Northeastern Section - 37th Annual Meeting (March 25-27, 2002)

Crustally derived granites are important probes of the unexposed cores of orogens. Although the structural record may be reconstructed in the field, questions remain regarding the ascent mechanisms, and the timing of magmatism vs. deformation and metamorphism. In southern Maine a variety of magmatic and deformational textures characterize the diverse granites that crop out within and adjacent to the Norumbega shear zone system (NSZS). Existing regional maps do not address relations between granites of various scales and their host structures. Field evidence for synkinematic transport and emplacement of granite is abundant, hence we seek to define the extent to which regional metamorphism and deformation accompanied granite magmatism, and whether these processes were phased or diachronous. In order to consider such problems appropriately, we integrate thorough field and structural analysis with precise geochronology and isotopic/elemental geochemistry.

We focus on a ~70 km across-strike transect from the core of the Sebago pluton (ca. 293 Ma) SE into the highest strain portion of the NSZS in its deepest regional exposure near Brunswick. New mapping of the pluton, previously considered the largest exposed in New England (>1600 km²), shows a clearly separate central body of homogeneous granite (<400 km²) derived from Avalon-like sources. The body is flanked by a migmatite domain permeated by smaller bodies of more geochemically heterogeneous granite (the Sebago migmatite domain). Extant geochemical data are consistent with derivation of these subconcordant granites from materials similar to their host metasedimentary rocks (Central Maine belt source). In the migmatite domain, deformational fabrics in amphibolite facies country rocks are relatively variable in orientation. However, the area SE of the pluton, adjacent to the NSZS, contains penetratively parallel structures at all scales, including fabrics in metamorphic rocks, migmatites and concordant granite sheets. Fabrics are consistently NE-SW-striking- and steeply to moderately SE-dipping, similar in orientation to those in the NSZS suggesting control of granite ascent by deformation in the NSZS. Within this well-constrained structural framework, geochemical and geochronological studies can provide meaningful tectonic constraints.