The chemical composition of Ti-poor and biotites from Ryoke metamorphic rocks in the Yanai district are described by the substitution, 2Ti+ ?? ^VI=2Al^VI+R²⁺. By applying this substitution to eastonite, a new endmember of Ti-biotite, K₂ ?? R²⁺₃Ti₂Si₄Al₄O₂₀(OH)₄, is defined. The compositional space defined by phlogopite-talc-eastonite-muscovite and this Ti-biotite can express the composition of Ti-bearing biotite better than other definitions of Ti-biotite endmenber.

抄録全体を表示

東北日本弧の太平洋側から脊梁帯にかけての広い地域に南北に配列する鮮新世 (4～2 Ma) 火山岩111試料について,主要元素と微量元素 (Rb, Sr, Nb, Y, Zr, Ni, Cr) の分析を螢光x線法で行った。本研究結果と既発表の他の鮮新世火山岩のデータは,太平洋側から日本海側に分布する火出岩(玄武岩～デイサイト)にむかって, K₂O, Rb, Srが漸次増加することを示す。これらの火山岩は,苦鉄質斑晶鉱物組合せや岩石系列の点においても,東北日本弧の第四紀火山岩の場合とほぼ同様な水平変化を示す。このような化学組成と岩石学的性質の水平変化は, 8～6 Maの火山岩にも認められているので,東北日本弧においては,太平洋プレートの沈み込みに関連した火山活動が少なくても8 Ma前から現在まで継続して起っているものとみられる。

抄録全体を表示

The Oligocene Omoto pluton in the Ishigaki-jima Island, southern Ryukyus, is divided into six rock types : Miyaragawa, Chayama, Ishigaki, Naguragawa, Otake and Omoto. These rock types that make up zoned plutonic sequence are younger and more felsic inward. The most of the rocks are adamellite and alkali feldspar granite, but the outermost rock facies is quartz diorite, and innermost one is porphyrytic granite. The clearly boundaries between each rock type indicate that zoned mechanism of the Omoto pluton is multiple intrusion.
The Omoto type that is innermost and most felsic rock facies shows characteristics of S-type granitoids, while the other types exhibit those of I-type granitoids. Cr contents of the Omoto type are similar to those of more basic rock facies in this pluton. These matters suggested that the Omoto type and the other types were derived from different sorce material.
In conclusion, the Omoto pluton is composed of six rock types which are derived from two different magmas.

抄録全体を表示