granite. Fabrics in SMD granites are cut by cm-scale granite dikes. On the NW, the SMD is apparently part of the NE-SW-trending Devonian central Maine-New Hampshire migmatite belt. Thus the age of the SMD is uncertain. Ongoing work will address this unknown. Prior mapping assumes the SMD as part of the Sebago pluton, but recent mapping and geochemical data support a separation of the two. The eastern SMD is bounded on the SE by the crustal-scale NE-SW-striking dextral Norumbega shear zone system. Structure of the eastern SMB is consistent with that zone, so the eastern SMD may be key to the pluton history.

Rocks in the exposure are typical of the SMD, dominated by moderately SE-dipping medium-grained stromatic migmatite and diatexite. Granite bodies are generally cm- to m-thick, 2-mica granite to pegmatite, and concordant to moderately discordant to the fabrics in the migmatites. Leucosomes are typically concordant with the SE-dipping fabrics whereas pegmatitic augen gneiss bodies are discordant to the migmatites, but contain fabrics subconcondant to the migmatite fabrics. Migmatites also contain cm-scale pods of unfoliated granite. At outcrop scale, vertical views show coarse-grained granite apparently antiformally flanking the core migmatite. Discontinuous pavement exposure is dominantly diatextite and schlieric granite. Details of the fabrics suggest that migmatization was complete before deformation of the granite bodies making the migmatite-granite relations challenging to interpret.