ABSTRACT

The chemical compositions of biotite grains have been determined for early Paleozoic plutonic rocks from the Sør Rondane Mountains, East Antarctica. The plutonic rocks are divided into three groups of the type-I granitoids, type-II granitoids and the Mefjell Plutonic Complex. Two types of biotite are found in the groups: pale yellow to dark green biotite in the type-I granitoids and brown to dark brown biotite in the type-II granitoids and the Mefjell Plutonic Complex. Mineral chemically, biotites from the type-I granitoids have higher SiO₂ contents, F contents, and X_Mg values with lower TiO₂ and Cl contents relative to biotites from the type-II granitoids. In the type-II granitoids and the Mefjell Plutonic Complex, biotite strongly enriched in FeO has lower F contents. Biotites in the type-II granitoids and the Mefjell Plutonic Complex have much higher log(f_H2O)/(f_HF) than those from the type-I granitoids. The fluorine content in biotite from the type-I granitoids is similar to that from partial melting A-type granites in Lachlan Fold Belt of eastern Australia; and those from the type-II granitoids and the Mefjell Plutonic Complex are comparable to fractionated aluminous A-type granites from the Lachlan Fold Belt, and the Ambalavayal aluminous Atype granites of South India. Lower fluorine contents in biotite from the type-II granitoids and the Mefjell Plutonic Complex may be mainly controlled by late-magmatic fluid-rock interaction processes associated with melt and may not be indicative of original magma contents. Higher fluorine contents in biotite and whole-rock from the type-I granitoids may reflect fluorine-enriched magmatic source. Our results in biotites from the type-I granitoids as well as geochemistry support the models, which fluorine-rich A-type granites may be derived from partial molten crustal igneous rocks of tonalitic to granodiorite composition.