K-Ar年代に基づく弱変成付加体の年代学 : 西南日本におけるジュラ紀-白亜紀初頭付加体の例(付加体地質学の手法)(ジュラ紀付加体の起源と形成過程)  [in Japanese] Geochronology of low-grade metamorphic accretionary complexes based on K-Ar dating : examples of Jurassic-earliest Cretaceous accretionary complexes in Southwest Japan(Methods)(Origin and evolution of Jurassic accretionary complexes in Japan)  [in Japanese]

K-Ar年代測定法が付加体の年代学に応用され, 過去の付加体が受けた弱変成作用の年代決定が可能となった.これに伴って, (1)付加体の付加・変成履歴の解明, (2)地体構造境界の決定, (3)年代未詳弱変成地質体の実体解明, (4)砕屑性白雲母の混入および接触変成作用に伴うK-Ar年代への影響などに関する地質年代学的研究が進展した.本稿では, 西南日本各地のジュラ紀〜白亜紀初頭付加体の(1)に関した研究のレビューを行った.また, ジュラ紀〜白亜紀初頭付加体の広域的なユニット区分を行い, K-Ar年代に基づくユニットの変成年代を概観した.さらに, 付加年代と変成年代との年代間隔やK-Ar年代に基づく付加体のユニット区分・対比の有効性について検討を行った.

The advanced K-Ar age determination method has made useful constraints on the timings of subduction-related and low-grade metamorphism of the ancient accretionary complexes (AC) in Southwest Japan. By virtue of it, the following geochronological studies have been carried out for the AC : (1) analysis of accretionary and subsequent metamorphic process; (2) discrimination of older or younger AC from high-P/T metamorphic complex series and re-definition of geotectonic boundary between them; (3) age assignment and determination of geotectonic position for nonfossiliferous AC; and (4) effect of detrital white mica and contact metamorphism to K-Ar dating.Several studies on (1) for the Jurassic-earliest Cretaceous AC in Southwest Japan were briefly reviewed by showing reconstructed primary stratigraphies, that are, oceanic plate stratigraphies and K-Ar phengitic mica ages from the AC. On the basis of oceanic plate stratigraphies, the AC are subdivided into many geologic units. Time-lag between accretion estimated from the youngest geologic time of each stratigraphy and peak metamorphism represented by average of K-Ar ages is a range of 13-39 million years for every unit. This age relation mainly depends on a range of age for the youngest radiolarian assemblage from the unit and assemblage zone, selection of radiolarian zone and geologic time scale, error of K-Ar dating, effect of detrital white mica to K-Ar age, rejuvenation of K-Ar age caused by ductile deformation, defference on accretion-metamorphic process of each unit, and so on.