幌満かんらん岩に記録されたマグマチャネリング様式 : メルト成分の涸渇や付加で生じた置換性かんらん岩  [in Japanese] Modes of magma channeling recorded in the Horoman peridotites : Replacive peridotites formed by melt extraction and addition  [in Japanese]

幌満かんらん岩体上部には, 2つの異なるマグマチャネリング様式が認められる。斜長石レルゾライト中の苦鉄質岩タイプGBIの周囲で観察されるチャネリング様式は, 壁岩かんらん岩から部分融解メルトが抽出され, 壁岩にメルト成分の涸渇を生じた様式(1)である。もう1つは, スピネルダナイト中に貫入したウェブステライトダイクの周囲で観察される様式で, 壁岩かんらん岩の涸渇を伴わずに通過マグマから一方的にメルト成分が付加した様式(2)である。このような様式の違いは, チャネリングマグマの温度と壁岩かんらん岩のソリダス温度の差によって生じたと考えられ, 様式(1)のチャネリングマグマの温度は斜長石レルゾライト壁岩のソリダス温度よりも約80℃高かったこと, 様式(2)のチャネリングマグマの温度はスピネルダナイト壁岩のソリダス温度よりも低かったことが議論された。

Mafic layers in orogenic lherzolite have been generally explained as traces of magma channeling in the upper mantle peridotites. Interactions commonly occur between channeling magma and peridotite walls. The Horoman peridotite complex in Hokkaido, Japan, contains a large number of olivine gabbro and pyroxenite dykes and sills. The contrasting processes of melt extraction and addition can be detected as follows: Case 1 (melt extraction): Gabbroic-tupe GBI sills in plagioclase lherzolite : A gradational compositional layering of plagioclase lherzolite→spinel lherzolite→spinel harzburgite, 350 cm thick, is observed in the wall peridotite around the gabbroic sill. The modal variation and the chemical composition variation of bulk rocks and minerals suggest that the amount of melts extracted increases towards the gabbroic magma channels. Temperature of the channeling magma was estimated from the 10-15kb experiments on Hawaiian pyrolite to have been approximately 80℃ higher than the solidus temperature of the plagioclase lherzolite wall. Case 2 (melt addition): Websterite dykes in spinel dunite : Replacive harzburgite is observed as a narrow band, 5-7 cm wide, between the websterite dyke and the spinel dunite wall. Since the dunite wall has a simple assemblage of primary olivine+spinel, it is obvious that the replacement occurred by crystallization (modal addition) of orthopyroxene, clinopyroxene and pargasite. The replacive phases crystallized as interstitial grains among the residual phases from the infiltrated melts. The mineralogical variations across the channel further support this conclusion. No melt extractions have been detected in this channel. One way process of melt addition seems to be best explanation for the modification of high-Mg# spinel dunite wall, of which solidus was higher than the temperature of the channeling magma.