Oceanic plateau

Continental interiors are thought to be constructed from welded pieces of island arcs, metamorphosed sediments, oceanic plateaux and continental fragments (e.g. Hoffinan 1989). Modern-day analogues for this include the convergent zones of the Western Pacific (Hamilton 1979), and the South American-Caribbean plate boundary, where the Leeward Antilles arc is colliding with South America (Ave Lallemant 1997). To what extent these processes were occurring... 

Kerr, A. C, White, R. V. Saunders, A. D. 2000. LIP reading recognizing oceanic plateaux in the geological record. Journal of Petrology, 41, 1041-1056. 

In the oceans, hot-spots are recognised as areas that are shallower than normal ocean floor of the same age. Sometimes they are contemporaneous with spreading, as in Iceland. Other hot-spots postdate the ocean floor by tens of millions of years, as in the Hawaiian-Emperor Seamount Chain, or give rise to oceanic plateaux, such as the Ontong Java plateau in the southwest Pacific. 

Puchtel I. S., Hofmann A. W., Mezger K., Jochum K. P., Shchipansky A. A., and Samsonov A. V. (1998) Oceanic plateau model for continental crustal growth in the Archaean a case study from the Kostomuksha greenstone belt, NW Baltic Shield. Earth Planet. Sci. Lett. 155, 57-74. 

N. Lesotho (Nixon and Boyd 1973b) Monastery (Gurney et al, 1979), Jagersfontein (Hops et al, 1992), Kaapvaal craton The Malaita megacryst suite (Nixon and Boyd, 1979), occurs in an ocean plateau alnoite, but has many similarities with the kimberhtic low-Cr suite... 

Kerrich R., Wyman D., HoUngs P., and Polat A. (1999) Variability of Nb/U and Th/L.a in the 3.0-2.7 Ga superior province ocean plateau basalts implications for the timing of continental growth and lithosphere recycling, Earth Planet. Sci. Lett. 168, 101-115. 

Links between CTB Oceanic Plateau Volcanism and Environmental Perturbation... 

The most recent major phase of oceanic plateau formation was in the Cretaceous when the Ontong Java, Manihiki, Hess Rise, and the Caribbean-Colombian plateaus formed in the... 

Oceanic plateau Mean age (Ma) Area (10 km ) Thickness range (km) Volume (10 km )... 

Table 2 shows representative analyses of in situ and accreted portions of Cretaceous oceanic plateaus. Cretaceous oceanic plateaus are predominantly basaltic (<10 wt.% MgO) in composition (Figure 5), although more-MgO-rich lava flows are found in the Caribbean-Colombian oceanic plateau (CCOP). Typically, oceanic plateaus possess generally low levels of compatible elements (i.e., Ni < 300 ppm and Cr < 1,000 ppm see Figure 6). In terms of incompatible trace elements the majority of oceanic plateau lavas and intrusive sheets possess relatively flat rare earth... 

Figure 4 Map to show the main accreted outcrops of the Caribbean-Colombian oceanic plateau along with the locations of DSDP/ODP drill holes which penetrated the thickened crust of the Caribbean plate.

Table 2 Representative analyses of Cretaceous oceanic plateau lavas. 

Both chemical and geological features can be useful in the identification of oceanic plateaus. Condie (1999) and Kerr et al. (2000) have discussed the diagnostic features of oceanic plateaus in detail, and only a brief account will be given here. Table 3 summarizes the characteristics which are useful in distinguishing igneous rocks formed in an oceanic plateau from those which originated in other tectonic settings. 

Incompatible trace elements are only of limited use in distinguishing between volcanic successions formed in back-arc basins and those formed in oceanic plateaus (Table 3). However, the lower mantle temperature below a back-arc basin (Tp—1,280 °C) compared to a mantle plume (Tp> 1,400 °C) results in the eruption of few high-MgO lavas. An additional consequence of this lower mantle temperature is that back-arc basin lavas generally possess lower Ni and Cr contents for a given Mg number than oceanic plateau lavas (Figure 14(b)). Furthermore, because of their proximity to active subduction sites, back-arc basin sequences are also more likely to contain abundant volcaniclastic horizons than oceanic plateaus. 

The Chugoku and Chichibu belts in southwest Japan contain up to 30% basaltic material (greenstones) in thrust contact with limestones, cherts and mudstones (Tatsumi et ai, 2000). This lithological association, combined with preliminary major element data and a small range of trace element data, suggests that these basaltic assemblages are remnants of a plume-derived oceanic plateau. Tatsumi et al. (2000) proposed that this oceanic plateau formed in the Panthalassan Ocean (proto-Pacihc) in the Carboniferous (350-300 Ma). 

Figure 15 Map showing North American accreted oceanic terranes including the oceanic plateau sequences discussed in the text (after Tardy et al, 2001 Condie, 2001).

Some of the oldest preserved oceanic plateau sequences are those found in —3.5 Ga Barberton and Pietersberg belts of the Kaapvaal Shield of southern Africa (De Wit et aL, 1987 Smith and Erlank, 1982). These belts contain pillow basalts and komatiites, with chemical signatures (Lahaye et aL, 1995) suggesting a likely origin as part of an oceanic plateau. The Pilbara craton of Australia appears to possess some of the oldest oceanic plateau material so far identified (Green et aL, 2000) in the —3.5 Ga Coonterunah and Warrawoona Groups. 

Other Proterozoic oceanic plateau terranes have been identified in the Birimian province of western Africa (Figure 18) (Abouchami et aL 1990 Boher et aL, 1992), the Arabian-Nubian Shield (Stein and Goldstein, 1996), and the Flin Flon belt in Canada (Stem et aL, 1995). 

Elevated CO2 levels at the CTB may also be due to increased volcanic activity. Kerr (1998) has calculated that approximately lO kg of CO2 would have been released as a result of oceanic plateau volcanism around the CTB (Arthur et al., 1987). Additionally, LIP volcanism also releases substantial amounts of SO2, chlorine, fluorine and H2S which, when released into seawater, would have made the CTB oceans much more acidic (Kerr, 1998). The lack of carbonate at the CTB may be the result of increased dissolution by more acidic seawater, which would also release more CO2 to the atmosphere (Arthur et al, 1987). 

Increased concentration of toxic trace metals in the oceans, liberated by hydrothermal fluids from oceanic plateau lava piles, may well have been a contributory factor to the demise of some marine organisms around the CTB (Wilde et al., 1990). The upwelling of deep ocean trace metals and... 

Table 5 Correlations between black shale events and oceanic plateau volcanism over the last 250 Ma.

Age Black shales Oceanic plateau or volcanic rifted margin... 

The correlation between oceanic plateau formation and marine environmental catastrophes (characterized by mass extinction, oceanic anoxia and black shale deposition) throughout the Mesozoic period suggests a causal link between oceanic plateau formation and environmental crises. 

Desrochers J. P., Hubert C., Ludden J. N., and Pilote P. (1993) Accretion of Archean Oceanic Plateau fragments in the Abitibi Greenstone Belt, Canada. Geology 21, 451-454. 

Kerr A. C. (1998) Oceanic plateau formation a cause of mass extinction and black shale deposition around the Cenoma-nian-Turonian boundary. J. Geol. Soc. London 155, 619-626. 

Kerr A. C., Tarney J., Marriner G. F., Klaver G. T., Saunders A. D., and Thirlwall M. F. (1996b) The geochemistry and petrogenesis of the late-Cretaceous picrites and basalts of Curasao Netherlands Antilles a remnant of an oceanic plateau. Contrib. Mineral. Petrol. 124, 29-43. 

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