Island-arc

Equivalent igneous and metamorphic lithologies appear to weather about twice as rapidly in island-arc and younger volcanic... 

Fig. 9-4 Photograph of landslides (soil avalanches) that occurred following earthquakes in Panama on July 17,1976, near Jaque. In the background is a bay of the Pacific Ocean. The effects of this earthquake are described by Garwood et al. (1979), who estimated that about 42 km (about 10%) of the region near the epicenter of the earthquake was devegetated. The bedrock is mostly island-arc basalts and andesites. (Photography by N. C. Garwood.)...

Earth (Li, 1976). The high denudation rate is a reflection of the poorly lithifled, highly tecton-ized nature of the sedimentary rocks that compose the island. Sediment-yield data compiled by Milliman and Meade (1983) and Milliman and Syvitski (1992) indicate that island arcs and mountain belts in the tropical and subtropical west Pacific may contribute as more than 22% of all solid material discharged by rivers into the ocean. Furthermore, the tropical mountainous areas in southeast Asia and India may contribute another 33%. 

Figure 1.4. Comparison of quantities of ore deposits formed in late Cenozoic in NE and SW Japan. Weight per kilometer length of island arc (Ishihara, 1978).

Figure 1.5. Three island arc junctions in the Japanese Islands (Kubota, 1994).

Figure 1.6. Di.stribution and temporal and spatial relationship of late Cenozoic gold deposits in the Japanese Islands. 1 Quartz vein-type gold deposits with little to no base metals. 2 Gold-silver deposits with abundant base metals. 3 Distribution boundary of gold deposits formed during the Miocene. 4 Location of Plio-Pleistocene gold deposits at the actual island arc junctions. 5 Location of Plio-Pleistocene gold deposits in front of the actual island arc junctions. Numbers in the figure are K-Ar ages of epithermal Au-Ag veins (Kubota, 1994).

Several different hypotheses on the tectonic setting of the Kuroko mine area have been proposed. They include volcanic front of island arc (T. Sato, 1974 Horikoshi, 1975a), rifting of island arc (Cathles, 1983a), back-arc depression (Fujioka, 1983 Uyeda, 1983), and back-arc basin. 

In recent years, many hydrothermal solution venting and sulfide-sulfate precipitations have been discovered on the seafloor of back-arc basins and island arcs (e.g., Ishibashi and Urabe, 1995) (section 2.3). Therefore, it is widely accepted that the most Kuroko deposits have formed at back-arc basin, related to the rapid opening of the Japan Sea (Horikoshi, 1990). 

The summary of the bulk chemical compositions (major elements, minor elements, rare earth elements), Sr/ Sr (Farrell et al., 1978 Farrell and Holland, 1983), microscopic observation, and chemistry of spinel of unaltered basalt clarifies the tectonic setting of Kuroko deposits. Based on the geochemical data on the selected basalt samples which suffered very weak alteration, it can be pointed out that the basalt that erupted almost contemporaneously with the Kuroko mineralization was BABB (back-arc basin basalt) with geochemical features of which are intermediate between Island arc tholeiite and N-type MORE. This clearly supports the theory that Kuroko deposits formed at back-arc basin at middle Miocene age. 

In Fig. 1.85 iron contents of epidote from two different geologic environments, island arc and oceanic ridge or ophiolite, are summarized. It can be seen in Fig. 1.85 that the iron content of epidote from ridge basalt and ophiolite is generally lower than... 

The above-mentioned changes in paleogeography, volcanism, crustal movement (subsidence and uplift), and stress field clearly demonstrate that these features of back-arc volcanism in early-middle Miocene are quite different from those of Island arc volcanism in late Miocene to present. According to Yoshida and Yamada (2001), the age of change in volcanism from back-arc type to Island arc type in Northeast Honshu was 12.7 Ma and this age corresponds to the age of Kuroko formation. 

From late Miocene, uplift took place due to the collision of Pacific plate to North American plate under the Kuril arc and Japanese island arc. 

Cathles, L.M. (1983a) Kuroko-type massive sulfide deposits of Japan Products of an aborted Island-arc rift. Econ. GeoL Mon., 5, 96-114. 

Horikoshi, E. (1975b) Statistical distribution of strikes of the Neogene vein deposits in the NE Japan arc. In Horikoshi, E. (ed.). Island Arcs, Marginal Seas, and Kuroko Deposits. Mining Geology Special Issue, 11, 117-124 (in Japanese). 

Horikoshi, E. (1995) Sulfur of ore pyrite in island arc environments. Resource Geology, 45, 213-222 (in Japanese). 

Ishihara, S. and Sasaki, A. (1994) Sulfur isotopic characteristics of late Cenozoic ore deposits at arc junction of Hokkaido, Japan. The Island Arc, 3, 122-130. 

Kobayashi, K. (1983) Spreading of the sea of Japan and drift of Japanese island arc a synthesis and speculation. Mining Geology Special Issue, 11, 23-36. 

Kubota, Y. (1994) Temporal and spatial relationship and significance of island arc junction on the late Cenozoic gold deposits in the Japanese Islands. Resource Geology, 44, 17-24 (in Japanese). 

Niitsuma, N. (1979) Development of geologic structure of Northeast Japanese Island arc. Kagaku (Science), 49, 36-43 (in Japanese). 

Otofuji, Y. (1996) Large tectonic movement of the Japan Arc in late Cenozoic times inferred from paleomag-netism Review and Synthesis. The Island Arc, 5, 229-249. 

Shikazono, N., Shimizu, M., Inoue, A. and Utada, M. (eds.) (1999) The Japanese Island Arc Its Hydrothermal and Igneous Activity. Resource Geology Special Issue, 20. 

Sugimura, A. and Uyeda, S. (1973) Island Arcs Japan and Its Environs. Amsterdam Elsevier. 

Ueda, A. and Sakai, H. (1984) Sulfur isotope study of Quaternary volcanic rocks from the Japanese islands arc. Geochim. Cosmochim. Acta, 48, 1837-1846. 

Shinozuka et al. (1999) analyzed the host volcanic and intrusive rocks in the Minamidani mine district in the Maizuru tectonic Belt and found that these rocks formed in an island arc back-arc system near Laurasia during late Paleozoic. Probably the Yanahara deposits, one of the representative Hitachi subtype deposits, were formed in an island arc back-arc system as same as the Minamidani. Sato and Kase (1996) thought that the Hitachi-subtype deposits formed in back-arc rift or continental rift (Table 2.21). 

Hydrothermal Flux from Back-Arc Basin and Island Arc and Global Geochemical Cycle... 

Previous studies demonstrated that the CO2 fluxes by hydrothermal solution and volcanic gas from midoceanic ridges play an important role in the global CO2 cycle and affect the CO2 concentration in the atmosphere (e.g., Javoy, 1988). However, submarine volcanism and hydrothermal activity occur not only at midoceanic ridges but also at island arc and back-arc basins as already noted. 

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