南西諸島奄美大島中生界砂岩中の砕屑性ザクロ石の起源  [in Japanese] Origin of detrital garnet from the Mesozoic sandstones of Amami-Oshima, Nansei Islands  [in Japanese]

奄美大島の三宝山帯と四万十帯の砂岩中の砕屑性ザクロ石の化学組成を測定し, 砂岩の後背地解析を行った。三宝山帯砂岩中の砕屑性ザクロ石は, Mgを含むアルマンディンを主とし, Mn-Feザクロ石をわずかに伴う。一方四万十帯砂岩中のものは, 上記の種類の他に, CaとMgを含むアルマンディンを伴い, 三宝山帯とは異なる化学組成を示す。これらの砕屑性ザクロ石・岩片・重鉱物などから判断して, 三宝山帯の砂岩の後背地は, グラニュライト相に達した泥質岩を原岩とする変成岩からなる変成帯で花崗岩を伴う地域が推定される。また四万十帯の砂岩の後背地は, 泥質岩や塩基性火成岩を原岩とする高度変成岩と花崗岩を基盤とし, 酸性-中性火山岩が分布する地域が推定される。

Provenance analysis of sandstones from the Sanbosan and Shimanto Belts was carried out on the basis of the chemical composition of clastic garnets as well as sandstone petrology. The sandstones from the Early Cretaceous Naon Complex of the Sanbosan Belt are composed of feldspathic sandstone and chert arenite. Feldspathic sandstones from the late Albian to Cenomanian Koniya Complex and the Turonian Naze Complex of the Shimanto Belt generally have a similar composition to each other. Judging from the characteristics of rock fragments and heavy minerals, the provenance for the feldspathic sandstones from both Belts is considered to have been similar to each other, dominated by acidic igneous rocks. However, detrital garnets from these sandstones display a different range in chemical composition. Detrital garnets from the sandstones of the Sanbosan Belt consist mainly of almandine garnet with some Mg and rarely Fe-Mn garnet. On the other hand, those of the Shimanto Belt consist mainly of almandine garnet with some Mg, and minor Fe-Mn garnet and almandine garnet with some Ca and Mg. The detrital garnets from the Sanbosan Belt are regarded as being derived from high-grade, partly granulite facies, metamorphic rocks originally pelitic rocks and granite, and those of the Shimanto Belt from high-grade metamorphic rocks originally pelitic and basic igneous rocks and granite.