山陰地方中部の新第三系における変質帯の分布  [in Japanese] Distribution of alteration zones in the Neogene formations in the central part of Shan'in district, Japan  [in Japanese]

山陰地方中部の新第三系(砕屑岩を除く)の変質をX線粉末法により調べた.その結果,この地域の変質は続成作用,広域中性変質作用,ホルンフェルシック変質作用,熱水変質作用,風化作用によるものであることが区別された.ホルンフェルシック変質は分布が極めて小さく,風化作用については記載を省略した.続成変質岩は沸石類により特徴付けられ,広域中性変質岩はスメクタイト-緑泥石系によって特徴付けられる.熱水変質岩は多数の変質鉱物によって構成されているが,脈状のものと層状のものに大きく分けられる.続成変質岩には種々の層準で他のタイプの変質が重複しているが,その様相は地域により異なっている.変質岩の帯磁率の測定を行った結果,変質作用の種類によって有意の差異があることが判明した.

The Neogene formations are distributed in the central San'in district facing the Sea of Japan. They contain a large amount of felsic to andesitic volcanic and volcaniclastic materials. Alteration of these materials was investigated by means of X-ray diffractometry using CuKoc radiations. Five types of alteration are discriminated: diagenetic, regional intermediate, hornfelsic, hydrothermal alteration, and weathering. Diagenetic alteration is characterized by authigenic zeolites and classified into 4 zones: fresh glass, clinoptilolite-mordenite, (heulandite) -analcime, and laumontite zones. These zones are distributed in ascending order roughly according to stratigraphy. Regional intermediate alteration is characterized by smectite-chlorite series clay minerals. It is classified into 3 zones: the smectite, the smectite-chlorite mixture, and chlorite zones. Hornfelsic alteration is characterized by biotite and/or hornblend in addition to assemblage of the chlorite zone of regional intermediate alteration. However, this alteration is very rare in this district. Hydrothermal alteration is divided into two types by mode of occurrence of alteration minerals. One is the vein-type in which alteration minerals occur in veins and another is the layer-type in which alteration minerals occur as stratified layers showing zonal distribution. Detail description on weathering was omitted in this paper. Diagenetic alteration is overlapped by other types of alteration in various horizons. The extent of overlapping is different in each areas. The magnetic susceptibility of altered rocks was measured. There are significant differences among altered rocks of each type of alteration.