Incongruent melting of biotite to spinel in a quartz-free restite at El Joyazo (SE Spain): Textures and reaction characterization

In the Grt-Bt-Sil restitic xenoliths of El Joyazo (Cerro de Hoyazo), hercynitic spinel is a minor phase commonly associated with biotite. The possible reaction relationships among biotite and spinel are studied in reaction textures developed around biotites at their contact with patches of fibrolitic sillimanite and rhyolitic melt. In these textures, resorbed biotite crystals about 1 mm long are rimmed by a layer of glass < 200 mu m thick containing spinel and ilmenite; the same glass also fills embayments in biotite. Spinel forms euhedral crystals < 100 mu m in size, and ilmenite occurs as smaller anhedral crystals or needles, often intergrown with spinel. The homogeneous felt-like melt-sillimanite aggregate ("mix") is richest in glass close to the reaction rim around biotite. Plagioclase and garnet are located >5 mm away from the reaction texture. Biotite is chemically zoned. Cores (Bt(1)) have X-Mg = 0.35 +/- 0.02 and Ti = 0.58 +/- 0.01 atoms; whereas the outer rims (Bt(2)) have X-Mg = 0.45 +/- 0.01 and Ti up to 0.68 atoms. The hercynite-rich spinel (Spl) has low ZnO content (<0.80 wt%) and X-Mg = 0.26 +/- 0.04. The chemical compositions of the mix aggregate represent linear combinations between sillimanite and a silica-rich melt. This melt (melt(1)) is different from that of the layer around biotite (melt(2)), which is also richer in Ca and alkalis. Garnet rims (Grt) have low Ca and Mn, and X-Mg = 0.14. Plagioclase is characterized by large homogeneous cores (Pl(1), An(31) +/- 2) and more calcic rims (Pl(2), An(49) (+/- 6)). Matrix analysis in the 9-component (Al-Ca-Fe-K-Mg-Mn-Na-Si-Ti), 9-phase (Bt(1)-Bt(2)-GTt-Spl-Ilm-melt(2)-mix-Pl(1)-Pl(2)) system provides the mass balance (in mole units): 0.78Bt(1) + 1.95Pl(1) + 2.82mix + 0.32Grt = 0.25Ilm + 5.00melt(2) + 0.30Bt(2) + 2.53Spl + 1.03PL(2) This relationship is in excellent agreement with the observed textures and hence is considered a good model for the incongruent melting of biotite in the xenoliths. The mass-balance indicates that melt production is dominated by the availability of K from biotite, and that garnet and plagioclase must be involved as reactants, so that the reaction volume is larger than the melt production site. The melting of biotite, constrained at T = 900-950 degrees C and P greater than or equal to 5 kbar, is not a terminal reaction, as its variance in the reduced X-component multisystem is greater than or equal to 3.