名張斑れい岩体の風化過程 (1)

風化殻の分帯

抄録

Nabari gabbroic body, which belongs to Ryoke Complex, occurs in the mesa-like hill in the southern part of Iga basin, Mie Prefecture, Southwest Japan, and a thick weathering crust occurs at the top level of the hill.
Based on geological and mineralogical studies on neoformation minerals in saprolites and modes of occurrence of core stones with or without “shells”, weathering crust can be divided into three zones, I, IIa and IIb, as follows;
Zone I: kaolin+goethite in saprolite, presence of a core stone coated by shell with gibbsite, Zone IIa: smectite+kaolin±goethite in saprolite, presence of a core stone coated by shell free from smectite,
Zone IIb: smectite+kaolin±goethite in saprolite, presence of a core stone without shell.
Zoning of weathering crust is nearly horizontal and water table lies near the bottom of the zone IIa. Therefore, authigenic occurrence of neoformation minerals in each weathering zone is considered to reflect the hydrolytic decomposition of primary aluminosilicate minerals by percolating meteoric water and stagnant ground water in weathering crust.
The relationships among authigenic formation of minerals in weathering crust, parent rock composition and environmental variables, such as climatic and time factors, are briefly discussed in this paper.