THE STABILITY OF OSUMILITE IN METAPELITIC GRANULITES

A petrogenetic grid and related diagrams derived from KFMASH-system ex periments demonstrate that osumilite is stable in relatively magnesian bulk rock compositions (X(Mg) > 0.6) at temperatures in excess of 875 degrees C and pressures less than 11 kbar. The experiments, involving the dehydration melting of biotite in synthetic metapelites, were con ducted in the range 850-1000 degrees C. Both the mineral assemblages a nd phase compositions reported from well-documented natural examples o f osumilite-bearing rocks are reproduced by the experiments at P-T con ditions similar to those previously estimated for these occurrences. P eak metamorphic P-T conditions can be reliably inferred from distincti ve osumilite-bearing assemblages identified in the phase diagrams, the reby avoiding the problems of diffusional re-equilibration that often prohibits conventional geothermobarometry from recovering peak conditi ons. Integration of the experimental data with recent independent expe riments, after correcting the latter for an underestimated friction co rrection, allows extension of the petrogenetic grid to higher temperat ures. The extended grid is applied to assess and refine the metamorphi c history of the Napier Complex, East Antarctica: the high-P stability limit for osumilite in the Napier Complex is 9-10 kbar, the prograde P-T-t path is not necessarily anticlockwise and isobaric cooling in th e Scott and Tula mountains occurred, respectively, at pressures greate r and Less than reactions in the range 8-9 kbar. The stability range f or osumilite predicted by the KFMASH-system petrogenetic grid overlaps many more metamorphic terranes than osumilite is found in. Whilst osu milite is not distinctive in thin section and is prone to retrogressio n, it is possible that carbon dioxide present in the natural system st abilizes cordierite at the expense of osumilite.