Stability of osumilite coexisting with spinel solid solution in metapelitic granulites at high oxygen fugacity

Experiments were carried out in the quartz-saturated part of the system KFM ASH at hematite-magnetite buffer conditions in a piston-cylinder apparatus using synthetic biotite, K-feldspar, quartz, and sillimanite(80)-quartz(20) gel. Natural garnet and sillimanite were used as seeds. Three bulk composi tions were used: X-Mg = 0.81, 0.72, and 0.53, and the experiments were vapo r absent. In the bulk composition with the highest X-Mg, dehydration-meltin g of biotite produced osumilite coexisting with spinel solid solution withi n the P-T window 7-8.5 kbar and 850-1000 degreesC. However, this mineral pa ir coexisted with cordierite at P < 7.5 kbar, and with sillimanite above 7. 5 kbar. There is no appreciable change in the lower thermal stability of os umilite with lowering of X-Mg in the bulk composition, but both the upper p ressure and upper thermal stabilities are reduced. With increasing pressure , osumilite + spinel breaks down to an orthopyroxene + sillimanite assembla ge. There is no overlap of the stability fields of osumilite and sapphirine . Rather, sapphirine-bearing assemblages are produced at the expense of osu milite with increasing temperature. Sapphirine, however, did not appear in the bulk composition having X-Mg, = 0.53. Instead, a wide stability field o f spinel + cordierite + orthopyroxene + sillimanite is noted at P > 7 kbar and T > 950 degreesC. Garnet + orthopyroxene + sillimanite become stable at lower pressures with decreasing X-Mg in the bulk composition. The effects of additional components in natural assemblages, such as Zn in spinel and F and. Ti in biotite, on the KFMASH equilibria are evaluated qualitatively. The experimental results were applied to six high-grade terranes, where osu milite + spinel coexistence at peak. metamorphic conditions has been report ed. The deduced P-T window for the osumilite + spinel association is entire ly consistent with independent PT estimates for the natural occurrences. It is argued that osumilite + spinel assemblage should be characteristic of m etamorphism of highly magnesian pelites at high f(o2) along a prograde path of high dT/dP.