四国西部唐崎マイロナイトのSHRIMP U-Pb年代

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

Sakashima Toshihiko

寺田 健太郎

Takeshita Toru

Hayasaka Yasutaka

Sano Yuji

Hidaka Hiroshi

Takahashi Yoshio

地質学論集

The memoirs of the Geological Society of 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.

Karasaki mylonite

SHRIMP

U-Pb age

zircon

Paleo-Ryoke belt

jpn

日本地質学会

© The Geological Society of Japan 2000

[ISSN] 0385-8545

[NCID] AN00141779

[NAID] 110003026137