義城地域の白亜紀堆積物の地球化学と後背地 Geochemistry and provenance of Cretaceous sediments from the Euisong block, Gyeongsang Basin, Korea

韓半島東部の白亜系慶尚層群の河川成〜湖成堆積物は, 現在の朝鮮大陸の東縁に白亜紀初頭に発達した北北東方向の伸長性の山間盆地を埋積した堆積物である.義城(Euisong)地域の新洞(Shindong)および河陽(Hayang)層群から採集した81試料の全岩分析を行い, 元素組成を主要に規制する要素を明らかにし, 全体としての後背地構成と風化作用の経過を見積った.全岩化学組成は粒度に強く影響を受け, そのため岩相の変化に調和し, ほとんどの元素がSiO_2/Al_2O_3の変化に対応して系統的に変化する.Ti, Fe, Mg, K, P, Rb, Ni, Cr, Nb, V, Y, ZnとおそらくCe, Th, Gaの濃縮率は, SiO_2/Al_2O_3と粒度の増加にともなって減少し, このことは粘土基質が影響していることを示す.反対にCa, Na, SrとおそらくBaやPbは増加するが, これはこれらの元素が長石の粒子や炭酸塩鉱物基質に存在することを示す.Zrは特定のパターンを示さず, 基質とジルコン粒子の濃縮の両方に制約されているらしい.いくつかの試料では重鉱物の分級作用は, Ce, Th, Y, Ni, Crの濃縮率にも影響している.いくつかの判別図と移動しにくい元素の比に基づけば, 新洞層群の元素組成はIタイプやSタイプ花崗岩の平均化学組成をもち, この地層が地下深部まで風化が進んだ長石質の非活動的大陸縁に起源をもつことを示す.隣接する嶺南(Ryongnam)地塊の結晶質岩石も源岩として妥当である.河陽層群は, 判別図ではより未成熟であるが, 移動しにくい元素の比からは新洞層群のそれと類似する.しかし, 源岩の風化度は小さい.新洞層群に比べて, 河陽層群に見られる元素組成の異常は風化作用と続成作用の影響によるものであろう.

Fluvial-lacustrine sediments of the Cretaceous Gyeongsang Basin in east Korea represent the fill of a NNE-trending extensional intermontane trough which developed near the eastern margin of the present Korean continent in the Early Cretaceous. Whole-rock analyses of a suite of 81 samples from the Shindong and Hayang Groups in the Euisong area were carried out to determine the primary controls on composition, and to asses bulk provenance and weathering history. Whole-rock chemistry is strongly influenced by grain size, with most elements showing systematic variations with SiO_2/Al_2O_3 according to lithology. Ti, Fe, Mg, K, P, Rb, Ni, Cr, Nb, V, Y, Zn and probably Ce, Th and Ga abundances decrease with increasing SiO_2/Al_2O_3 and grain size, suggesting clay matrix control. Conversely, Ca, Na, Sr and possibly Ba and Pb increase, suggesting residence in feldspar and carbonate matrix. Zr shows no pattern, and is likely to be controlled both by matrix and zircon concentration. Heavy mineral sorting also influences Ce, Th, Y, Ni and Cr abundances in some samples.Discriminant diagrams and immobile element ratio plots show that the Shindong Group was derived from a deeply weathered felsic passive continental margin source, with a bulk composition close to that of average I- and S-type granite. Crystalline rocks of the adjacent Ryongnam Massif are a suitable source. Although discriminant diagrams indicate a less mature source for the Hayang Group, immobile element ratio plots show that the bulk source composition was similar to that of the Shindong Group. The source, however, was less weathered. Anomalies in Hayang chemistry compared with the Shindong Group are attributable to the effects of weathering and diagenesis.