MIGMATIZATION IN PIEDMONT ROCKS, SOUTHWESTERN MARYLAND AND NORTHERN VIRGINIA

Models of melt ascent in a contractional setting (Brown et.al., 1998) predict channelized melt migration by sheet-like flow and a large volume of residual rocks left behind. Field and textural evidence in metamorphic rocks along the Potomac River suggests that both the Mather Gorge and Sykesville Fms. have been subjected to partial melting. At Great Falls, relict microtextures in sillimanite-bearing migmatitic schist, including the occurrence of myrmekite, trains of feldspar subgrains, and old, ribbon quartz grains with recovery textures are suggestive of high-T deformation and consistent with the conclusion of Fisher (1970) of peak conditions near 650 C. Texturally, leucosomes (interpreted to represent former melt) are characterized by an increase in grain size, especially of feldspar, relative to the host, and coalescence of qtz+plag±kfsp into pods of more randomly oriented grains with disruption and fragmentation of host schistosity. In nearby sheared greenschists relict cumulate textures are observed in pods of leucosome as close-spaced coarse idioblastic feldspar with interstitial quartz and opaques that are interpreted as intercumulus phases. Melt textures are synchronous with peak metamorphism, are pre- or syn-tectonic with shearing, and pre-date retrogression.

Concordant to weakly discordant, tabular, white to pink cm- to metric-scale sheets occur parallel to early penetrative S_a schistosity, and are characterized by flow-banding and inclusions of boudinaged wall rock. Discordant bodies occupy high-strain zones including the axial planes of later upright F_b folds and C-surfaces of shear zones. Diffuse nebulous patches of leucosome occur along the sheared limbs of F_b and overprint earlier tabular sheets. Leucosomes also occupy low-strain pockets: in extension fissures and noses of F_b , and gaps between fragments of pulled-apart host rock. Large bodies of dark-banded saussuritized rock are interpreted to be sheared residual diatexite. Field observations suggest that migmatization involved in-situ melting, intrusion, and injection in a contractional deformation setting. It is proposed that Kfsp-absent leucosomes are residual rocks depleted in Kfsp by deformation followed by melt extraction.