THE TRANSITION OF METATEXITE TO DIATEXTITE MIGMATITE RECORDED AT MT. WASHINGTON, NEW HAMPSHIRE

Devonian pelitic migmatites in the Mt. Washington area of NH (S of Gorham; Central Maine belt) are part of a NE-SW-trending regional migmatite belt from central Maine through Massachusetts. Regional mapping delineated km-scale NNE-SSW belts of migmatite classes, including stromatic (layered) and dilation-structured metatexite migmatite, schollen (raft) diatexite migmatite, and schlieric diatexite migmatite (including the Wildcat “granite”). Previous work suggested static and post-tectonic migmatite formation overprinted on structures formed at pre-metamorphic to low-grade metamorphic conditions.

Our work reveals a more progressive history where migmatite formation was syn-tectonic during the Devonian Acadian Orogeny consistent with migmatites along strike in Maine. Migmatites are complex in appearance, including m-scale schollen that are either (1) isolated paleosome pods within the host schlieric diatextite migmatite, or (2) dominate the outcrops as the primary component. The latter is best exposed in Peabody River, Great Gulf Wilderness trailhead, where some schollen are rootless paleosomes without migmatite formation textures, but dominantly, they are a network of m-scale internally deformed stromatic- to dilatant-structured metatexite migmatites. Schollen boundaries are neosome-mantled, across which stromatic structures are discontinuous and discordant (5°-85°). Schollen were apparently melt-supported during passive block rotation at super-solidus conditions for a significant duration with apparently low melt evacuation rate. The result was protracted weak rheological behavior. This is the essence of diatexis where, in this example, the schollen diatexite migmatite formed with the breakup of stromatic structures into blocks, and where the schollen preserve the metatexite protolith within. By definition, metatexite migmatite is no longer present larger than m-scale.

Migmatites at Mt. Washington have recorded progressive transition from metatexite to diatexite migmatite in schollen-dominated outcrops whereas schlieric diatexite migmatite completed the process. The petrogenetic difference between schollen diatexite and schlieric diatextite migmatites suggests the schollen diatexite migmatite protolith had relatively stronger planar structure.