PETROGENESIS OF ACADIAN METAPELITIC MIGMATITES, WESTERN MAINE - QUANTITATIVE INVESTIGATIONS IN THE NCKFMASH MODEL SYSTEM

In Maine, Siluro-Devonian turbidities were metamorphosed under high-T-low-P facies series conditions during deformation within a Devonian crustal-scale shear zone system. At upper amphibolite facies grade, metapelites were partially melted, the onset of which is recorded by a migmatite front. The resulting migmatites are stromatic or heterogeneous, and have melt-depleted compositions relative to the inferred metapelitic protolith. Leucosomes are peraluminous and represent the cumulate products of fractional crystallization and variable loss of an evolved fractionated liquid. Small peraluminous granites have a range of chemistries that reflect variable entrainment of residual plagioclase and biotite, accumulation of products of fractional crystallization and loss of evolved liquid. Common leucogranite of the Phillips pluton has compositions that suggest crystallization of evolved liquids derived by fractional crystallization of the liquid products of volatile phase absent muscovite dehydration melting of a source similar to the migmatites. Late muscovite growth in the migmatites could be due to retrograde reaction with melt or water exsolved at the wet solidus may have caused both late muscovite growth and retrogression in sub-solidus rocks. To investigate progressive melting and the relative importance of different processes during the retrograde evolution in the migmatites, including the effects of melt loss and water-rich volatile phase exsolution during crystallization of melt, we use quantitative P-T and T-X pseudosections constructed for representative Central Maine Belt metapelite and residual migmatite bulk compositions. Melting relations and stabilities of assemblages for water-saturated and -undersaturated bulk compositions are calculated with the software THERMOCALC and the internally consistent thermodynamic data set of Holland and Powell (1998, J. Metamorphic Geol.) in the NCKFMASH model system, using the thermodynamic model for silicate melt from White et al. (2001, J. Metamorphic Geol.). The degree of preservation of peak mineral assemblages and the extent of retrogression are controlled by combinations of the amount of melt lost from the system, the distribution of residual melt within the system and the decreasing size of the equilibration volume as T declines.