[S-GL23] 日本列島および東アジアの地質と構造発達史

[SGL23-03] Tectonostratigraphic characterization of Maizuru back-arc basin during its closure: geochemical and U-Pb detrital geochronological approaches.

*Larissa NGOMBI MAVOUNGOU¹、Kaushik Das^1,2、Yasutaka Hayasaka^1,2、Haruna Kuriu¹、Kenta Kawaguchi³ (1.Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University、2.HiPeR, Hiroshima、3.Chonbuk National University, Korea)

キーワード：Maizuru back-arc basin, Tonoshiki breccia, Maizuru Group, Timing of basin closure

The rocks of the Maizuru terrane are considered to be related to a paleo back-arc basin that opened during early Permian and closed at the end of Permian. The deposits of this basin are exposed in the central zone of the present-day Maizuru terrane. They are mainly characterized by the Maizuru Group consisting dominantly of massive mudstone, and sandstone, with restricted occurrence of breccia that overly the Yakuno ophiolitic group considered as the basement of Maizuru back-arc basin. The breccia-dominated unit is a part of the Tonoshiki Formation with unclear stratigraphic position. The aim of this study is to understand the final stage of the evolution of Maizuru back-arc basin and for that, fixing the stratigraphic position of Tonoshiki Formation is of utmost importance. This may hold the key to defining the deformation style and mechanism of Maizuru back-arc basin rocks and the tectonic processes occurring prior to its closure. To address these issues, geochemical and geochronological studies have been conducted. Tonoshiki breccia is a collapse breccia with different sized extremely angular clasts. Clasts are made up of blocks of mafic and felsic rocks with minor but ubiquitously present mudstone. The mudstone clasts in Tonoshiki breccia and mudstone of Maizuru Group are both massive in nature and having similar mineral compositions (quartz and minor feldspar clasts in a pool of illite and chlorite-dominated matrix). Moreover, the bulk-rock chemical data indicate that those mudstones samples mostly sourced from felsic igneous rocks of the upper continental crust. The similar characteristics between the mudstone clasts in Tonoshiki breccia unit and the massive mudstone of Maizuru Group fix the stratigraphic position of the Tonoshiki Formation over the Maizuru Group. There is no known rock unit overlying this Permian sequence, making the Tonoshiki Formation as the topmost unit of Maizuru Group. The U-Pb detrital zircon geochronological studies conducted on two samples of Tonoshiki breccia show closed space probability peaks between ca. 285 Ma to 271 Ma for a sample with clasts from Yakuno ophiolites and one slightly younger single probability peak at 268 Ma for a sample with clasts of shallow crustal rocks. This indicates that there are two major possible sources for the Tonoshiki breccia. Its formation possibly occurred by debris flow during two episodes of tectonic activity that affected Maizuru back-arc basin at the end of Permian. The closure of the basin is characterized by multiple pulses of collapse breccia formation, probably induced by a faulting movement.