Island-arc tholeiites

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. 

The Ti-Zr-Sr discrimination diagram for basalts (after Pearce and Gann, 1973). Island-arc tholeiites plot in field A, calc-alkaline basalts plot in field B and MORE plot in field C The plotting coordinates for this diagram (extracted from Pearce and Cann, 1973 — Figure 4) are ... 

The different basalt fields are subdivided according to Ti/V ratio (Figure 5.10). MORB plote between Ti/V ratios of 20 and 50, although there is considerable overlap with the fields of continental flood basalt and back-arc basin basalts. Ocean-island and alkali basalts plot between Ti/V ratios of 50 and 100. Island-arc tholeiites plot between Ti/V ratios of 10 and 20 with a small overlap onto the field of MORB, Calc-alkali lavas plot with a near-vertical trend and with Ti/V ratios between 15 and 50. 

Lecolle (1989) This diagram has not yet been widely tested but offers another means of discriminating between different types of MORB. Elemental concentrations are plotted in ppm as La/10, YV15 and Nb/8 and the three main fields are further subdivided (Figure S.ll). Volcanic-arc basalts plot in field 1 and are subdivided into calc—alkali basalts (lA) and island-arc tholeiites (IQ. Field IB is where the two plot together. Field 2 characterizes continental basalts and field 2B may define continental back-arc tholeiites, although this subdivision is based upon a single... 

Figure S.9 The Th-Hf-Ta discrimination diagram for basalts (after Wood, 1980). The fields are A, N-type MORE B, E-cype MORE and within-plate tholeiites C, alkaline within-plate basalts D, volcanic-arc basalts. Island-arc tholeiites plot in field D where Hf/Th > 3.0 and calc-alkaline basalts where Hf/Th < 3.0. The broken lines indicate transitional zones between basalt types. The plotting coordinates for the boundary lines (extracted from Wood, 1980 — Figure 1) are ...

A plot of FI and F2 separates MORE, volcanic-arc basalts, shoshonites and within-plate basalts (ocean-island basalts and continental tholeiites) from each other (Figure 5.19a). A plot of F2 and F3 separates island-arc tholeiites, cak-alkali basalts... 

The MnO- Basalts and basaltic andesites in the silica range 45-54 wt % S1O2 can be subdivided TiOg-PjO on the basis of their MnO, Tt02 and P2O5 concentrations into the following types diagram of MORB ocean-island tholeiites ocean-island alkali basalts island-arc tholeiites ... 

The composition of average European Shale The activity (or fugacity) of oxygen Billion (10 ) years High Field Strength trace element FGgh p. mantle source region (see Island-Arc Tholeiite... 

Several volcanoes with calc-alkaline, tholeiitic (MORB and arc tholeiites) to Na-transitional and alkaline affinity coexist on the Tyrrhenian Sea floor. According to some authors (Savelli 1988 Locardi 1993), the calc-alkaline and shoshonitic seamounts developed along arcuate structures that become younger from west to south-east. The older volcanic cycle (4.5 Ma) of Ponza Island may represent the northern end of one of these arcs. 

Nicholls 1. A. and Ringwood A. E. (1973) Effect of water on olivine stability in tholeiites and the production of silica-saturated magmas in the island-arc environment. J. Geol. 81, 285-300. 

An excellent example of the use of discriminant analysis in igneous petrology is found in the work of J.A. Pearce (1976), who employed this technique in an attempt to classify basalts on the basis of their major element chemistry (see also Section 5.2.2). The study is based upon a collection of recent basalts taken from six different tectonic environments — ocean-floor basalts, tsland-arc tholeiites, calc-alkaline basalts, shoshonites, ocean-island basalts and continental basalts. The objective of the study was to see if there is a relationship between major element chemistry and tectonic setting. 

The K2O-H2O discriminiidon diagram for basalts after MuenOW et a/., 1990). The fields of MORB, OIB (ocean-island basalt), BAB (back-arc basin basalt) and Arc (volcanic-arc basalts) are Grom Muenow et al. (1990), The fields of fore-arc boninites and fore-arc tholeiites are taken from Bloomer and Stern (1990). 

The Bowers Terrane is located east of the Wilson Terrane and is separated from it by the Lanterman fault zone which has been traced from the Bowers Mountains on the Oates Coast to the Lady Newness Bay on the Borchgrevink Coast of the Ross Sea. The Bowers Terrane appears to be a sliver of a much larger land-mass that originally included an oceanic island-arc system. It is composed of oceanic tholeiites interbed-ded with and overlain by fossiliferous sedimentary rocks that were deposited in a marine environment. The rocks were compressed by westward-directed forces that caused the rocks to be folded and faulted. However, the metamorphic grade is lowermost green-schist facies. The eastern boundary of the Bowers... 

B. Island Arcs Japan Tholeiitic (pigeonitic or calcic), high alumina, and andesitic (hypershenic or calc-alkalic) series (from south to north)... 

Basalts, basaltic andesites, and andesites with this distribution are common in some island arcs (e.g., Jakes and Gill, 1970 Ewart et al., 1973 Taylor et al., 1969). Their presence is believed to result from melting of subducted oceanic crust. By and large, the sediment layers which lie above the ocean floor tholeiites and are derived mainly from continental material are not subducted but piled up against continental margins in some manner that prevents their modifying significantly the trace element and isotopic abundances of oceanic crustal matter in the production of this class of island arc volcanics. Nor does ocean water severely modify the lanthanide distributions in volcanics that are extruded under... 

The tholeiitic basalts of Japan that have light-lanthanide depleted distributions were collected on the eastern (Pacific Ocean) side. These islands, like the Caucasus geosyncline region discussed earlier (Ronov et al., 1974) show how lavas with ocean floor affinities and lavas with continental or oceanic island affinities both appear in island arcs. The basalts of the Crescent formation (northwestern Washington) are a good example of welding of materials partly of ocean floor affinity onto a continent (Glassley, 1974). The lower basalts of the... 

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. 

Arndt et al. (1997) compared the compositions of Archaean tholeiites with those of modern basalts from ocean-ridge, ocean-island, and arc environments (Fig. 3.22). They showed that Archaean tholeiites have higher SiOa and FeO, and lower incompatible trace element concentrations (although enriched in Rb and Ba relative to modern MORB), compared to their modern equivalents. Many of these geochemical features of Archaean basalts match those of modern arc basalts, but there are also important differences. This led Arndt et al. (1997) to the important conclusion that "the major- and trace-element characteristics of Archaean basalts are matched by no common type of modern basalt." The uniqueness of Archaean tholeiitic basalts requires that they are either from a source different from that of modern tholeiites, or are the product of a different melting process. Arndt et al. (1997) interpreted the low incompatible trace element concentrations and high Si, Fe, Ni,... 

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