日高変成帯南部オピラルカオマップ複合深成岩体の岩石学的特徴  [in Japanese] Petrological characteristics of Opirarukaomappu plutonic complex in the southern Hidaka metamorphic belt, Hokkaido, Japan  [in Japanese]

日高変成帯南部に位置するオピラルカオマップ複合深成岩体は記載岩石学的特徴から, トロクトライトーかんらん石はんれい岩グループ(TROG)・細粒かんらん石はんれいノーライト(FOGBN)・細粒はんれいノーライト(FGBN)・はんれい岩グループ(GB)・はんれいノーライト-角閃石はんれい岩グループ(GNHG)・閃緑岩-トーナライトグループ(DITO)の6岩型(群)に分類される。本岩体では西部にソレアイト系列であるはんれい岩類が分布し, 東部にカルタアルカリ系列である閃緑岩類が分布する。集積岩であるTROGは非常に枯渇した起源物質を持つ玄武岩質マグマの分別結晶作用によって形成された。全岩化学組成およびSr同位体組成の特徴からオピラルカオマップ岩体の主体をなすTROG, GNHG, DITOはそれぞれ異なる成因を持つと考えられる。

The Opirarukaomappu plutonic complex, southern Hidaka metamorphic belt, Hokkaido, Japan, consists of six rock types. They are troctolite-olivine gabbro group (TROG), fine-grained olivine gabbronorite (FOGBN), fine-grained gabbronorite (FGBN), gabbronorite-hornblende gabbro group (GNHG), gabbro group (GB), diorite-tonalite group (DITO). Gabbroic rocks occupy the western part of the complex, while DITO is distributed in the eastern part. None of the gabbroic rocks contains hydrous minerals. The bulk chemical composition of gabbroic rocks shows tholelitic trend. In contrast, the DITO includes hornblende and biofite with plagioclase and quartz and shows calc-alkaline trend. TROG and FOGBN have very depleted and uniform Sr isotopic compositions (0.7025-0.7027). In contrast, those of other rocks show broad compositional variations affected by crustal materials. The TROG is composed mainly of cumulus plagioclase (An_<82-59>), olivine (Fo_<84-69>) and intercumulus to subhedral clinopyroxene showing adcumulus-mesocumulus texture. On the basis of modal composition and occurrence of clinopyroxene, the TROG is divided eastward into three units: I (troctolite), II (olivine gabbro with subhedral to euhedral clinopyroxene) and III (olivine gabbro with interstitial clinopyroxene). The modal proportion of clinopyroxene increases eastward and the chemical compositions of the minerals are also differentiated from unit I to unit II in the TROG. The mineral compositions of unit III are less differentiated than those of unit II. These characteristics indicate that the rocks of TROG were substantially formed by fractional crystallization from basaltic magma. The bulk chemical and Sr isotopic characteristics indicate no genetical relationships among the main constituent rock types (TROG, GNHG and DITO) in the Opirarukaomappu complex.