Petrogenesis of migmatites in Maine, USA: Possible source of peraluminous leucogranite in plutons?

In Main, Siluro-Devonian turbidites were metamorphosed under high-T-low-P f acies series conditions during deformation within a Devonian crustal-scale shear zone system, defined by kilometer-scale straight belts of apparent fl attening strain that anastomose around lozenges of apparent constrictional strain. At upper amphibolite facies grade, metaphelites are partially melte d, the onset of which is recorded by a migmatite front. The resulting migma tites are stromatic or heterogeneous, and smaller-volume granites from shee ts or cylinders according to the structural zone in which they occur, sugge sting 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 ge ochemical characteristics similar to those of the host Siluro-Devonian succ ession. Migmatites have melt-depleted compositions relative to metapelites. Leucomes are peraluminous and represent the cumulate products of fractiona l crystallization and variable loss of evolved fractionated liquid. Among t he heterogeneous migmatites are schlieric granites, the geochemistry of whi ch 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 an d biotite, accumulation of products of fractional crystallization and loss of most of the evolved liquid. Common leucogranite of the Phillips pluton a nd large granites in the migmatites have compositions that suggest crystall ization 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 expos ed nearby. Water transported through the shear zone system dissolved in mel t was exsolved at the wet solidus to cause retrogression in sub-solids rock s and retrograde muscovite growth in migmatites.