秩父累帯南帯砂岩の地域的組成変化  [in Japanese] Local variation of sandstone compositions in the southern belt of the Chichibu Terrane  [in Japanese]

関東山地, 四国中・西部, 九州東部に分布する秩父累帯南帯の斗賀野層群相当層の砂岩組成を検討した。各地域の砂岩は, 非常によく似た組成を持ち, 長石と石英が多く, 岩片と基質の少ない, 長石質アレナイトである。主に花崗岩質岩からなる源岩城が想定される。関東山地の海沢層では, 中期ジュラ紀と後期ジュラ紀の砂岩で組成に違いが認められる。カリ長石に地域的変化が認められ, 四国中・西部の斗賀野層群と俵津層の砂岩は, 他地域のものに比べ正長石が多いことから, それらの源岩である花崗岩質岩は, 関東山地の海沢層や九州東部の尺間山層の砂岩の供給源に比べ, より高温で形成されたと考えられる。斗賀野層群相当層の砂岩は, 現在の南海トラフの堆積物とは異なり, 当時の主要な海底谷を通って海溝まで運ばれた砂粒が底層流によって均質化され形成されたが, 運搬経路ごとの違いは残していると考えられる。

Modal composition of sandstones from the chert-clastic stratigraphic units in the southern belt of the Chichibu Terrane of the Kanto Mountains, central and western Shikoku and eastern Kyushu have been analyzed. The units correlate with the Togano Group of central Shikoku, and the age of the clasticsis Middle to Late Jurassic. Most sandstones of the Togano Group and its correlatives are feldspathic arenite, and are derived from an area in which granitic rocks were widely distributed. Ternary Q-F-L and Qm-P-K plots of the sandstone compositions from each area show that they almost plot in the same field. Among sandstones from the Unazawa Formation, the composition of sandstones from the Middle Jurassic are different from those of the Late Jurassic. It is recognized that the amount and variety of potash-feldspar in those areas differ. It means that granitic rocks, which provideds and grains to central and western Shikoku, were emplaced under higher temperature conditions than in the other areas. Sandstones of the Togano Group and its correlatives have been transported through submarine canyons to the trench. Though they are uniformalized by the bottom current along the trench axis, the local variation of sandstone compositions are preserved as a result of being transported via different submarine canyons.