CHEMICAL VARIATIONS OF BIOTITE IN MELANOSOMES FROM THE ECHINOS MIGMATITES (THRACE, GREECE)

In the anatectic in situ migmatites of the Echinos area, marginal and internal melanosomes are observed. Most of the melanosomes are margina l and occur in the form of selvages separating leucosome and mesosome layers. The melanosomes are mainly composed of biotite, quartz and fel dspars. The biotites from the Echinos melanosomes show a compositional range from Mg/(Mg+Fe) = 0.38 to 0.56, while the mutual substitution o f Mg for Fe2+ is operative. All biotites contain several octahedral va cant sites and low abundance of interlayer cations. They also present a deviation from the ideal stoichiometry of 12 to 30 % and combined ef fects of the Al- and Ti-Tschermak's substitutions. Many analyses conta in Al-VI not solely balanced by Al-IV, while the talc component does n ot exceed of 20 %. Chemical composition and substitutions comparison i n melanosomatic biotite and biotite from the adjacent unmigmatized gne isses reveals that the biotite chemistry was remained more or less una ffected during the partial melting event.