Marked change in sandstone composition during the Middle Jurassic in Jurassic accretionary complexes of SW Japan, and geologic significance

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  • 西南日本のジュラ紀付加体砂岩におけるジュラ紀中世の組成変化とその意義
  • セイナン ニホン ノ ジュラキ フカタイ サガン ニオケル ジュラキ チュウセイ ノ ソセイ ヘンカ ト ソノ イギ
  • Marked change in sandstone composition during the Middle Jurassic accretionary complexes of SW Japan, and geologic significance

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To clarify the signatures and provenance evolution of Jurassic accretionary complexes, we studied the petrographic features and bulk chemistry of sandstones in the Southern Chichibu Belt and northern margin of the Shimanto Belt, eastern Kyushu, and in the Tamba Belt (Kuga Group), eastern Yamaguchi Prefecture. The sandstones are classified as high-Ti and high-Zr types based on a Zr/Nb-Ti/Nb diagram. The high-Ti sandstones are characterized by relatively low SiO2 contents, high contents of TiO2, MgO, Na2O, and V, and high concentrations of volcanic rock fragments. In contrast, the high-Zr sandstones are characterized by high SiO2 contents, low contents of TiO2, MgO, Na2O, and V, and low concentrations of volcanic rock fragments. The high-Ti and high-Zr sandstones were derived mainly from a magmatic arc and an area dominated by crystalline rocks without volcanic rocks, respectively. The compositional change from high-Ti to high-Zr sandstones occurred during the Middle Jurassic. The crystallization ages of granitic rocks of the Korean, Liaodong, and Jiaodong peninsulas reveal a northwestward migration of the magmatic arc during the Early-Middle Jurassic and a magmatic hiatus during the Late Jurassic and early Early Cretaceous. The extinction of the magmatic arc and rapid unroofing of granitic and continental basement rocks upon the Korean Peninsula during the Late Jurassic and early Early Cretaceous could have produced the high-Zr sandstones. The inland migration of the magmatic arc and subsequent magmatic lull could be explained by a rapid shallowing of the subduction angle and the occurrence of flat-slab subduction, respectively. Flat-slab subduction developed in response to the subduction of a large, buoyant oceanic plateau during the Jurassic.

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