MINERALOGICAL AND TEXTURAL VARIATIONS IN THE MIGMATITE-GRANITE COMPLEX NEAR THE WESTERN CONTACT OF THE SEBAGO PLUTON, SW MAINE

Mapping in the western part of the migmatite-granite complex at its contact with the Sebago pluton in SW Maine (Kalczynski and Solar, NEGSA08) showed significant mineralogical variation and structural complexity. Field relations show the area rocks are complexly deformed migmatites and granites that are challenging to understand structurally, or to place within a coherent regional structure. Building on the previous mapping, we analyzed collected specimens for mineralogical and textural variation in order to better understand field relations.

Analyzed specimens are mostly migmatitic meta-pelitic schist, of both stromatic migmatite and diatexite types. Metamorphic and migmatitic fabrics are defined by strongly-developed mica+Sil foliations. Leucosomes are clearly distinct and separate in stromatic migmatites, with biotite+garnet+sillimanite melanosome selvedges. Other rocks are categorized into three groups. The first group is granite of the Sebago pluton (from its western contact), a homogenous two-mica granite that is typically medium grained with a mosaic texture. The second group is meta-psammitic rocks, bodering on semi-pelite. These are defined by their high quartz and plagioclase compositions. Fabrics are made obvious in hand specimen by disjunctive mica foliations. The third group is the biotite-rich meta-psammite. Biotite fabrics are penetrative, and strongly planar with weak to no lineation. Microstructures indicate penetrative solid-state fabrics in all samples, consistent with hand-specimen and field observations. From laboratory analysis, field relations are augmented and clearer, however field relations remain challenging to interpret structurally, but are better constrained on a mineral texture basis.