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1 Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10W8, Kita-ku, Sapporo, 060-0810, Japan
2 Department of Geological Sciences, Jadavpur University, Calcutta, 700 032, India
Experiments were carried out in the quartz-saturated part of the system KFMASH at hematite-magnetite buffer conditions in a piston-cylinder apparatus using synthetic biotite, K-feldspar, quartz, and sillimanite80-quartz20 gel. Natural garnet and sillimanite were used as seeds. Three bulk compositions were used: XMg = 0.81, 0.72, and 0.53, and the experiments were vapor absent. In the bulk composition with the highest XMg, dehydration-melting of biotite produced osumilite coexisting with spinel solid solution within the P-T window 7–8.5 kbar and 850–1000 °C. However, this mineral pair 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 osumilite with lowering of XMg in the bulk composition, but both the upper pressure and upper thermal stabilities are reduced. With increasing pressure, osumilite + spinel breaks down to an orthopyroxene + sillimanite assemblage. There is no overlap of the stability fields of osumilite and sapphirine. Rather, sapphirine-bearing assemblages are produced at the expense of osumilite with increasing temperature. Sapphirine, however, did not appear in the bulk composition having XMg = 0.53. Instead, a wide stability field of spinel + cordierite + orthopyroxene + sillimanite is noted at P > 7 kbar and T > 950 °C. Garnet + orthopyroxene + sillimanite become stable at lower pressures with decreasing XMg 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 osumilite + spinel coexistence at peak metamorphic conditions has been reported. The deduced P-T window for the osumilite + spinel association is entirely consistent with independent P-T estimates for the natural occurrences. It is argued that osumilite + spinel assemblage should be characteristic of metamorphism of highly magnesian pelites at high fO2 along a prograde path of high dT/dP.
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