岐阜県中津川市北部地域の濃飛流紋岩と火山豆石
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概要
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The Nohi Rhyolite is a representative product of the Late Cretaceous acid volcanism in central Japan, composed mainly of a large number of pyroclastic flow deposits. Recent investigations have made clear that these pyroclastic flow deposits being principally welded tuffs, can be stratigraphically divided into five stages (Stages I-V) according to the intervening sedimentary formations between them (COLLABORATIVE RESEARCH GROUP FOR THE NOHI RHYOLITE, 1976). The studied area is situated on the southern part of the Nohi Rhyolite mass. Here the four welded tuff units and associated sedimentary formations are cropped out, together with granodiorite porphyry and granitic rocks which intrude into these volcanic rocks with evident thermal effect. Stratigraphic succession, thickness, and lithology of the Nohi Rhyolite in this area are summarized in the following table. Many accretionary lapillis have found in a non-welded crystal-rich tuff layer of the Sodebora Formation at more than eleven localities in this area and also in other tuffaceous sediments intercalated within the Nohi Rhyolite at more than twenty localities. The occurrence, form and internal structure suggest that they grew in an ash-charged volcanic cloud produced by rather small-scale steam eruption somewhat later than the eruption of large-scale pyroclastic flow, and then fell to the ground or lake water and were sometimes reworked in lake water. After deposition, they were deformed into oblate spheroids in some extent and later they were thermally metamorphosed.[table]
- 1978-09-25