Abstract
The Neogene formations in the eastern San’in district facing the Sea of Japan contain a large amount of felsic to intermediate volcanic and volcaniclastic materials. Alteration of these materials was investigated by means of X-ray diffractmetry using Cu Ka radiations. Five types of alteration are discriminated: diagenetic, caldera-type alkaline, regional intermediate, hydrothermal, and weathering.
Diagenetic alteration zone is characterized by authigenic zeolites and classified into three zones: the fresh, the clinoptilolite-mordenite, and the analcime zones. Three zones are distributed in ascending order according to stratigraphy. Caldera-type alkaline alteration zone in this district (Teragi Caldera) is nearly same mineral assemblage to diagenetic alteration zone. However, zeolites occur at shallower burial depths in response to higher geothermal gradients. Regional intermediate alteration zone is characterized by smectite-chlorite series clay minerals. This zone is classified into three zones: the smectite, the smectite-chlorite mixture, and the chlorite zones, and is the most widespread. 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 or amygdals. Another is the massive type in which alteration minerals occur as massive in whole rock. Alteration minerals are silicates, carbonates, sulfates, sulphides, oxides, hydroxides and others. Weathering is characterized by halloysite.
The diagenetic alteration is variously overlapped by other types of alteration in each area. The caldera-type alteration is restricted in the Teragi Caldera only. The weathering crust covers the surface of post-Pliocene volcanic rocks.
The magnetic susceptibility of altered rocks was measured. There are significant differences in the magnetic susceptibility among the altered rocks of each of the alteration type.
