幌満かんらん岩体で見られるマグマ分離現象  [in Japanese] Melt segregation processes observed in the Horoman mantle diapir  [in Japanese]

北海道, 日高帯の幌満かんらん岩体は, かつてのマントルダイアピルの断片であり, その物理条件の違いから, 様々な段階でのマグマ分離現象が凍結されている。高温部では上昇の最終課程まで部分溶融していたため形成されたメルトとそのメルトが小規模クラックに分離して形成された斜長石に富む脈が見られ, 大規模なメルト分離現象は現在SDWで占められているかつてのmelt conduit周辺部に凍結されている。規模の違いこそあれ, 両者の基本的なマグマ分離メカニズムは, 溶融メルトのクラックへの吸い込みであった。

The Horoman peridotite complex in the Hidaka belt, Hokkaido, northern Japan, has a record of various types of melt segregation process in the upper mantle. The small-scale melt segregation processes are preserved as plagloclase-rich veins, which are oblique to the foliation and associated with surrounding depleted zones. This type of vein is commonly present in the Upper Zone of the Horoman complex. The veins are composed mainly of coarse-grained plagioclase and minor orthopyroxene, olivine, and rare hydrous minerals. The sodic nature of plagioclase grains occurring at the center of the veins, a probably magmatic zoning preserved at the core of coarse plagioclase, microstructure and the orientation of the veins suggest that the veins were sealed by minerals crystallized from a segregated melt that migrated from the surrounding partially molten peridotites. This partial melting may have occurred when the Horoman complex cut the H_2O-under-saturated solidus during the ascent as a mantle diapir. The Ca enrichment toward the rim of plagioclase grains and the margin of veins suggest that the decompression reaction among pyroxenes and spinel producing Ca-rich plagioclase occurred during and after the crystallization of plagioclase from the segregated melt. The larger scale melt segregation processes are preserved as thick dunite zones sandwiched between the most depleted peridotite layers. The variations of modal composition and mineral chemistry indicate partial melting of the surrounding peridotite and migration of the partial melt into a magma conduit that was later filled by dunite layer. These observations indicate that melt migration by sucking a partial melt towards fractures is an essential segregation mechanism in the upper mantle.