THE RECORD OF GRANITE MAGMA FLOW THROUGH DEFORMING ROCKS, COMPARATIVE METER-SCALE MAPPING OF CONTRASTING TYPES OF MIGMATITE IN WESTERN MAINE, USA

Evidence for granite magma flow during transpression may be observed indirectly by examining textures and contact relations between migmatites and associated granite bodies found in the cores of ancient orogenic belts. In the northern Appalachians, migmatite in western Maine is interpreted to have formed syntectonically during the Devonian. Previous work has shown that granite magma transfer was coeval with migmatization, supported by (1) the positive correlation between the geometries of both mineral fabrics in the migmatite and the granite bodies with geometries that define the regional structure, and (2) regional age data. Within the regional structure, two types of migmatite correspond with two zones of contrasting finite strain (apparent flattening v. constriction). Accordingly, one type has a strong planar fabric (foliation-dominated; S >L) and concordant cm- to meter-scale sheet-like granite bodies, whereas the other type has a strong linear fabric (lineation-dominated; L > S) and cm- to meter-scale cylindrical granite bodies. Since each type corresponds with the regional structure, these data are used to suggest control of the deformation on the shape of the syntectonic granite flow network. As a test of this hypothesis, meter-scale mapping of 2 ~300 m² pavement outcrops, representing each type of migmatite, reveal intimate relations between fabrics and the shapes of the associated granite bodies. Mineral fabric of the L > S migmatite tends to wrap around meter-scale cylindrical granite bodies, two of which dominate the outcrop (5 to 7 m in diameter) with the long dimensions plunging sub-vertically concordant with the penetrative matrix linear fabric. The foliation is variable, however, showing higher intensity where granite is less dominant. Four foliation attitudes are identified to illustrate a wavy pattern typical of rocks of this type. In contrast, S > L migmatites are structurally planar and sheet-like. However, similar to L > S migmatite, fabrics in the host rock tend to wrap around the granite bodies, but in a planar fashion. In the context of the regional structure, the strong concordance shown by our mapping, regardless type of migmatite, supports the interpretation that deformation apparently controlled granite flow through the crust by controlling the shape of the plumbing at all scales.