Tectonic setting

Japan is situated along the boundary between Eurasia and the Pacific plate (Fig. 1.1). The metallogeny, geology, geophysics and geochemistry of Japan are largely controlled by this tectonic setting. 

It is clear in Fig. 1.10 that the distribution of Kuroko deposits is restricted in a narrow zone in the Green tuff region which was called a Kuroko belt by Inoue (1969). This belt was formed by rapid subsidence under the extensional stress regime and is thought to have been a back-arc depression zone at middle Miocene age. The relationship between tectonic setting and formation of Kuroko deposits is discussed in section 1.5. 

General geology, country rocks and tectonic setting... 

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. 

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. 

Fig. 2.30. Location and tectonic setting of Okinawa Trough. The solid circles mark the hydrothermal fields listed in Table 2.12. The broken lines show contours of 1000 m water depth. The triangles indicate Quaternary volcanoes (Ishibashi and Urabe, 1995).

Tectonic settings, geologic structure and volcanic rocks... 

Fig. 2.36. Tectonic setting of Kyushu, Japan, showing location of (north) Hishikari and Nansatsu (south) deposits in Kirishima arc. Triangle volcanoes younger than Middle Pleistocene stipple accretionary prism of Cretaceous and Tertiary age W-B zone isobath of deep Wadati-Benioff seismic zone MTL Median Tectonic Line BTL Butsuzo Tectonic Line K-P Ridge Kyushu-Palau Ridge (Mitchell and Leach, 1991).

Sato and Kase (1996) summarized geological characteristics and inferred tectonic settings of the Besshi-type deposits as shown in Table 2.21. These geological characteristics are summarized below. 

Tectonic setting Back-arc rift or continental rift Sediment-barren midoceanic Sediment-covered... 

Mariko, T. and Kato, Y. (1994) Host rtx k geochemistry and tectonic setting of some volcanogenic ma.ssive sulfide deposits in Japan Examples of the Shimokawa and the Hitachi ore deposits. Resource Geology. 44, 353-367. 

Shinozuka, M., Furuno, M. and Mariko, T. (1999) Host rock geochemistry and tectonic setting of volcanogenic massive sulfide Cu deposits Example of the Minamidani deposits, Hyogo prefecture. Resource Geology, 49, 29-41. 

As mentioned already in Chapter 2, submarine volcanism occurs not only at midoceanic ridges but also at subduction-related tectonic settings such as the Shikoku and Daito Basins, Farce Vela Basins, and Mariana Trough, Okinawa Trough and Izu Bonin Arc (e.g.. Wood et al., 1980 Dick, 1982 Delaney and Boyle, 1986). 

Bulk rock chemistry of hydrothermally altered midoceanic ridge basalt has been well studied and used to estimate the geochemical mass balances of oceans today (Wolery and Sleep, 1976 Humphris and Thompson, 1978 Mottl, 1983). In contrast, very few analytical data on hydrothermally altered volcanic rocks that recently erupted at back-arc basins are available. However, a large number of analytical data have been accumulated on the hydrothermally altered Miocene volcanic rocks from the Green tuff region in the Japanese Islands which are inferred to have erupted in a back-arc tectonic setting (section 1.5.3). 

Lipman, P.W. (1982) Volcano-tectonic setting of Tertiary ore deposits, southern Rocky mountains. Mining... 

Observed U-series disequilibria in MORE relative to other tectonic settings... 

Figure 1. Solidus as a function of pressure and temperature for diy peridotite, wet peridotite, pyroxenite and eclogite. This diagram illustrates the range in solidus temperatures potentially found in the intraplate tectonic setting where compositional differences and differences in volatile contents are expected compared with the mid-ocean ridge (dry peridotite).

Most of the differences between arc magmas and those from other tectonic settings result from oxidation (Wood et al. 1990 Blatter and Carmichael 1998 Parkinson and Arculus 1999) and hydration (Blatter and Carmichael 1998) of arc magma sources by... 

Convergent margins are generally considered to be the principle present-day tectonic setting where new continental crust is formed (-1.1 kmVyr, Reymer and Schubert 1984). As illustrated on Figure 23, this new crustal material is characterized by Th/U ratios that are even lower than the Th/U ratio of the MORB mantle (2.6, Sun and McDonough 1989) yet the Th/U ratio of the bulk continental crust (3.9, Rudnick and Fountain 1995) is close to the Th/U ratio of the bulk silicate earth (see Bourdon and Sims 2003). There are several possible explanations for this paradox. Firstly, it is possible that the processes that formed the continental crust in the past were different to those in operation today. Since... 

Syme, E.C. Bailes, A.H. 1993. Stratigraphic and tectonic setting of early Proterozoic volcanogenic massive sulfide deposits, Flin Flon, Manitoba. Economic Geology, 88, 566 -589. 

The REE data suggest that these carbonate rocks cannot be carbonatites of mantle origin. However, it is noteworthy that the range of compositional variations of typical carbonatite family rocks overlaps with those of sedimentary, metamorphic, and endogenic carbonate rocks (Samoilov 1991). Samoilov (1991) explained that one reason for the significant compositional variations of the carbonatites were due to different types of the associated alkaline rocks under different geologic-tectonic settings. 

Pearce, J. A. Cann, J. R. (1973). Tectonic setting of basic volcanic rocks determined using trace-element analyses. Earth Planet. Sci. Letters, 19, 290-300. 

Hollings, P., Cooke, D.R., Clark, A. 2005. Regionai geochemistry of Tertiary voicanic rocks in Centrai Chiie impiications for tectonic setting and ore deposit genesis. Economic Geology, 100, 887-904. 

Fig. 1. Primitive mantle (PM) normalized whole rock PGE profiles for stratiform chromitites from the Stillwater Complex and for podiform chromitites from the mantle section of Thetford Mines Ophiolite (TMO). Note the similarity between IPGE contents of chromitites from these different tectonic settings. D.L.= detection limit.

We analyzed chromites from 1) podiform chromitite deposits from the mantle section of the Thetford Mines Ophiolite (TMO), southern Quebec Appalachians (Canada) 2) stratiform chromitite seams found within the Stillwater Complex, Montana (USA). In addition, we also analyzed chromite in lavas from different tectonic settings in order to 1) verify whether the IPGE solubility in chromite could be related to geological setting and... 

We have evaluated the IPGE (Os, Ir, Ru, Rh) content of chromite grains from chromitites and lavas in various tectonic settings using LA-ICPMS. 

Armstrong-Altrin, J.S., Yong, I.L., Verma, S.P., Ramasamy, S. 2004. Geochemistry of sandstones from the Upper Miocene kudankulam formation, southern india implications for provenance, weathering, and tectonic setting. Journal of Sedimentary Research, 74, 285-297. 

Bhatia, M.R. Crook, K.A.W. 1986. Trace element characteristics of graywackes and tectonic setting discrimination of sedimentary basins. Contributions to Mineralogy and Petrology, 92, 181-193. 

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