紀伊半島四万十累帯白亜系砂岩の重鉱物組成とその時代的変化  [in Japanese] The chronological transition of heavy-mineral composition of the Cretaceous sandstones in the Shimanto Belt of the Kii Peninsula, Southwest Japan  [in Japanese]

紀伊半島四万十累帯白亜系砂岩の重鉱物組成およびザクロ石の化学組成を検討した.重鉱物組成は別所(1997)の結果と併せると6タイプに区分でき, それらは時代的な変化を示すこと, ザクロ石の化学組成は時代が新しくなるつれてスペッサルティン成分に乏しくなり, パイロープ成分に富むアルマンディンが増加すること等が明らかになった.以上の変化は後背地の源岩構成の違いや火成活動の消長を反映したものと考えられ, 既知のデータと併せ考察すると, 紀伊半島四万十累帯白亜系の後背地では白亜紀中期には花崗岩や片麻岩が卓越していたが, 時代とともに酸性の火成活動が活発化していったと推定される.また, この陸側の後背地には現在の内帯と同様な岩石のほかに, グラニュライト相に達する変成岩が存在し, 白亜紀後期になると, こうした変成岩からの供給が増加したと推定される.

Heavy-mineral analysis of sandstones and chemical analysis of detrital garnets were carried out in the Cretaceous Shimanto Supergroup of the Kii Peninsula, Southwest Japan. Heavy-mineral assemblages of the sandstones were classified into six types (I, II, III, III', IV, V) on the basis of the ratios among euhedral zircon, rounded zircon, garnet, epidote, titanite and allanite. These heavy mineral types show a tendency of transition (V→IV→II→III') in the sediments of forearc basin and (II, I→III) in those of the vicinity of trench with time. On the other hand, the pyrope-rich almandine increases gradually with decreasing of the spessartine-rich almandine in the detrital garnets. Judging from the study of the assemblages of heavy minerals, chemical composition of detrital garnets and other published data (the composition of sandstones and conglomerates), the provenance of sandstones of Shimanto terrane was composed chiefly of granitic rocks and pelitic gneiss toward the end of the Early Cretaceous, and acidic magmatism increased in violence during Late Cretacous. The existence of Mg-rich garnets (MgO=10.85%, 41.7 pyrope mole %) suggests that there were granulite-facies metamorphic rocks in the source area of Shimanto Belt.