小笠原群島母島に産する初生的ソレアイトおよびカルク・アルカリ系列岩

Abstract

Hahajima, Bonin archipelago, is located approximately 1050 km away to the south-south east of Tokyo. Hahajima is the Tertiary volcano mainly composed of evolved island arc andesites and primitive volcanics also occur. The primitive volcanics can be classified into tholeiitic basalts and calc-alkaline basaltic andesites. Their genetical relationships were considered. Primitive tholeiites (TH-P) and calc-alkaline rocks (CA-P) show magnesian compositions (FeO*/MgO 700ppm, Ni>150ppm). They can be distinguished, however by assortment of groundmass pyroxenes. Euhedral olivines with Mg#>90 exist in the TH-P. The olivine should be the phenocrysts which crystallized from the liquid. Olivines in the CA-P could not be analyzed because of the serpentinized alteration. Their augite and Cr-spinel compositions are significantly magnesian. The CA-P is not distinguished from the TH-P by the Sr isotopic ratio. Cr-spinels in olivines in CA-P are systematically enriched in Cr than those in the TH-P. Therefore, the TH-P and CA-P would have originated from different primitive magmas. Judging from Cr-spinel compositions and REE modelings, the CA-P's parental mantle might be more depleted than the TH-P's. Hydrous partial melting experiments of lherzolite imply that the CA-P would have been produced under more hydrous condition than the TH-P. This inference is not contradictory to the CA-P's alkali and alkali-earth elements enrichment. The TH-P might have been produced at the deeper region of oceanic mantle where was not so refractory, while the CA-P might be produced at the shallower region where is relatively deplete. Incompatible trace elements characteristics of the TH-P and CA-P are very similar to those of the tholeiite and calc-alkaline volcanics from Funagata volcano (North east Japan arc). This feature might imply that the geochemical characteristics of relatively unevolved island arc volcanics could be explained by immutable process not according to the age or region of subduction.