Garnet-bearing spinel harzburgite xenolith from Arato-yama alkali basalt, southwest Japan.

Abstract

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).