Anatexis in K-poor and Si-rich migmatites

K-poor and Si-rich pelites are potential protoliths of migmatites, but often lack clear mineralogical indicators for anatexis. This is due to the limited number of peritectic phases, and because the products of melting reactions tend to be erased by retrograde deformation and recrystallization. Thus the pre- and post-peak assemblages may be almost identical and evidence for migmatization is overlooked. The Roded migmatites of southern Israel were previously thought to form by subsolidus metamorphic differentiation based on the lack of K-feldspar in leucosomes and similarity of plagioclase compositions in coupled leucosomes and melanosomes. Geochemical analysis of carefully sampled palaeosomes shows that the protoliths of Roded migmatites were K-poor and Si-rich metapelites. Nonetheless relict textural evidence for melting is found in the Roded migmatites including quartz-filled embayments in plagioclase crystals, and newly formed euhedral crystals of plagioclase in leucosomes. Likewise lenticular K-felsdpar occurs within melanosome biotite indicating muscovite dehydration melting. Thermodynamic analysis of specific palaeosome compositions show that anatexis should have occurred at peak P-T conditions estimated for the Roded migmatites (Fig.1; 4.5 kbar; approximately 650 degrees C). Petrographic and microstructural observations suggest that potassium was mobilized from the leucosomes to the melanosomes during melt crystallization: Leucosome K-feldspar was replaced by myrmekite and melanosome sillimanite was replaced by symplectites of muscovite and quartz. Wider neosomes are characterized by diatexitic microstructures and by lower amount of myrmekite suggesting channeled influx of water resulting in higher degree of anatexis and dilution of potassium in the melt. K-feldspar and muscovite+quartz symplectites in melanosomes, and myrmekite in leucosomes are established as criteria for inferring partial melting in heavily overprinted and retrogressively deformed K-poor, Si-rich migmatites.