Dunite channels within a harzburgite layer from the Horoman peridotite complex, Japan: Possible pathway for magmas

Two discordant dunite channels in a polished slab were collected from a harzburgite layer in the Horoman orogenic lherzolite massif, Hokkaido, Japan. The dunite channels show extreme grain-size variations, and include very large olivine megacrysts up to 1.6 m long. Spinel-rich pyroxenite veins or veinlets occasionally cross-cut the olivine megacrysts. Mineral compositions differ between channels, and the mineral compositions and textures of the dunite channels and the host harzburgite suggest a replacive origin for the dunites. The Sr and Nd isotopic ratios of clinopyroxene separates from the pyroxenite veins in two dunite channels are different, but these ratios are generally similar to those of mid-ocean ridge basalts. Assuming the channels formed by the through-flow of melts, then the trace element compositions of the melts estimated from the clinopyroxenes are broadly similar. The melts have similar features such as enriched in incompatible elements and depleted in heavy rare earth elements, but there are differences in the depletions in Ti and Ba, indicating melts that were different but of similar origins. A Rb-Sr crystallization age (~ 52 Ma) was obtained for clinopyroxene-orthopyroxene pairs in a dunite channel and its host harzburgite. The distinctive dunite channels with megacrysts formed as a result of the through-flow of mafic magma(s), and those magmas had trace and minor element contents that are indicative of contributions of a melt derived from subducted oceanic lithosphere.