F- and Cl-bearing Calcic Amphiboles in Mafic Granulite from Tonagh Island in the Napier Complex, East Antarctica: Implications for Stability of Amphibole during UHT Metamorphism

Tsunogae, T., Osanai, Y., Toyoshima, T., Owada, M., Hokada, T., and Crowe, W.A. (2000)
EOS, Vol. 81, No. 22, V31A-04.


Abstract

The Napier Complex of Enderby Land, East Antarctica, is an example of late-Archean (ca. 2.5 Ga) high-grade terrane which has suffered granulite-facies metamorphism at temperature higher than 1000°C. Amundsen Bay area located in the western part of the Napier Complex is known as the highest grade region of the complex where Spr-Qtz and osumilite-bearing mineral assemblages are common. Tonagh Island, which is located at the southern end of Amundsen Bay, is composed mainly of layered gneisses of various rock chemistry (e.g. quartzofeldspathic gneiss, Grt-gneiss, Grt-Opx gneiss, mafic to ultramafic gneisses, and Mag-Qtz gneiss).

Mafic granulites are present as layers of about several cm to 100 m in thickness within the gneisses. They are composed mainly of Opx, Cpx, Pl, Qtz with or without Ilm, Bt, Mag, and Grt. Calcic amphibole is a common hydrous mineral in UHT mafic granulite of Tonagh Island. They are texturally subdivided into two types; high-grade brownish or colorless amphibole and retrograde greenish amphibole.

Brownish amphibole in sample B98020801 is present as coarse-grained and subhedral mineral coexisting with Opx, Cpx, and Pl. It is compositionally pargasite and characterized by high Na+K content up to 1.0. Its F content is up to 1.60 wt.% which corresponds to F/(F+Cl+OH) ratio of 0.40. Calcic amphibole in Opx-Cpx mafic granulite (sample B98012802F) shows the highest F content up to 2.62 wt.% (F/(F+Cl+OH)=0.60). The F content has positive correlation with Mg/(Fe+Mg). Cl content in the high-F amphiboles is generally very low (<0.3 wt.%).

Sample B98020802B shows a similar granoblastic texture with that of sample B98020801, yet brownish amphibole in the sample is rimmed by fine-grained aggregates of Opx, Ilm, Qtz, and Pl. The texture suggests an evidence of devolatilization reaction of calcic amphibole.

@@Ca-Amp + Pl (An57-61) -> Opx + Qtz + Pl (An77-79) + Bt + V

Although brownish amphibole in the sample shows slightly lower F content (0.21-0.52 wt.%; F/(F+Cl+OH)=0.05-0.13) than the other brownish amphiboles, its Cl content is higher (up to 1.56 wt.%; Cl/(F+Cl+OH)=0.22) than that of sample B98020801. Cl content in the calcic amphiboles has positive correlation with K.

According to the experiment of Holloway and Ford (1975), fluor-pargasite is stable up to 1100°C at 5 kbar, and F/(F+OH) increases with increasing temperature. F-rich calcic amphibole discussed in this study is therefore a stable mineral during UHT metamorphic condition. It has to be noted that brownish amphibole which fluorine content is less than 0.2 wt.% has corona texture. The evidence that the corona has anhydrous mineral assemblage and that product plagioclase shows higher An content than matrix indicates that the corona formed during prograde metamorphism.

In contrast, F and Cl contents in retrograde greenish amphiboles are very low. This implies an infiltration of aqueous fluid during retrograde metamorphism.