Fluorine-rich calcic amphiboles in ultrahigh-temperature mafic granulite from Tonagh Island in the Napier Complex, East Antarctica: Preliminary report.

Tsunogae, T., Osanai, Y., Toyoshima, T., Owada, M., Hokada, T., and Crowe, W.A. (2000)
Polar Geoscience, 13, 103-113.


Abstract

Fluorine-rich calcic amphiboles occur in ultrahigh-temperature mafic granulite from Tonagh Island in the Napier Complex, East Antarctica. The amphiboles are subdivided into two types: high-grade brownish to pale brownish amphibole and retrograde greenish amphibole. The brownish amphibole is pargasitic occurring as a subhedral grain in two-pyroxene mafic granulite. It contains up to 2.2 wt.% fluorine, which corresponds to an F/(F+Cl+OH) ratio of 0.54 (sample B98020801). In sample B98020802B, brownish amphibole is rimmed by an orthopyroxene-plagioclase-quartz corona, probably formed by a devolatilization reaction. It has a lower fluorine content of 0.21-0.52 wt.% (F/(F+Cl+OH)=0.04-0.13). Pale brownish amphibole is a minor phase in metapyroxenite. It has the highest fluorine content (2.6 wt.%; F/(F+Cl+OH)=0.60; sample B98012802F). Greenish amphibole is present as fine-grained aggregates with quartz, rimming ortho- and clinopyroxenes. Fluorine is almost absent in this amphibole.

Holloway and Ford (1975) experimentally determined that pargasite with a F/(F+Cl+OH) ratio of 0.48 is stable up to 1100°C at 5 kbar, and that F/(F+Cl+OH) increases with increasing temperature. The fluorine-rich calcic amphiboles in mafic granulites are therefore stable under UHT metamorphic conditions. The formation of an orthopyroxene-plagioclase-quartz corona in sample B98020802B can therefore be explained by breakdown of the amphibole due to its low-fluorine content. The origin of the fluorine is not known, but it is probably derived from basic magma at the stage of protolith formation.