Abstract
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 Cu-Kα 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.
