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The stability of osumilite in high grade metamorphic rocks

✍ Scribed by B. J. Hensen

Publisher
Springer
Year
1977
Tongue
English
Weight
458 KB
Volume
64
Category
Article
ISSN
0010-7999

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✦ Synopsis

Osumilite, approximate composition K(Mg,Fe)2 A15SiloO3o, has been reported recently from two granulite localities. The mineral has been synthesised in a model pelitic composition at 1000 and 1100 ~ C and 3.6-6.3 kb under conditions of low water and oxygen fugacity. Osumilite coexists, apparently stably, with hypersthene, cordierite and quartz(?) thus duplicating the mineral assemblage of one of the natural occurrences. Osumilite is in a divariant reaction relationship with cordierite and hypersthene i.e. osumilitecordierite +hypersthene + orthoclase +quartz. This reaction runs to the right with increasing pressure. Experimental data and field observations suggest that the joins osumilite-garnet and osumilite-sillimanite are not stable. It is suggested osumilite is involved in an invariant point in the system K20-MgO-FeO-A1203 -SiO 2 with the phases cordierite, hypersthene, sapphirine, spinel, orthoclase and quartz. The invariant point should occur at 1000 _+ 100 ~ C and 7 +_ 2 kb.

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Coexisting Na-plagioclases from greenschists both in the thermal aureole of the Kasugamura Granite, Japan, and in the low-P metamorphic zone of Yap Island, western Pacific were analyzed in great detail; the peristerite solvus was determined for each suite. The asymmetric solvus has steep albite-rich