SHRIMP U-Pb ages of the Karasaki mylonites in western Shikoku, Japan

唐崎マイロナイトの優白質角閃岩マイロナイトについて, ジルコンのSHRIMP U-Pb年代測定を行った結果, 1試料からの8粒子は114.2±3.9～102.3±4.5Ma(±1σ)の年代値を示した.マイロナイト化した優白質角閃岩は, SiO_2含有量が低い(49wt.%)ことと, 主として角閃石, 曹長石, 緑泥石および緑簾石のモード比の違いによる数百μm～数mm幅の顕著な層状構造を有することから, 玄武岩質火山岩または火山砕屑岩に由来する砂岩を原岩としている可能性が高い.優白質角閃岩マイロナイト中の丸みを帯びたジルコンは, ある程度遠方より運搬される過程で円磨された可能性が高いこと, マグマから晶出したジルコンに特徴的なオシレトリー累帯構造が認められることから, 砕屑性起源のジルコンであると判断される.また, その年代値が約110Maの狭い範囲を示す事実は, ジルコンが"古領家帯"に帰属すると考えられる真穴帯の大島変成岩や肥後帯の約110Ma深成岩類のみに由来する可能性を示唆する.したがって, 本論において初めて"古領家帯"中にアルビアン以降の堆積岩源変成岩が存在していることが明らかとなった.Zircon U-Pb isotope ages have been analyzed by the Sensitive High Resolution Ion MicroProbe (SHRIMP) in an leucocratic amphibolite mylonite from the Karasaki mylonites, western Shikoku, Japan which has been considered to belong to the "Paleo-Ryoke" belt. Eight zircon grains from the sample yielded U-Pb ages of Albian (Early Cretaceous) ranging from 114.2 ± 3.9 Ma to 102.3 ±4.5 Ma (± 1σ errors). The protolith of the mylonitized leucocratic amphibolite has been inferred to be basaltic volcanic sandstones, because the SiO_2 content is low (49 wt.%), and the leucocratic amphibolite (consisting of albite and epidote rich, and chlorite and epidote rich layers alternates with amphibole rich and chlorite rich layers on the order of a few mm, suggestive of original sedimentary layering. All the zircon grains can be inferred to be detrital based on both the facts that they were abraded possibly due to sedimentation processes, and that they show oscillatory zoning indicating igneous (primary) texture. The small range of U-Pb ages varying ca. 110 Ma suggests that the zircon grains in the volcanic sandstones were supplied only from the Oshima metamorphic rocks of the Maana belt and the Higo plutonic rocks of the Higo belt whose U-Pb zircon ages were dated as ca. 110 Ma. The fact clearly indicates that the metamorphic rocks (Karasaki mylonites) originated from post-Albian sediments also constitute as a new member of the "Paleo-Ryoke" belt

Stratigraphy, geologic structures, and K-Ar ages of the Ryuhozan metamorphic rocks in western Kyushu, Japan

熊本県, 肥後帯南縁部に位置する竜峰山変成岩の原岩堆積場は, 暗灰色(泥質)石灰岩や砕屑岩を挟在する火山岩および火山砕屑岩優勢な原岩構成に基づき, 受動的大陸縁辺部もしくは島孤縁辺部であった可能性が推察される.竜峰山変成岩中で東西走向・鉛直傾斜のペネトレーティブな片理形成で示される主要な変成・変形作用は, K-Ar鉱物年代測定から白亜紀前期末頃(100 Ma前後)に生じたことが推測された.また, その運動像が北盤水平西ずれのトランスプレッションであったことが大構造および非対称変形微細構造の解析から示唆された.竜峰山変成岩と地体構造的に対比可能な地質体としては, 石炭紀からペルム紀頃の陸棚古生層を原岩とし, 白亜紀に変成作用および左ずれ変形作用を被った南部北上帯や日立変性岩が推察される.The Ryuhozan metamorphic sequence in the southern Higo belt, Japan is lithologically divided into two formations ; the lower formation (total thickness, ca. 950 m) consisting of limestone and chloritoid rocks intercalated by metamorphosed basic-acidic volcanic and volcanoclastic rocks, and pelites and psammites, and the upper formation (total thickness, ca. 850 m) consisting of slates, and bedded and massive limestone. The geologic structure is characterized by an E-W trending and vertically dipping schistosity which was truncated by sinistral trunscurrent faults of NE-SW strike. The sinistral sense of shear can be inferred from more than 70% of the asymmetric microstructures (n=48) such as shear band, shape and lattice fabric of quartz and calcite (i.e. shear sense indicators). Considering also the development of isoclinal folds with E-W trending axis, a large amount of shortening in NS direction and E-W trending left-lateral shear seem to have concurrently occurred in the zone (a left-lateral transpression). Both type I and type II crossed girdle c-axis fabric patterns are developed in the elongated quartz aggregates parallel to the schistosity. The fact suggests that the deformation occurred at the temperature conditions equivalent to the greenschist to amphibolite facies, being consistent with the mineral assemblages (up to hornblende+andesine) observed in the closely associated basic rocks. Hornblende, muscovite and biotite forming schistosity give ca. 100 Ma in K-Ar age, suggesting that the Ryuhozan metamorphic rocks were formed in a Cretaceous regional tectonometamorphic event which affected the whole Higo metamorphic belt. The host lithologies for the Ryuhozan metamorphic rocks were a sedimentary sequence dominant in volcanic and volcanoclastic rocks with bimodal compositions intercalated by impure limestones and laterites, which suggests the depositional setting either at a volcanic arc or at a passive margin which occurred temporarily from Carboniferous to Permian in the Asian continent. This Ryuhozan metamorphic sequence is correlative with the South Kitakami belt and the Hitachi metamorphic sequence based on their protolithic and tectonometamorphic similarities.日本地質学会第104年学術大会にて一部講演．本研究は第一著者（TS）が広島大学で修士論文として行った研究の一部である

Finding of chloritoid from the Ryuhozan zone in the northeastern part of the Yatsushiro City, Kumamoto Prefecture

日本地質学会第101，102年学術大会にて一部講演．野外調査の一部に鹿児島大学援助会援助費（平成4年度）を使用した