Several reaction-zones occur in the contact between impure calcite marble and corundum-bearing high-grade metapelite in the basement, Alboran Sea (site 976, ODP Leg 161, western Mediterranean). From the metapelite (corundum + biotite + anorthite + K-feldspar + or - garnet + or - sillimanite) to the marble (calcite + anorthite + biotite + amphibole + clinopyroxene), the sequence of mineral assemblages in these reaction zones is: corundum + hercynite + biotite + anorthite + K-feldspar (Zone I), hercynite + biotite + anorthite + K-feldspar (Zone II), biotite + garnet (Grs (sub 15-30) ) + anorthite (Zone III), amphibole + biotite + garnet (Grs (sub 25-33) ) + anorthite + K-feldspar (Zone IV), anorthite + clinopyroxene (Zone V), and clinopyroxene + garnet (Grs (sub 49-65) ) + anorthite (Zone VI). These reaction zones are interpreted as having developed by diffusion metasomatism of Al (sub 2) O (sub 3) , CaO, and SiO (sub 2) between the marble and metapelite at 705<T<730 degrees C and 4.3<P<5.1 kbar. Corundum grew by a muscovite-breakdown reaction in the absence of quartz, at T>650 degrees C and P<5 kbar. Phase-relation analysis with chemical potential projections (mu (sub Al2O3) -mu (sub SiO2) -mu (sub CaO) ) is consistent with the observed sequence of reaction zones, the mineral assemblages, and the observed variation in the chemical composition of garnet, hercynite, and biotite among the different zones. The behavior of the three diffusing components is characterized by a slight mean variation in mu (sub SiO2) and a strong variation in mu (sub Al2O3) and mu (sub CaO) between the metapelite and the calcite marble. In spite of this, strong gradients in chemical potential are deduced for SiO (sub 2) and Al (sub 2) O (sub 3) through Zone III. In our opinion, this zone was formed under non-ideal conditions of diffusion owing to the heterogeneous character of the protolith, derived partly from a corundum metapelite and partly from a quartz-bearing metapelite.