九州黒瀬川帯に産するザクロ石-単斜輝石グラニュライトおよびザクロ石角閃岩の原岩と同位体年代(古領家帯と黒瀬川帯の構成要素と改変過程)  [in Japanese] Protolith and Sm-Nd geochronology of garnet-clinopyroxene granulite and garnet amphibolite from the Kurosegawa Belt in Kyushu, southwest Japan(Geotectonic evolution of the Paleo-Ryoke and Kurosegawa Terranes)  [in Japanese]

九州・八代地域の黒瀬川帯には, 高度変成作用を受けた苦鉄質変成岩類が堆積岩中のテクトニックブロックとして多数存在する.今回検討した八幡滝産ザクロ石-単斜輝石グラニュライトおよび坪木ノ鼻産のザクロ石角閃岩と単斜輝石角閃岩は, 島弧あるいは陸弧のソレアイト玄武岩起源とみなされる.ザクロ石-単斜輝石グラニュライトからは, Sm-Nd鉱物アイソクロン年代として約420Maが得られ, ホルンブレンドの形成を伴う後退変成時期(約600MPa, 750℃)を示すと考えられる.この岩石のピーク変成条件は, 約1000MPa, 950℃とみなされる.ザクロ石角閃岩は, 約600MPa, 800℃の変成条件を示し, Sm-Nd鉱物アイソクロン年代として約490Maが得られた.単斜輝石角閃岩からは, Sm-Nd全岩アイソクロン年代として約540Maが得られた.これらのデータは, 日本列島におけるゴンドワナフラグメントの存在, および関連するパンアフリカン変動を検討する上で重要な意味を持つ.

The western end region of the Kurosegawa Belt in Kyushu island (Yatsushiro area), southwest Japan, contains high-grade mafic metamorphic rocks (garnet-clinopyroxene granulite, garnet amphibolite and clinopyroxene-bearing amphibolite) as tectonic block in sedimentary rocks. These metamorphic rocks have been considered to have strong relationship with mafic metamorphic rocks from the Higo metamorphic rocks. The mafic metamorphic rocks from the Kurosegawa Belt (Hachinan-daki and Tsubokinohana areas) could be derived from mafic igneous rocks of volcanic arc (continental arc) tholeiite, which presumed by using major and trace elements discrimination diagrams, while those of the Higo metamorphic rocks show MORB-like precursor.The garnet-clinopyroxene granulite from Hachiman-daki has a Sm-Nd internal isochron age of late Silurian period (c. 420Ma). The estimated peak metamorphic P-T conditions of garnet-clinopyroxene granulite indicate metamorphic evolution from high-pressure granulite-facies (c. 1000 MPa, 950℃) to lower-granulite (high-amphibolite) facies (c. 600MPa, 750℃) along clockwise P-T path. The garnet amphibolite and clinopyroxene-bearing amphibolite from Tsubokinohana give Sm-Nd internal isochron age of early Ordovician (c. 490Ma) and Sm-Nd whole-rock isochron age of middle Cambrian (c. 540Ma), respectively. No sign of high-pressure granulite-facies conditions can be detected for the mafic metamorphic rock of Tsubokinohana (c. 600MPa, 800℃). Metamorphic conditions of mafic metamorphic rocks from the Kurosegawa Belt, including high-pressure granulite-facies condition, indicate the similarity of metamorphic evolution of the Higo metamorphic rock. However, metamorphic or protolith age of Higo-rocks (c. 280-250Ma) are clearly different.