Plagioclase lherzolite from the Horoman complex, Hokkaido, Japan, contains four types of plagioclase, seam type, inclusion type, isolated type and vein type, with different mode of occurrence and texture. The occurrence of each type is systematically related to the lithostratigraphy of the complex, most typically represented by the contrast between the Lower and Upper Zone, and is mostly due to the continuous spatial change in P-T history suggesting a diapiric structure for the complex. The seam type plagioclase commonly occurs as one of minerals forming seam like fine-grained aggregates that are divided into two sub-types: clinopyroxene-free seam and clinopyroxene-bearing seam. Clinopyroxene-free seams are common in the Lower Zone (cooler marginal zone of the diapir) and are inferred to have been formed by the subsolidus reaction among spinel and pyroxenes, which were originated from garnet and olivine. The bleb-like inclusions of sodic plagioclase associated with quartz, orthopyroxene, and Ti -pargasite only occur in the middle of the Lower Zone, and are inferred to represent an incipient partial melt formed and trapped in clinopyroxene. Seams containing a certain amount of clinopyroxene, isolated plagioclase grains, and sodic plagioclase-rich veins fringed by orthopyroxene are commonly occur in the Upper Zone (hotter central zone of the diapir). They indicate that a significant extent of partial melting took place in this zone. The veins may correspond to fractures, toward which incipient partial melt was segregated and crystallized. The contrasts between the two zones demonstrate that partial melting of plagioclase lherzolite was extensive to permit segregation of the partial melt into fractures in the Upper Zone, but that the melting and segregation was fairly limited in the Lower Zone. In the Nikanbetsu complex, segregation veins are frequently observed forming a network, which is interpreted to be a more advanced stage of partial melting associated with melt segregation than that presumed for the Upper Zone of the Horoman complex.

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The basic and ultrabasic metavolcanics, occurring within the about 1.0 Ga Kuanping Group in the Qinling orogenic belt, China, have been investigated for bulk-rock major and trace-element data. Studied rocks fall into three geochemical categories: (1) komatiites, resembling Barberton-type Al-depleted komatiites; (2) OIB-like basalts, spatially associated with and geochemically similar to komatiites; and (3) MORB-like tholeiites, resembling mid-ocean ridge and back-arc basin basalts. Combining our observations and recent high-pressure experimental data, we accordingly considered an unusual pressure condition at the depth of mantle transition zone and a low-degree critical melting (5-6%) event of a majorite mantle source for the generation of komatiitic initial melt. The OIB-like basaltic melt may have been produced by another low degree melting event of the komatiite-derived source or by high-pressure segregation of initial komatiitic magma at the depth greater than 300 km, whereas the MORB-like tholeiite by 15-30% partial melting of a depleted shallow spinel-peridotite source. An unusually hot rising plume would have controlled the generation of komatiitic and basaltic magma and the magmatic evolution. This plume magmatism may have occurred either in a marginal ocean basin where the mantle plume would also controlled the development of the marginal basin, or in a major ocean basin which had developed between the North-China and the South China platform at about 1.2 to 1.0 Ga.

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