Petrogenesis of Migmatites in Maine, USA: Possible Source of Peraluminous Leucogranite in Plutons?

In Maine, Siluro-Devonian turbidites were metamorphosed under high-T–low-P facies series conditions during deformation within a Devonian crustal-scale shear zone system, defined by kilometer-scale straight belts of apparent flattening strain that anastomose around lozenges of apparent constrictional strain. At upper amphibolite facies grade, metapelites are partially melted, the onset of which is recorded by a migmatite front. The resulting migmatites are stromatic or heterogeneous, and smaller-volume granites form sheets or cylinders according to the structural zone in which they occur, suggesting that migmatites and granites record syntectonic melt flow through the deforming crust. Common leucogranite of the nearby coeval Phillips pluton, which was emplaced syntectonically, was sourced from crustal rocks with geochemical characteristics similar to those of the host Siluro-Devonian succession. Migmatites have melt-depleted compositions relative to metapelites. Leucosomes are peraluminous and represent the cumulate products of fractional crystallization and variable loss of evolved fractionated liquid. Among the heterogeneous migmatites are schlieric granites, the geochemistry of which suggests melt accumulation before fractional crystallization and loss of the evolved liquid. Smaller-volume granites are peraluminous with a range of chemistries that reflect variable entrainment of residual plagioclase and biotite, accumulation of products of fractional crystallization and loss of most of the evolved liquid. Common leucogranite of the Phillips pluton and larger granites in the migmatites have compositions that suggest crystallization of evolved liquids derived by fractional crystallization of primary muscovite dehydration melts. We infer that the leucogranite represents the crystallized fugitive liquid from a migmatite source similar to that exposed nearby. Water transported through the shear zone system dissolved in melt was exsolved at the wet solidus to cause retrogression in sub-solidus rocks and retrograde muscovite growth in migmatites.