STRUCTURAL, TEXTURAL AND PETROGRAPHIC VARIATIONS IN ROCKS ON BRUCE HILL, EASTERN SEBAGO MIGMATITE DOMAIN, SOUTHERN MAINE

Part of a larger multi-disciplinary study of the rocks of the N Appalachian migmatite-granite belt, this study is one of several concurrent and related projects that together form a database of fine-scale documentation of the Sebago migmatite domain (SMD) and related rocks in S Maine and New Hamphire. For this work, we document variations of mineral content, structures and fabric at the m-scale in rocks of Bruce Hill, Cumberland Ctr, ME. Bruce Hill is in the center of the E part of the SMD that mantles the 400 km² Permian Sebago granite pluton on its W, N and E.

The SMD is in the NE-SW-trending Devonian central Maine-New Hampshire migmatite belt, and is defined by metapelitic stromatic migmatite and diatexite. Granite bodies are local, cm- to m-scale, and range from medium-grained 2-mica granite to pegmatite, with mineral textures ranging from no fabric to schlieric granite and augen gneiss. Prior mapping grouped the SMD and the Sebago pluton together, but age in the SMD is uncertain because more recent mapping and geochemical data support a distinctive SMD-pluton separation. The SMD on the SE is within the crustal-scale NE-SW-striking dextral Norumbega shear zone system; structure of the E SMB is consistent with that zone.

Rocks at Bruce Hill are of 5 SMD-typical units; semipelitic schist, stromatic migmatite (locally gneissic), 2-mica granite, pegmatite and granitic augen gneiss. Where fabrics exist, they are consistently 048-055-striking and moderately SE-dipping throughout the exposure. Augen gneiss with S-C fabrics dominates the hill's S slope. The SW slope is migmatite with Bt melanosomes and concordant cm-scale leucosomes. Pegmatite is subordinate on the SW, but dominates the top of the hill. The N slope is Grt-Tm-bearing pegmatite that has increasing grain size with elevation. The E and NE slopes are dominantly semipelitic schist with discordant cm-scale granitic veins and subordinate migmatite and granite.

Questions from Bruce Hill data involve the significance of two granites, one an S-C augen gneiss, and one a 2-mica granite with no apparent solid-state fabric (similar to that of the Sebago pluton to the W). Since the Sebago pluton is Permian, and the country rock is Devonian in the migmatite belt, the granitic rocks could be different in age by ~100 Ma, the augen gneiss recording deformation in the Norumbega system.