STRUCTURAL AND MINERALOGICAL VARIATIONS ASSOCIATED WITH THE SOUTH-WESTERN CONTACT OF THE SEBAGO PLUTON WITH THE SEBAGO MIGMATITE DOMAIN, SW MAINE: RESULTS FROM NEW MAPPING

Significant mineralogical variation and structural complexity is revealed by new mapping in the western Sebago migmatite domain of Maine. The domain encompasses the homogeneous L. Pennsylvanian 2-mica Sebago pluton (~ 400 km²). Work was conducted on rocks in the area near/along the pluton's SW contact in order to complement work done on the pluton east contact with the domain (e.g., LaFleur et al., this volume).

Petrologic variation in the migmatite domain is dominated by high-grade meta-pelitic schists and gneisses to migmatites including diatexites. Tourmaline crystals are penetrative and large in the migmatites, ~1-4 cm, and show no preferred orientation in areas of largest structural complexity and/or highest apparent degrees of migmatization. They do define, however, strong lineations locally, but only where they are < 5 mm. Throughout the area pegmatitic 2-mica to Kfs-rich granitic rocks are typically associated with cross-cutting granitic bodies, but are locally sub-concordant to mica+Sil foliations in migmatites. Some pegmatites with no apparent fabrics are m-long veins (a few cm thick), and some larger, more irregular bodies contain Grt up to 2 cm.

Structures vary on the km scale where foliations within 2 km of the SW contact of the pluton range from very steep to shallow dips, and with no apparent pattern in map view. However a weak correlation is apparent on stereograms when grouping stations into 1 km wide domains (N-S-trending). In the domain in contact to the pluton (~ 2 km wide, domain 1), the pattern suggests a general shallowly plunging WNW-trending foliation fold-hinge line system, consistent with field data, where open to tight m to 10 m scale foliation folds are found with shallowly plunging hinge lines to the WNW-ESE. Domain 1 mineral lineations are local, but have similar orientation to the hinge lines. In contrast, structures in the next domain to the W (~ 2 km wide, domain 2) show a more N-S trend to foliation strikes. The next domain to the W (~ 3 km wide, domain 3) shows distinctly greater complexity featuring a distinctive crenulation cleavage (1-2 cm scale) with local shear bands (10-15 cm wide). Domain 3 rocks differ also by their strongly residual composition suggesting a high degree of melt loss during migmatization in the domain. We interpret these variations to reflect pluton emplacement into the migmatites.