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. 

Wyman, D. 1999. A 2.7 Ga komatiite, low Ti tholeiite, arc tholeiite transition, and inferred proto-arc geodynamic setting of the Kidd Creek deposit evidence from precise trace element data. Economic Geology Monograph, 10, 511-528. 

Tyrrhenian Sea Floor (12 Ma to Present) Comacya (12), Magnaghi (3), Mar-sili (1.8-0), Vavilov, Aceste, Anchi-se, Lametini, Palinuro, older Ponza, etc. - Coexisting intraplate (oceanic tholeiites, Na-transitional and alkaline) and arc-type (arc-tholeiitic, calc-alkaline, potassic) rocks. 

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. 

Rock compositions range from mafic to silicic, and show a calc-alkaline (CA), high-potassium calc-alkaline (HKCA) to shoshonitic (SHO) affinity. A few potassic alkaline rocks with a composition close to the Roman potassic series (KS) occur at Vulcano and Stromboli (e.g. Keller 1982 Francalanci et al. 2004). Arc tholeiites have been dredged along some seamounts (Beccaluva et al. 1982). 

The SiC>2 vs. K2O plot (Fig. 7.8) shows that the Lipari volcanics define a steep increase in K2O from mafic to intermediate rocks, straddling the limits between arc tholeiitic, calc-alkaline and shoshonitic series. Basaltic andesites and andesites have porphyritic textures with phenocrysts of zoned... 

There are several seamounts located to the northwest (Sisifo, Enarete, Eolo) and to the northeast (Lametini, Alcione, Palinuro) of the Aeolian archipelago that define a horseshoe-type pattern around the Marsili basin. Ages range from 1.3 to less than 0.1 Ma. Compositions range from arc-tholeiitic (Lametini and southern Marsili basin), calc-alkaline and F1KCA (Marsili, Sisifo, Eolo, Panarea slope, Palinuro, Stromboli canyon and Alcione) to shoshonitic (Enarete, Eolo, Sisifo, and northern Stromboli slope). Description of rocks from Aeolian seamounts is given in Chap. 9. 

Aeolian Seamounts (Sisifo, Enarete, Eolo, Lame-tini, Alcione, Palinuro) 1.3 to <0.1 - Arc tholeiites (Lametini and southern Marsili basin) to calc-alkaline and shoshonitic mafic to intermediate rocks. 

Fig. A.l. K20 vs. Si02 classification grids for arc rocks. 1 arc tholeiitic series 2a calc-alkaline series 2b high-K calc-alkaline series Field 3 shoshonitic series. Modified from Peccerillo and Taylor (1976).

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 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 Ti y Ti and V are adjacent members of the first transition series in the periodic table and diagram of yet in silicate systems they behave in different ways. This is the basis of the Shervais (1982) discrimination diagram of Shervais (1982), which is used to distinguish between volcanic-arc tholeiites, MORB and alkali basalts. 

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 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 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... 

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