山陰地方東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 eastern part of San' in district, Japan  [in Japanese]

山陰地方東部の新第三系の変質(砕屑岩を除く)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,カルデラ型アルカリ性変質作用,広域中性変質作用,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.続成作用岩およびカルデラ型アルカリ性変質岩は沸石類により特徴付けられ,後者は前者に比べ,より高い地温勾配条件下で生成し,本地域では照来カルデラ内にのみ見られる.広域中性変質岩はスメクタイト-緑泥石系粘土鉱物により特徴付けられ,本地域に最も広く分布する.熱水変質岩は脈状のものと塊状のものがあり,ケイ酸塩など多様な鉱物により構成されている.風化岩はハロイサイトにより特徴付けられている.続成変質岩には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差異があることが判明した.

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.