Migmatite
Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks.
[2] Components exsolved by partial melting are called neosome (meaning ‘new body’), which may or may not be heterogeneous at the microscopic to macroscopic scale.
[3] These form segregations of leucosome, light-colored granitic components exsolved within melanosome, a dark colored amphibole- and biotite-rich setting.
If present, a mesosome, intermediate in color between a leucosome and melanosome, forms a more or less unmodified remnant of the metamorphic parent rock paleosome.
The resulting leucosome layers in stromatic migmatites still retain water and gas[5] in a discontinuous reaction series from the paleosome.
Bowen 1922, p184[6] described the process as being ‘In part due to … reactions between already crystallized mineral components of the rock and the remaining still-molten magma, and in part to reactions due to adjustments of equilibrium between the extreme end-stage, highly concentrated, "mother-liquor", which, by selective freezing, has been enriched with the more volatile gases usually termed "mineralizers," among which water figures prominently’.
[6] If the temperature attained only just surpasses the solidus, the migmatite will contain a few small patches of melt scattered about in the most fertile rock.
The network of channels through which the melt moved at this stage may be lost by compression of the melanosome, leaving isolated lenses of leucosome.
Conduction is the principal mechanism of heat transfer in the continental crust; where shallow layers have been exhumed or buried rapidly there is a corresponding inflection in the geothermal gradient.
[3] The melanosome is a dark, mafic mineral band formed in migmatite which is melting into a eutaxitic texture; often, this leads to the formation of granite.
[12] The coincidence of schistosity with bedding gave rise to the proposals of static or load metamorphism, advanced in 1889 by John Judd and others.
[15] A later paper of Edward Greenly in 1903 described the formation of granitic gneisses by solid diffusion, and ascribed the mechanism of lit-par-lit occurrence to the same process.
and (although it specifically included partial melting and dissolution) he considered magma injection and its associated veined and brecciated rocks as fundamental to the process.
[17] The upward succession of gneiss, schist and phyllite in the Central European Urgebirge influenced Ulrich Grubenmann in 1910 in his formulation of three depth-zones of metamorphism.
Granites were absent nearby, so he interpreted the patches and veins to be collection sites for partial melt exuded from the mica-rich parts of the host gneiss.
Sederholm later placed more emphasis on the roles of assimilation and the actions of fluids in the formation of migmatites and used the term ‘ichor’, to describe them.
Persuaded by the close connection between migmatization and granites in outcrop, Sederholm considered migmatites to be an intermediary between igneous and metamorphic rocks.
[22] Read considered that regionally metamorphosed rocks resulted from the passage of waves or fronts of metasomatizing solutions out from the central granitization core, above which arise the zones of metamorphism.
There is a close connection between migmatites and the occurrence of ‘explosion breccias’ in schists and phyllites adjacent to diorite and granite intrusions.
