Volcanic subaerial

These different sites of hydrothermal and ore-forming activity may have resulted from the mode of subduction of the Pacific Plate. Mariana-type subduction (characterized by a steep angle of subduction and back-arc basin formation Uyeda and Kanamori, 1979) during middle Miocene caused WNW-ESE extension, submarine hydrothermal activity, thick accumulation of bimodal (basaltic and dacitic) volcanic activity (Green tuff) and Kuroko-type formation (Shikazono and Shimizu, 1993). Plio-Pleistocene Chilean-type subduction (shallow-dipping subduction zone, E-W compression Uyeda and Kanamori, 1979) and oblique subduction of the Pacific Plate beneath the North American Plate led to uplift and expansion of land area, subaerial hydrothermal activity accompanied by meteoric water circulation, subaerial andesitic volcanic activity and formation of vein-type deposits. 

Figure 1.62. Location of epithermal-type deposits in Japan (Shikazono and Shimizu, 1988a). 1 Green tuff and subaerial volcanic region of Tertiary/Quaternary ages, 2 Main Paleozoic/Mesozoic sedimentary terranes, 3 Main metamorphic terranes. TTL Tanakura tectonic line, ISTL Itoigawa-Shizuoka tectonic line, MTL Median tectonic line. Open circle epithermal Au-Ag vein-type deposits, solid circle epithermal base metal vein-type deposits, open triangle epithermal Au disseminated-type deposits.

Leaching of sulfide sulfur from subaerial young (Miocene-Pliocene) volcanic rocks (Shikazono, 1987b). 

The type of volcanic activity in and around the Japanese Islands changed throughout Tertiary. In early Tertiary subaerial andesitic activity was intense. For example, in... 

Northeast Honshu, subaerial andesitic volcanic activity was dominant at Daijima stage (mostly early Miocene). From middle Miocene bimodal basaltic-dacitic activity started with rapid subsidence in Northeast Japan. The production of volcanic activity was probably greater than today. 

From late Miocene to present, subaerial arc-volcanic activity (calc—alkali rocks, andesite, tholeiitic and high alumina basalt) started associated with uplift of the Japanese Islands. This volcanic activity is different from that at middle Miocene age. 

It is inferred that in the northern part of the province submarine volcanic rocks are thick in the central zone, while at marginal zone it is thin and the Plio-Pleistocene subaerial volcanic rocks are exposed. The vein-type deposits occur widely in the province. The precious vein-type deposits occur in relatively young (Plio-Pleistocene) volcanic rocks, while large base metal vein-type deposits (e.g., Toyoha, Inakuraishi, Ohe) and Kuroko deposits (e.g., Kunitomi) occur in central zone where thick Miocene submarine volcanic rocks are distributed (Figs. 1.149 and 1.150). Small base metal vein-type deposits occur in Paleozoic rocks in the southern part. 

If the submarine vs. subaerial hypothesis mentioned above is correct, it is expected that Kuroko deposits are found in the marine sedimentary and volcanic horizons whose... 

In this tectonic situation, intense bimodal volcanism and associated seawater circulation occur, resulting to the formation of Kuroko deposits on the seafloor and formation of vein-type mineralization under subaerial condition and intense hydrothermal and volcanic CO2 fluxes to ocean and atmosphere. Such fluxes affect the long-term environmental changes (see Chapter 4). 

In contrast, in Southwest Japan, polymetallic veins (so-called xenothermal-type deposits in the sense of Buddington (1935) or subvolcanie hydrothermal type in the sense of Cissartz (1928, 1965) and Schneiderhohn (1941, 1955) occur. Examples of these deposits are Ashio, Tsugu, Kishu and Obira. All these vein-type deposits have formed at middle Miocene age in western part of Tanakura Tectonic Line under subaerial environment. In these deposits, many base-metal elements (Sn, W, Cu, Pb, Zn) and small amounts of Au and Ag are concentrated. These deposits are associated with felsic volcanic and plutonic rocks along the Median Tectonic Line (MTL) or south of MTL. 

This submarine vs. subaerial hypothesis for the origin of the two types of deposits (Kuroko deposits, epithermal vein-type deposits) can reasonably explain the difference in metals enriched into the deposits by HSAB (hard-soft acids and bases) principle proposed by Pearson (1963) (Shikazono and Shimizu, 1992). Relatively hard elements (base metal elements such as Cu, Pb, Zn, Mn, Fe) are extracted by chloride-rich fluids of seawater origin, while soft elements (Au, Ag, Hg, Tl, etc.) are not. Hard elements tend to form chloro complexes in the chloride-rich fluid, while soft elements form the complexes in H2S-rich and chloride-poor fluids. Cl in ore fluids is thought to have been derived from seawater trapped in the submarine volcanic and sedimentary rocks. 

Tsunogai, U., Ishibashi, J., Wakita, H., Gamo, T., Watanabe, K., Kajimura, T., Kanagawa, S. and Sakai, H. (1994) Peculiar features of Suiyo seamount hydrothermal fluids, Izu-Bonin arc Differences from subaerial volcanism. Earth Planet. Sci. Lett., 126, 289-301. 

Intense submarine and subaerial volcanic activities during the Tertiary at Green tuff regions took place not only at the Japan Sea but also at marginal basins in the circum-Pacific Region. 

Sano and Williams (1996) calculated present-day volcanic carbon flux from subduction zones to be 3.1 x 10 mol/year based on He and C isotopes and C02/ He ratios of volcanic gases and fumaroles in circum-Pacific volcanic regions. Williams et al. (1992) and Brantley and Koepenich (1995) reported that the global CO2 flux by subaerial volcanoes is (0.5-2.0) x lO mol/m.y. and (2-3) x 10 mol/m.y. (maximum value), respectively. Le Guern (1982) has compiled several measurements from terrestrial individual volcanoes to derive a CO2 flux of ca. 2 x 10 mol/m.y. Le Cloarec and Marty (1991) and Marty and Jambon (1987) estimated a volcanic gas carbon flux of 3.3 X 10 mol/m.y. based on C/S ratio of volcanic gas and sulfur flux. Gerlach (1991) estimated about 1.8 x 10 mol/m.y. based on an extrapolation of measured flux. Thus, from previous estimates it is considered that the volcanic gas carbon flux from subduction zones is similar to or lower than that of hydrothermal solution from back-arc basins. 

Th- U dating of samples from subaerial volcanoes. When several volcanic rocks covering a significant period of the eruptive activity of a volcano can be dated (either by mineral isochrons or by other dating methods), the evolution through time of the ( °Th/ Th)o or ( °Th/ U)o initial ratios will be revealed (see section 3.4). If these ratios remain nearly constant, then they may be used to calculate the ages of other lavas... 

Ra- Th (and ( Ra)/Ba) dating of subaerial volcanics. Systematic analyses of Ra- °Th disequilibria in recent and well dated rocks from active volcanoes allow studies of magmatic evolution as explained in section 3.4. If the variations through time of ( Ra/ Th)o or ( Ra)o/Ba ratios are sufficiently well constrained, one may use the... 

In the northern part of the TCZ, the Chaleurs Group comprises subaerially deposited. Late Silurian bimodal volcanic rocks that overlie early Silurian shallow marine fine- to coarse-grained clastic and carbonate sedimentary rocks. The Chaleurs Group is overlain disconformably by bimodal volcanic rocks and interlayered sandstones and siltstones of the Dalhousie Group. 

We summarize noble gas amounts in deep-sea and subaerial sediments in Figure 5.1. From the data displayed here, we calculated median values which are shown in Table 5.1. Both Figure 5.1 and Table 5.1 show that even though there is little difference in the lighter noble gas concentration between subaerial and deep-sea sediments (He, Ne, and Ar), heavier noble gases are much more abundant in subaerial sediments than in deep-sea sediments. As in volcanic rocks (cf. Section 6.6), most sediments, either deep-sea or subaerial, show fractionation toward the heavier ones relative to air noble gas, although the mechanism for the fractionation may be different. Figure 5.2 shows noble a gas elemental abundance pattern relative to the air abundance subaerial sediments show much more severe fractionation. 

Study of specific regions in which iron cherts and volcanic rocks are spatially unconnected naturally led to the development of hypotheses of an exogene source of the iron, unrelated to volcanism. Thus there arose the hypothesis that the iron cherts were formed from material supplied to the sedimentary basin in the course of intensive subaerial weathering. Svital skiy (1924), Piatnitskiy (1924), Strakhov (1947), James (1954), White (1954), Belevtsev (1957) and Plaksenko (1966) shared this hypothesis. 

The question whether the role of volcanic processes in the formation of the iron cherts diminished from Archean to Proterozoic, and correspondingly the role of products of subaerial weathering increased, remains controversial. 

This volcanism rate is the same as the average rate of subaerial volcanism on Earth and is —5% of the terrestrial plate creation rate of 20km yr . The required sulfur eruption rate to maintain SO2 on Venus at steady state is —28 Tg yr . This is similar to estimates of 9 (subaerial), 19 (submarine), and 28 (total) Tgyr for SO2 emissions from terrestrial volcanism (Charlson et al., 1992). 

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