Published experimental, thermodynamic, and other geochemical data on H2O-bearing silicic melts are used to obtain relationships which show that: (1) the rate of exsolution of H2O (vesiculation) from silicic magmas that contain more than a few tenths of 1 wt percent H2O is sufficiently rapid to contribute to the explosivity of pyroclastic eruptions; (2) the exsplution of only a few tenths of 1 percent H2O from a typical rhyolitic magma by the second boiling reaction—H2O-saturated melt crystals + H2O vapor releases sufficient mechanical energy (ΡΔV work of expansion)—to cause tensional fracture failure of wall rocks at pressures corresponding to ocean depths of at least 10 km; (3) the ΡΔV energy released by the exsolution of additional H2O, as a result of decompression following Wall-rock failure, is fully adequate to produce pyroclastic eruptions, even at these great ocean depths; (4) the crystallinity and vesicularity of the juvenile pyroclasts of the tuff units that host and underlie the Kuroko ores in the Hokuroku district of Japan are consistent with their having been erupted onto the sea floor at an ocean depth of 3.5 ± 0.5 km, from a magma chamber situated 1.1 ± 0.3 km beneath the sea floor; and (5) the submarine caldera model of Ohmoto (1978) for the formation of volcanogenic massive sulfide deposits appears, therefore, to be viable, at least for the deposits in the Hokuroku district. Application of these same relationships to the 1980 eruption of Mount St. Helens suggests that the March through May 18, 1980, eruptive sequence, including the intrusion of magma into the northern bulge and the landslide-triggering earthquake, was initiated by the second boiling reaction at a snbstantial depth beneath the summit. Furthermore, the devastating blast conld have been, but probably was not, cansed entirely by the virtually instantaneons exsolution of H2O from a higher level magma upon sudden decompression that accompanied the landslide.
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This paper consists of three parts. The first is an overview of the geologic history of the Green Tuff region where all Kuroko deposits occur. The second part presents a description of the stratigraphy and an interpretation of the structural and igneous history of the Hokuroku district, the most important Kuroko mining district. The third part is an analysis of the role of submarine calderas in Kuroko genesis.
The sequence and causes of the major geologic events that have occurred in Japan and its vicinity since the Cretaceous are interpreted as follows: (1) an active but shallow-dipping north-northwestward subduction of the Pacific plate under the Asian continent during a period from approximately 130 to 65 m.y. ago resulted in ilmenite series magmatism in the outer zone of Japan, then still a part of mainland Asia; (2) about 65 to 40 m.y. ago, the direction of the subducted Pacific plate changed to westward and the angle of subduction steepened, initiating back-arc spreading in the Japan basin province and migration of Japan away from the Asian mainland until about 30 m.y. ago; (3) during the period 65 to 30 m.y. ago, the basaltic crust created in the Japan basin province was subducted eastward under the Yamato Ridge province, resulting in calc-alkaline and magnetite series igneous activity in the inner zone of Japan; (4) about 25 m.y. ago, the first sea (proto-Japan Sea) was formed in the Japan basin province as a result of the eustatic rise of the sea following cessation of spreading there about 30 m.y. ago; (5) back-arc spreading was active in the Yamato basin province during the period between 25 and 5 m.y. ago, cansing bimodal volcanism and subsidence in the flanking Inner Honshu and Yamato Ridge provinces [the Hokuroku basin (i.e., a Kuroko-bearing basin), Niigata oil field basin, and Akita oil field basin were all fault-bounded, deep (>2,500 m) marine basins created by rapid subsidence of crustal blocks within a few million years around 17 m.y. ago, although Kuroko mineralization and the accumulation of organic matter were not synchronous]; and (6) the dip of the subducted Pacific plate returned to a shallow angle about 5 m.y. ago, causing the cessation of back-arc spreading and the initiation of subsidence of the Yamato basin province and uplift of the flanking Inner Japan and Yamato Ridge provinces. The Green Tuff activity is, therefore, synonymous with the tectonic and igneous activity that accompanied the formation of the Japan Sea and the Japanese islands during the period from ~65 m.y. ago to the present.