東北日本弧，男鹿半島の寒風山に産する角閃石含有同源集積岩と輝石グラニュライトの岩石学的，同位体的特徴：高圧下における角閃石の結晶作用と下部地殻物質の交代作用

抄録

Two suites of xenoliths and an associated scoria-bearing pyroclastic fragment were sampled near Kampu volcano on Oga Peninsula of the northeast Japan arc. These scoria-hosted xenoliths consist of hornblende-bearing clinopyroxenite, hornblendite, and hornblende gabbro cumulates, hornblende megacrysts, and minor amounts of lower crustal pyroxene-plagioclase granulites. The minerals within the cumulates are euhedral and the interstitial glass is interconnected with the hosting scoria. The cumulates and their hornblende megacrysts have ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios (0.70316-0.703390 and 0.512877-0.512974) similar to those of the Kampu magmas (0.702917-0.703409 and 0.512883-0.512988). However, these samples are not in isotopic equilibrium with each other or with the hosting scoria, with the latter having relatively constant Sr isotopic compositions (average of 0.703187). This is exemplified by two hornblende megacrysts with unusually high Al₂O₃ contents (< 17 wt%) that have different Sr isotopic ratios (0.703039 and 0.703235) and are isotopically homogenous from core to rim. This suggests that each melt that coexisted with the megacrysts was separated as a discrete batch without any isotopic mixing near the base of the lower crust. The host scoria was derived from discrete batches of magma having different isotopic compositions, but the scoria itself is isotopically homogeneous, indicating magma mixing and homogenization during the rapid ascent and transportation of these xenoliths from the lower crust to the Kampu volcano. This was most likely caused by the injection of upper-mantle-derived mafic magma into the base of the lower crust, as evidenced by micro-hornblende within the hosting volcanic glass that contains more Mg than the hornblende megacrysts.
  The granulite xenoliths have mosaic textures, contain plagioclase with deformation twins, are closely inter-locked, and do not contain any hosting glass material. They have Sr and Nd isotopic compositions (0.70501-070532 and 0.51260-0.51263) that contrast with the cognate hornblende megacrysts, the cumulates, and the Kampu lavas, but are similar to less metasomatized lower-crustal amphibolites (amphibole with compositions of 0.70524 and 0.51261) from Ichinomegata maar near Kampu volcano. Rare hornblendes within these granulites record the onset of metasomatism, although the isotopic composition of these samples remained relatively unchanged until a subsequent cryptic metasomatic event that generated the wide range of isotopic and trace element compositions present within the Ichinomegata amphibolites.