斑れい岩∼閃緑岩からなる日置苦鉄質岩脈が南九州鹿児島県摩半島中部の中期中新世日置花崗岩体に伴う。斑れい岩岩脈は，orthocumulusなカンラン石含有輝石斑れい岩からなり，化学組成上，MgO・Ni・Cr等のcompatible元素とK₂O・Rb・Sr等のincompatible元素に富む。閃緑岩岩脈は，intercumulusなサリック鉱物とadcumulus斜長石を伴うorthocumulusな黒雲母含有輝石閃緑岩からなり，化学組成上，斑れい岩と花崗岩の中間の組成をもつ。産状，岩石記載，全岩組成及び鉱物組成は，カルク・アルカリ安山岩質マグマの結晶分別作用を通して，日置苦鉄質岩脈がキュムラス相から形成され，花崗岩類が残液からもたらされたことを示す。

抄録全体を表示

Pyrope-rich garnets were found in a harzburgite xenolith from Arato-yama, one of necks of the Cenozoic alkali basalt distributed in southwest Japan. The harzburgite contains 85.9% olivine, 13.8% orthopyroxene, 0.2% clinopyroxene, 0.1% spinel and trace amount of garnet in modal composition. Chemical composition of the garnet is similar to that in lherzolite and harzburgite xenoliths from China and Baikal but different from those in websterite, pyroxenite and gabbro xenoliths reported from other localities near Arato-yama.
     Equilibration temperatures of Japanese lherzolite and harzburgite xenoliths range widely from 900 to 1200°C and can be divided into the high and low temperature groups. The Arato-yama mantle xenoliths have lower equilibration temperatures, close to those from On-yama, Oki-Dogo, Kurose and Megata, than Fukue-jima, Kawashimo and Noyamadake. Both groups may be equilibrated to different geotherms, since the equilibration temperature and pressure of the Arato-yama garnet-bearing spinel harzburgite (951°C, 3.1 GPa) is out of the equilibration field of Fukue-jima spinel lherzolite and harzburgite. It is believed that a cold lithospheric mantle underlies southern side of Chugoku district (Arato-yama and On-yama), north Kyushu (Kurose) and Japan sea (Oki-Dogo), while a hot one does northern side of Chugoku district (Kawashimo and Noyamadake) and northwest Kyushu (Fukue-jima).

抄録全体を表示

Rb-Sr whole rock isochron and Sm-Nd whole rock-mineral isochron ages were determined for the Yoshiwa quartz diorite in the Maizuru belt, northwestern part of Hiroshima Prefecture. The Yoshiwa quartz diorite intrudes Triassic to Jurassic Kuga Group. Eight whole rock samples give a Rb-Sr isochron age of 161.6±15.2 Ma with an initial Sr isotope ratio of 0.70624±0.00010. Whole rock, felsic fraction and hornblende show an internal Sm-Nd isochron age of 169.8±13.9 Ma with an initial Nd isotope ratio of 0.512371±0.000012. These middle to late Jurassic ages are comparable with previously reported ages for gabbro-diabase and granitic rocks from the northern zone of the Maizuru Belt in the Maizuru-Ooe district, Kyoto prefecture.

抄録全体を表示