THERMOBAROMETRIC CONSTRAINTS ON THE FORMATION OF SAPPHIRINE-SPINEL-PLAGIOCLASE SYMPLECTITES IN KYANITE ECLOGITES, AND THE PROGRADE AND RETROGRADE P-T PATH, CENTRAL RHODOPE MASSIF, BULGARIA

Sapphirine often occurs as a symplectite phase surrounding kyanite in eclogites, and suggests retrogression in the granulite facies. The pressure-temperature significance of this occurrence, however, is poorly understood. Textures of the symplectites indicate incomplete reactions. Based upon chemical and textural evidence, omphacite and kyanite were the likely reactants. Here we present balanced chemical reactions for the production of sapphirine-spinel-plagioclase symplectites after kyanite, and using available activity models and thermodynamic data, calculate the locus of the reactions in P-T space. Reactions in the Na₂O-CaO-MgO-Al₂O₃-SiO₂ system include:

kyanite + omphacite (Jd₆₇)=plagioclase (An₃₃) + spinel (1)

2Al₂SiO₅ + CaMgSi₂O₆ + 2NaAlSi₂O₆=CaAl₂Si₂O₈ + 2NaAlSi₃O₈ + MgAl₂O₄

kyanite + omphacite (Jd₆₀)=plagioclase (An₄₀) + sapphirine (2:2:1) (2)

8Al₂SiO₅ + 4CaMgSi₂O₆ + 6NaAlSi₂O₆=4CaAl₂Si₂O₈ + 6NaAlSi₃O₈ + Mg₄Al₈Si₂O₂₀

kyanite + omphacite (Jd₇₀)=plagioclase (An₃₀) + Al-sapphirine (3:5:1) (3)

8Al₂SiO₅ + 3CaMgSi₂O₆ + 7NaAlSi₂O₆=3CaAl₂Si₂O₈ + 7NaAlSi₃O₈ + Mg₃Al₁₀SiO₂₀

Inspection of these reactions indicates that a relict low-Na clinopyroxene (diopside-hedenburgite-CaTs solid solution) should result. Sapphirine analyses yield high Al contents (Si_1.2 for 20 oxygens). These reactions were calculated using THERMOCALC to obtain information on the late P-T conditions required by formation of sapphirine and spinel with plagioclase. Estimates of the P-T conditions of symplectite formation are in the range of 700ºC at 12.6 kbar up to 800ºC at 14.5 kbar, and possibly higher. These P-T are transitional between the high-pressure granulite and eclogite facies, and suggest that sapphirine symplectites form at high pressures following the formation of the eclogite facies assemblage. Both kyanite and garnet cores contain numerous inclusions of quartz, rutile, apatite, epidote, and hornblende, and likely record prograde amphibolite facies conditions. Inclusions in garnet become sparser in the rims and change in mineralogy to the eclogite assemblage (omphacite (Jd₄₂), kyanite, quartz, and rutile). Omphacite is present only as inclusions in garnet. P-T conditions of the eclogite assemblage are T=~650-675ºC and P > 14 kbar. Matrix hornblende and plagioclase record the final amphibolite facies assemblage during retrogression.