Calc-alkaline rocks

Kubota, 1991). For example, in the south Kyushu (Nansatsu district), the southwest Hokkaido and the north Hokkaido epithermal Au deposits are hosted by andesitic rocks (calc-alkaline rocks) and ages of mineralization are close to those of andesitic volcanic activity (Watanabe, 1990). 

This is divided into a western, a central and an eastern sector. The western Aeolian arc (Alicudi, Filicudi, Salina) consists of calc-alkaline rocks with typical island arc signatures. Mafic and intermediate rocks dominate the volcanic sequence, with minor silicic volcanics. The central islands (Vul-cano and Lipari) are dominated by calc-alkaline to shoshonitic mafic to silicic rocks mafic rocks from this sector show isotopic compositions and incompatible trace element ratios similar to the western islands. The eastern arc (Panarea and Stromboli) consists of calc-alkaline to potassic alkaline rocks. Stromboli shows geochemical and isotopic signatures akin to the Neapolitan volcanoes. The Island of Panarea, located between Stromboli and Lipari, has intermediate characteristics between these two volcanoes (Calanchi et al. 2002a). 

This province consists of central volcanoes, basaltic plateaux and monoge-netic centres composed of tholeiitic to Na-alkaline rocks (basalt to rhyolite, basanite and trachybasalt to phonolite and trachyte). It overlies calc-alkaline rocks of Oligo-Miocene age. 

Fig. 2.6.87Sr/86Sr vs. Sr for San Vincenzo and Roccastrada rhyolites. The curved line is a mixing trend between Tuscany metamorphic rocks and the calc-alkaline rocks from Capraia. Dashed lines are fractional crystallisation trends. For further explanation, see text. 

The volcanoes in the Campania Province, Pontine Islands and Vulture are composed of a wide variety of magma types. Silica undersaturated ultrapo-tassic volcanism is restricted to Somma-Vesuvio, whereas mildly undersaturated to oversaturated potassic rocks occur at Campi Flegrei, Ischia, Procida and Ventotene. In the latter two islands, low potassium compositions close to calc-alkaline basalts are found among lavas and lithic ejecta. Pliocene calc-alkaline rocks are found as rhyolites at Ponza and as basalts and basaltic andesites beneath the Campanian Plain. At Vulture, volcanism is highly enriched in both Na and K, a composition that is distinct from any other volcano in the Italian peninsula. 

The Vulture, Campania and Pontine volcanoes are characterised by magmas that vary from calc-alkaline to ultrapotassic compositions. Calc-alkaline rocks occur as rhyolites at onza and as basalt to basaltic andesites beneath the Campanian plain. Both these occurrences are older than the potassic and ultrapotassic magmas, indicating an overall time-related increase in potassium on a regional basis. 

Calc-alkaline rocks at Stromboli have lower K20, P2O5 and incompatible trace element contents than the associated shoshonitic and KS rocks. However, REE and the mantle-normalised incompatible element patterns have similar shapes for all rocks, with positive spikes of Ba and negative anomalies of HFSE (Fig. 7.14). Glass inclusions with primitive compositions (MgO 7.8 wt %) contained in olivine (F091-84) from basaltic scoriae of the present-day activity, have the same type of incompatible element patterns as the whole rocks (Bertagnini et al. 2003). Sr-isotope ratios are... 

Francalanci L, Taylor SR, McCulloch MT, Woodhead J (1993b) Geochemical and isotopic variations in the calc-alkaline rocks of Aeolian Arc (Southern Italy) constraints on the magma genesis. Contrib Mineral Petrol 113 300-313... 

The main importance of this diagram, however, is that it shows komatiites clearly as a separate field from basalts and from calc-alkaline rocks, and so is useful for Archean metavolcanics. The original diagram of Jensen (1976) was slightly modified by Jensen and Pyke (1982), who moved the komatiitic basalt/komatiite field boundary to a lower Mg value. This is the version presented here (Figure 3.11). The plotting parameters of the field boundaries are taken from Rickwood (1989). 

Examples of the trends characteristic of the tholeiitic and calc—alkaline rock series are also plotted in Figure 3.18. The tholeiitic trend is illustrated by Thingmuli volcano in Iceland and the calc-alkaline trend is for the average compositions of the Cascades lavas Carmichael, 1964). 

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