西九州, 黒瀬川地帯の下部白亜系砂岩の組成  [in Japanese] Petrography of the Lower Cretaceous sandstones from the Kurosegawa Terrane in western Kyushu, Southwest Japan  [in Japanese]

西九州黒瀬川地帯の下部白亜系は, 四国の物部川層群に対比される三峰山層群と, それとは二枚貝化石相および岩相が異なる先外和泉層群に区分されている.両者の砂岩の全岩化学組成, 砕屑性ザクロ石およびクロムスピネルの化学組成を検討した結果, 1)BIダイヤグラム上での検討では全岩化学組成の時代の経過にともなう変化パターンが両者で逆であること, 2)砕屑性ザクロ石の化学組成では先外和泉層群中のものに, スペッサルティン成分に富むザクロ石が少し多いこと, 3)砕屑性クロムスピネルは先外和泉層群砂岩中にのみ多く認められ, 蛇紋岩に由来するものが多数を占めるが, プレート内玄武岩を起源とするものも少数含まれること, が判明した.三峰山層群と先外和泉層群とは砂岩組成上でも相差が明確であり, 両者は堆積環境や後背地が異なった.離れた場所で形成されたと考えられる.

The Lower Cretaceous strata of the Kurosegawa Terrane exposed in the eastern part of the Yatsushiro area is divided into the two groups, namely, the Mitsumineyama and Pre-Sotoizumi Groups on the basis of the paleontological studies. The sandstone compositions were compared between the two groups to understand the cause of this difference. The analyses were carried out for major elements as whole rock and chemical composition of detrital garnets and chromian spinels. The chemical characteristics of sandstones from the Early Cretaceous two groups were discussed on the B.I. diagram. Roughly, the sandstone compositions of the Mitsumineyama Group migrated from EIA (evolved island arc) field to CA (continental arc) & DA (dissected arc) field, while those of the Pre-Sotoizumi Group migrated from CA & DA field to IIA (immature island arc) field. Detrital garnets of the sandstones from the Mitsumineyama and Pre-Sotoizumi Groups are mostly almandine with a minor amount of Mg and Mn. Mg-rich almandine, Mn-rich almandine and grandite (grossular+andradite), with minor difference of Mn-rich almandine content between the both groups. Detrital Cr-spinels are found only in the sandstones of the Pre-Sotoizumi Group. Judging from the sandstone geochemistry, it can be concluded that there are significant differences in provenance between the Mitsumineyama and the Pre-Sotoizumi Groups.