High fluorine pargasites in ultrahigh temperature granulites from Tonagh Island in the Archean Napier Complex, East Antarctica

Tsunogae, T., Osanai, Y., Owada, M., Toyoshima, T., Hokada, T., and Crowe, W.A. (2003): Lithos, 70, 21-38.


Abstract

Pargasites (F/(F+Cl+OH) ratio (XF) of up to 0.48) from Tonagh Island in Enderby Land, East Antarctica, are closely associated with typical high-grade minerals such as orthopyroxene in quartzo-feldspathic, mafic, and ultramafic granulites, and is regarded as a stable mineral at the peak metamorphic conditions (>1100°C) calculated for the ultrahigh-temperature Archean Napier Complex. Although experimental investigations have suggested that the upper thermal stability limit of F-free pargasite is below 1050°C, thermodynamic calculations for the present pargasite + quartz assemblage indicate that the thermal stability limit of pargasite with XF=0.5 is about 150°C higher than that of the hydroxyl end member. Fluorine substitution in the pargasite therefore allowed the mineral to survive the ultrahigh-temperature metamorphism at Tonagh Island.

A positive correlation between the F content of pargasite and coexisting biotite indicates that the minerals approach chemical equilibrium in terms of F-OH distribution. Although the fluorine composition of pargasites (XF = 0.12-0.48) and bulk rock (300-2500 ppm) varies widely, the log(fH2O/fHF) values calculated for these rocks are relatively constant (3.2-3.7), which is consistent with infiltration of a F-bearing fluid during prograde metamorphism. The infiltration of such a fluid is also supported by the higher bulk F content of most of the analyzed samples compared to those of continental and oceanic basaltic rocks, that is, F had been added from an external source. A positive correlation between bulk MgO and F content suggests that F may have been selectively trapped in high-XMg pargasite in MgO-rich rocks.