岡山県に分布する下部白亜系から得られた砕屑性クロムスピネルの化学組成と供給源 Composition and provenance of detrital chromian spinels from Lower Cretaceous sediments, Okayama Prefecture

岡山県西部には河川成堆積物からなる下部白亜系硯石層が点在しており, 重鉱物としてクロムスピネルを含んでいる。本研究ではクロムスピネルの化学組成をEPMA分析によって測定し, 組成の特徴からこれらのクロムスピネルを供給した地質さらに岩体を検討した。測定結果から, これらは貫入岩や島弧の火山岩よりもオフィオライトを構成する超塩基性岩からもたらされたものであることが考えられる。由来した岩石は主にハルツバ-ジャイトとダナイトと推定される。蛇紋岩化したダナイト-ハルツバ-ジャイトからなる岩体は調査地域北方に分布しており, 特にその内の大佐山岩体と足立岩体は供給源として可能性が高い。

Lower Cretaceous sediments (Kenseki Formation) which consist of fluvial sediments are distributed sporadically in western Okayama Prefecture, Japan. The detrital materials were derived from the north and contain chromian spinels as the most dominant heavy mineral. The detrial spinels are characterized by a rather narrow spectrum of chemical compositions. Cr_2O_3 contents range from 30.54 to 49.80 wt%. Al_2O_3 contents which are negatively correlated to Cr_2O_3 range from 19.63 to 36.15 wt%. FeO contents range from 12.46 to 22.91 wt% and are also negatively correlated to MgO which range from 8.43 to 15.86 wt%. The Fe_2O_3 contents (estimated on the basis of structural formula) reach up to 5.03 3wt%. All the analyzed spinels also contain measurable amounts of MnO which range from 0.14 to 0.78 wt%. The chemical compositions of the analyzed spinels suggest derivation from peridolites of an ophiolite suite rather than island-arc volcanics, or a stratiform intrusion. The lithology of the source was most probably dominated by harzburgite and dunite. Serpentinized dunite-harzburgite masses exposed north of the study area, particularly the Ohsa-yama and Ashidachi complexes, are the most likely sources for the detrital chromian spinels.