Origin of Plagioclase Lherzolite from the Nikanbetsu Peridotite Complex, Hokkaido, Northern Japan: Implications for Incipient Melt Migration and Segregation in the Partially Molten Upper Mantle

Abstract

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.