Abstract
Seven cauldrons of Paleogene age occur in the San'in district, western Honshu, Japan. They are mainly constituted by andesitic and felsic volcanic and volcaniclastic rocks. Alteration of these rocks was investigated by means of X-ray diffractometry using Cu-Kα radiation. They are classified into 4 types of alteration: caldera-type intermediate alteration, hornfelsic alteration, hydrothermal alteration, and weathering.
The first type of alteration that is characterized by the presence of chlorite-smectite series is widespread in all cauldrons. It is further classified into 4 zones: weakly alteration, smectite-type, mix-type (smectite and chlorite), and chlorite-type zones. These 4 types of zones are distributed irrespective of stratigraphic order. This feature is different from diagenetic zoning. The second type of alteration (hornfelsic) is characterized by the presence of hornblende or biotite in addition to the assemblage of the chlorite-type zone. It is usually distributed surrounding to intrusive masses of rocks. Both alteration of caldera-type intermediate and hornfelsic may have been formed under heterogeneous distribution of heat, partly relating to intrusive masses. The third type of alteration (hydrothermal) is superimposed upon caldera-type intermediate and hornfelsic alteration. It is classified into 10 zones. Eight zones are characterized by the presence of hydrothermal silicate minerals and other two are characterized by the presence of carbonates and sulfates, respectively. The kind and frequency of above zones are different in each cauldron.
The magnetic susceptibility of altered rocks in all cauldrons significantly decreases in the course of each alteration. That of hornfelsic altered rocks is the largest in averages, whereas that of the hydrothermally altered rocks is the smallest. That of the caldera-type intermediate altered rocks is intermediate, although there is no clear difference among 4 zones of this type of alteration.
