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In the Okoppe area of North Hokkaido, Japan, the eruption of adakitic and calc-alkaline dacites was followed by high-Mg andesite (HMA) and calc-alkaline dacite during the Middle Miocene (12–10 Ma). Adakitic dacite is characterized by high Sr/Y and low Y, with Sr and Nd isotopic compositions identical to those of mid-ocean ridge basalt. It has higher MgO contents than adakites generated by experimental melting of metabasalt and amphibolite, and higher Ni and Cr contents than either Archean trondhjemite–tonalite–granodiorite or Early Cretaceous adakitic granites, which are considered to represent partial melts of subducted oceanic crust. This provides compelling evidence that adakitic dacite magma from Okoppe resulted from interaction of a melt derived from subducted oceanic basaltic crust and the overlying mantle wedge peridotite, with little modification to the adakitic melt signature and Sr and Nd isotopic values. The compositional variations in the Toyono adakitic dacite and associated calc-alkaline dacite probably resulted from mixing of the reacted magma and an evolved silicic dacite magma formed by fractional crystallization of the reacted magma. A disequilibrium phenocryst assemblage in the HMA may result from mixing of boninite and silicic andesite that resulted from crustal melting. Calc-alkaline dacites associated with the HMA were derived by fractional crystallization of silicic andesite and assimilation of crust with an enriched Sr isotopic signature. The most likely tectono-magmatic model for the production of adakitic dacite and HMA involves upwelling of hot asthenosphere into the subcontinental lithosphere beneath North Hokkaido and the back-arc side of the NE Japan arc, coincident with the spreading of the Kurile back-arc basin and Japan Sea back-arc basin. This resulted in a high geothermal gradient in the mantle wedge beneath North Hokkaido. The subsequent melting of a limited part of the cool oceanic crust subducting beneath Hokkaido produced adakitic magmas, which interacted with the overlying mantle wedge peridotite. These magmas subsequently reacted with an evolved calc-alkaline melt en route to the surface. Boninitic magma derived from the ascending hot asthenosphere in part reacted with crust-derived silicic andesitic magma, undergoing simultaneous fractional crystallization. [ABSTRACT FROM PUBLISHER]

The ascent history of the Horoman peridotite complex, Hokkaido, northern Japan, is revised on the basis of a detailed study of large ortho- and clinopyroxene grains ∼1 cm in size (megacrysts) in the Upper Zone of the complex. The orthopyroxene megacrysts exhibit distinctive M-shaped Al zoning patterns, which were not observed in porphyroclastic grains less than 5 mm in size described in previous studies. Moreover, the Al and Ca contents of the cores of the orthopyroxene megacrysts are lower than those of the porphyroclasts. The Upper Zone is inferred to have resided not only at a higher temperature than previously suggested but also at a higher pressure (∼1070°C, ∼2·3 GPa) than the Lower Zone (∼950°C, ∼1·9 GPa), in the garnet stability field, before the ascent of the two zones. The Horoman complex probably represents a 12 ± 5 km thick section of lithospheric mantle with an ∼10 ± 8°C/km vertical thermal gradient. The current thickness of the Horoman complex is ∼3 km, which is a result of shortening of the lithospheric mantle by ∼0·25 ± 0·1 during its ascent. The Upper Zone appears to have experienced a heating event during its ascent through the spinel stability field, with a peak temperature as high as 1200°C. The effect of heating decreases continuously towards the base of the complex, and the lowermost part of the Lower Zone underwent very minor heating at a pressure higher than ∼0·5 GPa. The uplift and associated deformation, as well as heating, was probably driven by the ascent of a hot asthenospheric upper-mantle diapir into the Horoman lithosphere. [ABSTRACT FROM AUTHOR]

Dunite formation processes in highly depleted peridotites are discussed based upon a detailed study of the Iwanaidake peridotite, Hokkaido, Japan, which consists mainly of harzburgite with a small amount of dunite. In the harzburgites, the Mg# [= 100 × Mg/(Mg + Fe2+)] of olivine ranges from 91·5 to 92·5, and the Cr# [= 100 × Cr/(Cr + Al)] of spinel from 30 to 70; in the dunites, the Mg# of olivine ranges from 92·5 to 94 and the Cr# of spinel from 60 to 85, respectively. The NiO wt % of olivine in harzburgites ranges from 0·38 to 0·44, and in dunites from 0·35 to 0·37. The Mg# and Cr# are higher and NiO wt % is lower in the dunites than in the harzburgites surrounding the dunites. The Mg# and Cr# exhibit normal depletion trends expected from simple partial melting, whereas the NiO wt % shows an abnormal trend. On the basis of mass balance calculations, dunites are considered to be derived from the harzburgites by a process involving incongruent melting of orthopyroxene (orthopyroxene olivine + Si-rich melt). Hydrous conditions were necessary to lower the solidus, and thus melting of harzburgite was probably triggered by the introduction of hydrous silicate melt. The dunite in this massif may have formed in the mantle wedge above a subduction zone. [ABSTRACT FROM PUBLISHER]

The Nikanbetsu peridotite complex, Hokkaido, Japan, is composed of mainly fertile lherzolite, which shows several lines of evidence for incipient partial melting in the spinel–plagioclase facies. There are petrological, textural and mineral chemical variations in plagioclase-free and -bearing lherzolites from the base to the top of the complex within the total thickness of 1400 m. Two-pyroxene and spinel symplectites occur only at the base of the complex. Mass-balance calculations on their bulk compositions suggest that they lost the Al component from pyropic garnet. The Wo content of orthopyroxene cores continuously increases, whereas the Al content decreases from the base upward. Ca–Na zoning patterns of plagioclase in the plagioclase lherzolites characteristically change from W-shaped patterns at the base to oscillatory patterns in the upper part of the complex. These lines of petrological, textural and mineral chemical evidence indicate that incipient partial melting occurred everywhere in the complex, with an increase in the degree of melting from the base toward the top, in proportion to a monotonous rise of the equilibrium temperature from 1100°C to 1250°C. The systematics of plagioclase zoning provides evidence for simultaneous incipient partial melting, melt migration, decompression and melt crystallization in the ascending upper-mantle rocks. [ABSTRACT FROM PUBLISHER]