地向斜堆積物の続成・変成,とくに沸石質変質について

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In the Izumi group, a thick sequence of upper Cretaceous geosynclinal sediments in Southwest Japan, there is a widespread distribution of authigenic zeolites such as analcite, laumontite, stilbite, clinoptilote and mordenite. They occur mainly as an alteration product of glassy material in acidic tuffs which are interlayered with other sedimentary rocks. The authigenic minerals including the zeolites show a distinct zonal arrangement. In the middle part of the Izumi mountain range, the authigenic mineral zone can be divided into two zeolite-bearing zones and one zeolite-free zone as shown in Fig. 1. Because the sedimentary beds overlap successively to a horizontal direction, i.e. to the east, these mineral zones generally cross the stratigraphical boundaries and a rearranged depending on thickness from the uppermost bed at each section which is nearly equivalent to depth of burial. The lower limit of the zeolite zone is at depth of about 3500 m below the top of the sedimentary beds. Concerning the vertical zoning of the zeolitic assemblages in the area, it is noteworthy that the depth of the lower limit of the zeolite zone is relatively shallow and that the range of mordenite, one of the most abundant zeolites, some what extends to deeper depth than the ranges of analcite and laumontite. The mordenite at greater depth seems to be richer in Ca than alkali. Moreover, most analcites from the Izumi group are rich in silica and there is a general tendency that silica content of these analcites decreases with increasing in burial depth. The above-mentioned zonal arrangement of the zeolite s in the Izumi group is compatible with their stability relations experimentally determined, but temperature ranges of synthetic zeolites are relatively high. Authigenic mineral assemblages in sedimentary rocks including those from the Izumi group are controlled by original character of the host rock, chemical condition and age as well as burial depth and geothermal gradient. The rocks containing each zeolite-species have the characteristic range of chemical composition under the influence of their primary compositions (Fig. 2). Especially the chemical composition of the tuffs which contain silica-rich analcites nearly resemble the average composition of rhyolites namely the original rocks. However, the compositional differences between the zeolitized rocks and their original rocks reveal that considerable changes in chemical composition took place during the zeolitic alteration, and the changes seem to be remarkable in the deep burial rocks. To clarify the zeolitic alteration of geosynclinal sediments which extends from diagenesis to metamorphism, more detail field and laboratory works on the subject should be carried out.
日本地質学会の論文
1968-07-31

地向斜堆積物の続成・変成,とくに沸石質変質について

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