Amphiboles and coexisting ferromagnesian silicates in granitic rocks in Mahe, Seychelles セイシェル諸島マヘ島産花崗岩中の角閃石と苦鉄質鉱物の岩石学的・鉱物学的研究
Amphiboles and coexisting ferromagnesian silicates in granitic rocks in Mahe, Seychelles
A variety of granitic rocks from granodiorite to alkaline granite is developed in Mah'e island, Seychelles. Microprobe analyses were made on amphiboles and coexisting minerals. Amphibole constitutes the most prominent ferromagnesian minerals in the Seychelles granitic rocks. Its chemical composition ranges widely from calcic through sodic-calcic to alkali amphiboles and amphibole composition evolves systematically from Fe-poor to Fe-rich: magnesiohornblende →ferrohornblende →ferroeaenite →silicic ferroedenite →ferrorichterite and ferrowinchite →riebeckite. Riebeckite occurs abundantly in the alkaline rocks as subsolidus minerals. Throughout the evolution two types of isomorphous substitution, Mg Fe2 + and A1+ Ca Si + Na principally took place. Compositions of clinopyroxene and biotite also evolve from Fe-poor variety to Fe-rich variety. All these compositional evolutions of the constituent minerals suggest a comagmatic origin of the Seychelles granitic rocks studied. In the Seychelles alkaline magma, ferrorichterite crystallized at the late-magmatic stage under conditions of 650-700℃ in temperature and of slightly above the QFM-buffer in oxygen fugacity. With falling temperature, oxidizing condition prevailed and riebeckite crystallized. Generally, in alkaline granite and quarts syenite magmas, ferrorichterite evolves continuously to arfvedsonitic compositions when oxygen fugacity is defined by QFM-buffer even during subsolidus stage. On the other hand, ferrorichterite evolves to riebeckite composition when oxidizing condition prevails. But, in this case, continuous solid-solution between ferrorichterite and riebeckite is not found, presumably owing to an existence of a compositional gap between them.
名古屋大学博士学位論文 学位の種類:理学博士 (論文) 学位授与年月日:昭和61年1月14日