Distribution of Weathering Crust of Granitic Rocks and Alteration of Biotite in the Middle of Abukuma Mountains, Northeastern JAPAN

Abstract

Weathering crust of granitic rocks in the middle part of Abukuma Mountains are divided into five classes, fresh granites, weathered granites, “masa” A, “masa” B and reddish “masa”, according to the weathering degree. Fresh and weathered granites are distributed limitedly and narrowly along large rivers, and “masa” A, “masa” B and reddish “masa” cover extensively all over the area. The thick weathering crusts are distributed restrictedly on the low relief surfaces. The boundary surfaces between weathered granites and “masa” is parallel to the plane connecting the swells of low relief surfaces, and is nearly horizontal. These facts suggest that the weathering crusts of the middle part of Abukuma Mountains are fossil weathering crusts in which were formed from middle Miocene to early Pleistocene.
Clay minerals produced from biotite during the weathering process, were identified by X-ray diffraction examination. The experiment revealed that fresh biotite was first altered into intergradient chlorite-vermicurite, and then changed into interstratified biotite or kaolinite.
Gibbsite exists only along the land surface. This fact suggests that gibbsite has been produced under the present conditions after the current surface was formed.