Arc-type Rocks

Cornacya Seamount. This seamount recently was discovered, named and studied by Mascle et al. (2001). It is an about 12 Ma old volcano located SE of the southern Sardinian coast. Samples from Cornacya consist of strongly altered lavas that contain enclaves of mica-rich lamprophyres. The lavas have varied porphyritic textures with ubiquitous phenocrysts of zoned plagioclase and biotite plus some amphibole and clinopyroxene that are surrounded by a glass-rich matrix containing minor Na-rich plagioclase, anorthoclase, biotite and secondary products. Accessory minerals include Fe-Ti oxides, apatite and zircon. The lamprophyric enclaves are por- 

206pb/204pb 18 83 t0 18 92. 207pb/204pb 15 64 to Jg 208pb/204Pb  

Central Tyrrhenian arc. Very little information is available on these volcanoes, except for Anchise (see previous paragraphs), the buried andesitic rocks of the Campanian plain (Parete-2 well) and Ponza rhyolites (see Chap. 6). 

The OIB-type rocks have variable isotopic and trace element characteristics, which appear to be related to different processes and/or sources. 

---

Volcanic rocks of the Annidale Group are mostly arc type but an unusual group of high-Mg, high-Cr basalts with... 

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. 

The Sardinia Province contains a wide variety of rocks, ranging from subalkaline to Na alkaline and nephelinitic, and from mafic to felsic. Most rocks have relatively unradiogenic Sr and Pb isotopic signatures, resembling EMI mantle compositions. A few outcrops from southern Sardinia have isotopic and trace element signatures very close to the rocks of the Sicily Province. Plio-Quatemary volcanism in Sardinia overlies Oligo-cene-Miocene arc-type tholeiitic to calc-alkaline volcanism. 

Figure 14 Locations of selected major Phanerozoic continental flood basalts worldwide. References as in Table 3, with exception of Madgascar (Storey et al., 1997), Emeishan (Chung and Jahn, 1995 Zhou et al, 2002), North Atlantic Igneous Province (Saunders et al, 1997), Ethiopian and Yemeni traps (Menzies et al, 1997 Pik et al, 1999), and Central Atlantic magmatic province (CAMP) (Hames et al, 2000). The separation of CEB provinces into high Ti/Y ( P or plume type) and low Ti/Y ( A or arc type) from Puffer (2001). However, most provinces include examples of both types of basaltic rocks (cf. the Parana-Etendeka provinces). A more complete compilation of terrestrial large igneous provinces can be found...

Calcium—Silicon. Calcium—silicon and calcium—barium—siUcon are made in the submerged-arc electric furnace by carbon reduction of lime, sihca rock, and barites. Commercial calcium—silicon contains 28—32% calcium, 60—65% siUcon, and 3% iron (max). Barium-bearing alloys contains 16—20% calcium, 9—12% barium, and 53—59% sihcon. Calcium can also be added as an ahoy containing 10—13% calcium, 14—18% barium, 19—21% aluminum, and 38—40% shicon These ahoys are used to deoxidize and degasify steel. They produce complex calcium shicate inclusions that are minimally harm fill to physical properties and prevent the formation of alumina-type inclusions, a principal source of fatigue failure in highly stressed ahoy steels. As a sulfide former, they promote random distribution of sulfides, thereby minimizing chain-type inclusions. In cast iron, they are used as an inoculant. 

Watanabe (1986, 1989, 1990a,b, 1991) studied the vein pattern, the age of vein-type deposits and the volcanic rocks in southwest Hokkaido and showed that the major veins such as those at the Toyoha and Chitose have been formed at dextral strike-slip movement of an E-W trend, and those veins are situated at the west-southwest extension of the maximum displaced zone within the dextral shear belt along the Kuril arc. Watanabe (1990b) also showed that the veins in the Sapporo-Iwanai district strike E-W and are oblique to the NW-SE volcanic chains which are sub-parallel to the maximum principal stress estimated in southwest Hokkaido during Late Miocene to Holocene and oblique subduction of Pacific Plate was active during the Plio-Pleistocene age. 

The differences in base metal concentrations in the two types of hydrothermal solution are unclear, probably because of scarcity of data. However, it seems obvious that wpe/tWMn ratio of midoceanic ridge hydrothermal solution is higher than back-arc hydrothermal solution (Table 2.15). This may be due to the differences in Fe and Mn contents of volcanic rocks at back-arc basin and midoceanic ridges and temperature of fluids. 

The CO2 concentrations of present-day geothermal waters in terrestrial environment have been also interpreted in terms of the interaction of hydrothermal solutions with country rocks (Giggenbach, 1981 Shikazono, 1978,1985). For example, as noted in section 2.4.3, Shikazono (1985) estimated /CO2 for epithermal Au-Ag and base-metal vein-type deposits in Japan which formed in terrestrial environments at Miocene-Pliocene age and showed that fco2 controlled by the alteration minerals (Fig. 3.6). Estimated /coi" temperature range for epithermal Cu-Pb-Zn vein-type deposits are clearly similar to those for the Kuroko and back-arc deposits in which base metals (Cu, Pb, Zn) are concentrated. 

Alteration is always a cause for concern in geochemical investigations and the best approach will always be to avoid samples with visual or chemical evidence for alteration. The differential fluid mobility of U, Th, Pa and Ra undoubtedly provides the potential for weathering or hydrothermal circulation to disturb the U-series signatures of arc lavas. In a study of lavas from Mt. Pelee on Martinique, Villemant et al. (1996) found that domeforming lavas were in U-Th equilibrium whereas plinian deposits from the same eruptions had small U-excesses which they interpreted to reflect hydrothermal alteration. However, whilst the addition of U could be due to hydrothermal alteration, the plinian deposits were also displaced to lower °Th/ Th ratios which cannot. Instead, the two rock types may just be from separate magma batches. 

---