THE WILDCAT “GRANITE,” NEW HAMPSHIRE: PLUTON OR DIATEXITE?

Meta-sedimentary rocks, stromatic migmatite, and diatexite associated with the Peabody River Valley, New Hampshire (Mt. Washington and Wildcat areas), record syntectonic Devonian metamorphism, migmatite formation and granite magmatism. Some uncertainty is associated with the origin of the Wildcat “granite.” Previous work suggests it is a pluton. However, on inspection field relations and rock textures permits the interpretation that the body is diatexite, a result of a high melt fraction present during deformation. New mapping was performed to document textures in the “granite” to test these ideas.

Wildcat rocks are adjacent to stromatic migmatite to the west. Beyond is the migmatite front, and in turn the proposed protolith belt of sillimanite zone schists. Structural and belt grain is N-S. The contact between the Wildcat rocks and the stromatic migmatite is not sharp most likely due to the Wildcat rocks containing shlieric tonalite, with biotite schlieren and calc-silicate schollen throughout. Internally, schollen resemble the sillimanite zone rock structure.

The area of study is within and E of the Peabody River that roughly follows the belt trend. Mapping was dominantly traverse mapping. At each station, trend/plunge of long axes and size of biotite schlieren and calc-silicate schollen were collected and recorded. A base map for the Peabody River Valley was overlayed with orientations of structures in Wildcat rocks, allowing for approximate transitional boundaries between meta-sedimentary rock, migmatite, and the Wildcat rocks to be located. Plots of structural data show a preferred orientation trending to 016. Thin sections were cut according to fabrics for petrographic analyses of microtextural and mineralogical variations. Results show schlieren represent material entrained during melting, and schollen represent unmelted portions of host rock that flowed while rocks were liquid-supported. At all scales of observation, the Wildcat rocks record an origin of diatexis rather than plutonism.