PINKHAM NOTCH MIGMATITE, NEW HAMPSHIRE: EVIDENCE OF HIGH MELT PERCENTAGE DEFORMATION DURING DIATEXIS, AND THE TRANSITION FROM STROMATIC MIGMATITE TO DIATEXITE

Anatectic pelitic rocks in Pinkham Notch, NH (Mt. Washington area, S of Gorham; Devonian, Central Maine belt) are part of a N-S-trending regional anatectic zone from central Maine through Massachusetts. Stromatic migmatite here are among the most impressive examples in the world, but most anatectic rocks in the area are diatexite (Wildcat granite). Outcrops in the Peabody River at the Great Gulf Wilderness trailhead have apparently layered and stromatic migmatite. Leucosome percent is higher than expected (25% or more) suggesting super-solidus conditions for a significant duration with low melt evacuation rate. Cm- and m-scale granite sheets are intrusive. Diatexite (schlieric granite) with separate, m-scale metasedimentary rock schollen are found in outcrop less than 100m to the SSW. Interestingly, the trailhead outcrops are less than 150m from the migmatite front (to the west), across which rocks are at amphibolitie facies (staurolite zone), suggesting a steep thermal gradient or a fault at the migmatite front.

Examination from a distance suggests the trailhead outcrops are stromatic migmatite with classic tripartite components. Closer inspection, however, reveals more complex structure at the m- to cm scale. The stromatic structure is not throughgoing, and the rock structure is instead defined by sub-aligned, schlieric-granite-bounded schollen of stromatic migmatite (raft-like blocks; NNE-SSW-trending).

We documented the trailhead rock structures, including schollen shapes and attitudes, internal structures, and migmatite component percent. Results show the NNE-SSW structural grain is defined by moderate schollen shape-preferred orientation, and not by the internal stromatic structures. Schollen fabrics are flattened and include augen, boudinage, and folds, none of which transect schollen boundaries. The schollen are discordant relative to each other suggesting non-cohesive deformation and passive block flow that was melt supported. We conclude these rocks are not stromatic migmatite sensu stricto, but diatexite (sensu lato) where the protolith stromatic structure is preserved inside the schollen only. These rocks differ relative to the regional diatexite in opposite proportionality of diatexite and schollen, and appear frozen in the transition from metatexite to diatexite.