目潟火山のかんらん岩捕獲岩に見られる島弧マントルの加水作用

Abstract

Peridotite xenoliths from the Ichinomegata crater, the Megata volcano, have a wide spread of Cr# (Cr/(Cr+Al) atomic ratio) of chromial spinet, from 0.07 to 0.53. They frequently have secondary hydrous minerals, pargasite and phlogopite, replacing or veining primary minerals. The amount of pargasite is negatively correlated with the amount of clinopyroxene. Phlogopite occurs only in pargasite-rich rocks. The K/(K+Na) ratio of pargasite increases with an increase of degree of hydration of the rocks. Pargasite and phlogopite are depleted in TiO2 (<1.6 wt. %). Primary minerals have been chemically modified depending on the amounts of the hydrous minerals added ; olivine and pyroxenes have been enriched with Fe (down to Fo83), and chromian spinel, with Al and Fe3+. Metasomatic formation of the hydrous minerals was accompanied with addition of, at least, Na, Ti and K. The metasomatism was caused by infiltration of hydrous arc magmas, which consolidated as phlogopite-bearing hornblendite veinlets, into initially anhydrous peridotites. The fluids subsequently released were diffused into the surroundings to produce hydrous minerals. Peridotite xenoliths from the Sannomegata crater have a very narrow spread of Cr # of chromian spinel, from 0.10 to 0.25; i.e., all is lherzolitic. They usually have pyroxene-spinel symplectite or its coarsened equivalent, with or without relic plagioclase. They are almost free from hydrous minerals and their primary minerals have been devoid of the metasomatic modification. The petrographical contrast between the Ichinomegata and the Sannomegata peridotites suggests a strong heterogeneity, in terms both of the anhydrous primary mineralogy and of the degree of hydration, for the upper mantle beneath the Northeast Japan arc.