幌満かんらん岩体の苦鉄質岩層の起源  [in Japanese] Origin of mafic layers in the Horoman peridotite complex  [in Japanese]

幌満かんらん岩体に挟在するはんれい岩質苦鉄質岩は, 鉱物組み合わせや産状および化学組成の違いによって, GBI・GBII・GBIII・GBIVの4タイプに区分される。苦鉄質岩の産状は, 一般にかんらん岩の構造に調和的な層状ないしシル状であるが, GBIIIおよびGBIの一部はダイク状の形態を示す。パイロキシナイトは苦鉄質岩と壁岩かんらん岩との境界に産する。全岩および構成鉱物の化学組成から, 4タイプの苦鉄質岩およびパイロキシナイトの起源は次のように推定される。GBIは, 幌満岩体のMHL系列かんらん岩の部分融解メルト(N-MORBマグマ)からつくられた。GBI周縁部の一部は未分化なメルト組成を示し, それ以外のGBIは分化したマグマから生じたキュームレイトの性質を示す。GBIIは主にかんらん石, 斜長石および単斜輝石の分別結晶, GBIIIは輝石とかんらん石の分別結晶および分化メルト, GBIVはかんらん石, 輝石および斜長石分別結晶からなるキュームレイトであった。パイ口キジナイトは, 苦鉄質岩層をつくったマグマの早期の分別結晶からなるキュームレイトである。

The Horoman peridotite complex in the Hidaka Metamorphic Belt, Hokkaido, contains a small amount of olivine gabbroic and pyroxenitic mafic layers. Four distinct olivine gabbroic types, GBI, GBII, GBIII, and GBIV, are identified based on the mineral assemblages and the chemical compositions. The GBI, which is the most common type, is characterized by the presence of Ti-rich minerals such as titaniferous clinopyroxene, titaniferous pargasite, kaersutite and ilmenite. The GBII is marked by the existence of Ti-poor clinopyroxene and pargasite. Both the GBI and GBII are poor in orthopyroxene. The GBIII contains both cliinopyroxene and orthopyroxene. The GBIV contains abundant orthopyroxene and spinel. The GBI, GBII, and GBIV are observed as sills that are concordant with the layering of peridotites, where as the GBIII cross cuts the peridotites. Pyroxenites commonly occur as thin layers and lenses intercalated by the gabbroic rocks and peridotites. Regression lines of major element abundance for the Horoman peridotites suggest that the peridotites can generate a MORB-type magma. The gabbroic and pyroxenitic rocks do not have geochemical characteristics as primary melts generated from the peridotites, except for margins of the GBI. The GBI is considered to have been crystallized from the N-MORB magma derived from the main peridotite suite. The GBII is explained as cumulate that is composed mainly of cumulus olivine, plagioclase and clinopyroxene. The GBIII is considered to be cumulate that is composed of cumulus pyroxenes and olivine with small amounts of evolved melt. High-Mg# and Cr_2O_3 feromagnesian minerals of the GBIII and pyroxenite are probable to have been crystallized from the GBIII magma in association with magma-wall peridotite interactions. The GBIV is explained as cumulate with cumulus olivine, pyroxenes and plagloclase. Pyroxenites accompanied by all the gabbroic types are cumulates, which are composed of cumulus minerals crystallized from the channeling magma at the early stage.