四国西部三波川帯北縁部の唐崎マイロナイトのK-Ar年代と起源(古領家帯と黒瀬川帯の構成要素と改変過程)  [in Japanese] K-Ar ages and origin of the Karasaki mylonites from the northern part of the Sambagawa belt, western Shikoku, Japan(Geotectonic evolution of the Paleo-Ryoke and Kurosegawa Terranes)  [in Japanese]

四国西部三波川帯北縁部に分布する唐崎マイロナイトは低圧型角閃岩相の変成作用を受けた変成岩類を起源とし, 緑色片岩相条件下でマイロナイト化作用を受けている.角閃石および再結晶白雲母のK-Ar年代測定結果から, 変成作用の年代(約510℃の冷却年代)とマイロナイト化作用の時期はそれぞれ104Ma頃と71〜75Maとみなされる.唐崎マイロナイトは, 変成作用の性質・年代が肥後帯構成岩類のそれらに一致することから肥後帯に属し, さらに原岩岩相の類似性から肥後変成岩に対比される可能性が高い.唐崎マイロナイトは三波川変成岩類上にナップとして存在し, 三波川帯に衝上した肥後帯下底の境界剪断帯を構成する岩石とみなされる.衝上の時期はマイロナイト化作用の時期から白亜紀後期末である.九州の肥後帯では, 下位に三波川帯〜御荷鉾帯が伏在していると推測され, 低角度の水平的な構造が初生的であると考えられる.

The Karasaki mylonites (KM), distributed in the northern part of the Sambagawa belt in western Shikoku, are composed of mylonites originated from main amphibolite, and minor pelitic and qaurtzo-feldspathic gneisses. Mineral composition and thermobarometric studies indicate that the protoliths of mylonites have suffered low pressure amphibolite facies metamorphism. Syntectonic minerals forming a fine-grained matrix record the mylonitization under greenschist facies conditions.Hornblende separates from three types of amphibolites were dated by K-Ar method. An undeformed amphibolite gives 117 Ma in age and a garnet amphibolite mylonite, 120Ma. Two different fractions of hornblende separates from a garnet amphibolite protomylonite yield 161 and 170Ma. These ages including the previously reported ones range greatly from 104Ma to 170Ma for the amphibolites. We suggest that they have contained different amount of excess argon and not reset completely by the metamorphism because their protoliths were mainly mafic volcanic sands. Therefore, the youngest age of 104Ma should be close to a cooling age of metamorphic rocks predating mylonitization. Recrystallized muscovites in ultramylonites originated from pelitic and quartzo-feldspathic gneisses yield K-Ar ages of 71-75 Ma, and microclines from qaurtzo-feldspathic gneiss mylonites, 52-59Ma. The former ages are assigned to timing of mylonitization, and the later one, to a cooling age below 150-200℃.Structural analyses reveal that the KM overlies tectonically as a nappe on the Sambagawa schists of Futami nappe and both have experienced large-scale folding of the Hijikawa phase. Shear sense in the KM deduced from asymmetric microstructures indicates that top-to-the-east shear is predominant.Based on type and age of the metamorphism predating mylonitization, the protoliths of the KM are well correlated with the metamorphic rocks of the Higo belt, in particular, with the Higo gneisses. The KM are regarded as representing a ductile shear zone at the sole of the Higo belt thrusting over the Sambagawa schists of Futami nappe which had already exhumed above the depth of 10 km from the deeper part of a subduction zone. Judging from the tectonic framework of western Shikoku, it is suggested that the Higo belt is tectonically underlain by the Sambagawa and Mikabu belts and a horizontal structure is primitive in the belt.