四国東部の四万十累帯白亜系の砂岩における斜長石粒子の曹長石化  [in Japanese] Albitization of detrital plagioclase in the Cretaceous sandstones from the Shimanto Belt in eastern Shikoku, Southwest Japan  [in Japanese]

四国東部の四万十累帯北帯は, 北亜帯, 中亜帯, 南亜帯の3つに細分され, 南側の地帯ほど地質時代が若くなる。3つの亜帯から採集した中粒〜粗粒の砂岩中の斜長石粒子の組成をEDXで分析したところ, 北亜帯では平均45%, 中亜帯では平均55%, 南亜帯では平均87%の斜長石粒子が曹長石であることが判明した。砕屑性の曹長石粒子はわずかなものと推定されるので, 曹長石粒子の大部分は斜長石粒子が続成的に曹長石化したものと考えられる。曹長石化の条件は, フッ石相における曹長石帯の条件とほぼ同じと推定される。南亜帯の砂岩はもっとも若い時代であるにもかかわらず, 中亜帯および北亜帯の砂岩よりも大きな度合で曹長石化している。このことは, 付加帯を形成する造構場において南側の亜帯の地層ほどより深い層準まで, 構造的に埋没されたことを示すものと考えられる。

The Cretaceous strata of the Northern Shimanto Belt, eastern Shikoku, consist mainly of black shale, gray wacke sandstone and alternating beds of sandstone and shale, associated with a small amount of chert, greenstone and acidic tuff. This thick sedimentary pile is regarded as an accretionary complex. The Northern Shimanto Belt is subdivided into three zones, namely, northern, middle and southern zones. The geological age of the zone becomes younger southward. Electron microprobe analysis (EDX) was performed on the detrital plagioclase in coarse to medium-grained sandstones from the belt. The Ab Index, the ratio of ablite grains to total plagioclase grains, of the sandstones from the southern zone is 87 per cent on average, and those from the middle and northern zones are 55 and 45 percent on average, respectively. Few detrital albite grains should exist because the clastic sediments of the Cretaceous Shimanto Group are considered to have been derived mainly from an acidic to intermediate volcanic-plutonic terrane, judging from the framework composition of sandstone and conglomerate. There are almost no igneous intrusions or thermal alteration in the area. Therefore, the albitization of the plagioclase may be due to adiagenetic change. The conditions of albitization seem to be nearly the same as that of the albite zone in zeolite facies. Detrital plagioclase in the sandstones situated in the southern zone are albitized more severely than those in the middle and northern zones, although the southern zone is younger. This suggests that the strata in the southern zone was buried tectonically to a deeper horizon during accretionary tectonics.