A new occurrence of retrogressed eclogite from the Sanbagawa belt of southwest Japan and its significance. (27th June 2019)

• Record Type: Journal Article
• Authors: Taguchi, Tomoki; Endo, Shunsuke; Igami, Yohei; Miyake, Akira
• Is Part Of: Island arc. Volume 28:Number 5(2019)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2019-06-27
• Subjects: eclogite -- FIB–TEM -- omphacite -- Raman spectroscopy -- Sanbagawa belt; Plate tectonics -- Periodicals; Island arcs -- Periodicals; Geodynamics -- Periodicals; 551.136
• DOI: 10.1111/iar.12317
• Languages: English
• ISSNs: 1038-4871

Abstract

Abstract: The present paper reports, for the first time, the occurrence of an omphacite‐bearing mafic schist from the Asemi‐gawa region of the Sanbagawa belt (southwest Japan). The mafic schist occurs as thin layers within pelitic schist of the albite–biotite zone. Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Garnet grains in the sample show prograde‐type compositional zoning with no textural or compositional break, and contain mineral inclusions of omphacite, quartz, glaucophane, barroisite/hornblende, epidote and titanite. In addition to the petrographic observations, Raman spectroscopy and focused ion beam system–transmission electron microscope analyses were used for identification of omphacite in the sample. The omphacite in the sample shows a strong Raman peak at 678 cm −1, and concomitant Raman peaks are all consistent with those of the reference omphacite Raman spectrum. The selected area electron diffraction pattern of the omphacite is compatible with the common P 2/ n omphacite structure. Quartz inclusions in the mafic schist preserve high residual pressure values of Δω1 > 8.5 cm −1, corresponding to the eclogite facies conditions. The combination of Raman geothermobarometries and garnet–clinopyroxene geothermometry gives peak pressure–temperature ( P – T ) conditions of 1.7–2.0 GPa and 440–540 °C for the mafic schist. The peak P–T values are comparable to those of the schistose eclogitic rocks in other Sanbagawa eclogite units of Shikoku. These findings along with previous age constraints suggest that most of the Sanbagawa schistose eclogites and associated metasedimentary rocks share similar simple P–T histories along the Late Cretaceous subduction zone. Abstract : We report, for the first time, the occurrence of an omphacite‐bearing mafic schist from the Asemi‐gawa region of the Sanbagawa belt (southwest Japan). Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Abstract : 四国三波川帯・汗見川地域からオンファス輝石を含む苦鉄質片岩を発見したので，その詳細及び地質学的意義について報告する．この岩石は泥質片岩中に薄層として産し，オンファス輝石はザクロ石の包有物としてのみ認められる．基質の主Na相は曹長石であり，本岩石は著しい後退変成を経験したエクロジャイトと考えられる．オンファス輝石のラマンスペクトルは，参照した標準試料とよく一致した．透過型電子顕微鏡による電子回折の結果，オンファス輝石は空間群 P 2/ n と同定された．変成条件は，ラマン温度圧力計及びザクロ石—オンファス輝石温度計により， P = 1.7–2.0 GPa， T = 440–540 °Cと見積もられ, 三波川帯におけるエクロジャイト質片岩の変成条件と調和的である．本結果は，エクロジャイト質片岩及びその周囲に産する変堆積岩の多くが，後期白亜紀の沈み込み帯において類似の変成履歴を共有していたことを示唆する．