Total-Alkali-Silica diagram

The lavas of the Erebus volcanic province are alkali-rich and silica undersatured and are classified most effectively on the total alkali-silica diagram of Le Bas et al. (1986) in Fig. 16.3. However, Kyle et al. (1979) preferred the classification of Coombs and Wilkinson (1969) and identified most Cenozoic lavas in the Transantarctic Mountains as basanite, phonolite, or trachyte. Goldich et al. (1975, 1981), Sun and Hanson (1975,1976), Stuckless andEricksen (1976), Stuckless et al. (1981), and Weiblen et al. (1981) classified the lavas as trachybasalt and identified low-silica varieties as basanitoids. We will use the classification of Le Bas et al. (1986) which was also preferred by LeMasurier (1990). 

Fig. 16.3 The alkali-rich volcanic rocks of the Erebus volcanic province are classified on the total alkali-silica diagram of Le Bas et al. (1986). The rocks in this diagram can be arranged into three series with decreasing alkali content (1) Basanite (Bn), tephrite (Tp), phonotephrite (Ptp), tephriphonolite (Tpp), and phonolite (P) (2) Hawaiite (H), mugearite (M), benmoreite (Bm), and trachyte (T) (3) Basalt (B), basaltic andesite (Ba), andesite (A), and dacite (D). The boundary between the alkali-rich and subalkalic rocks defined by Macdonald and Katsura (1964) is shown as a dashed line (Adapted from LeMasurier (1990) and Wilson (1989))...

Fig. 16.16 The chemical compositions of volcanic rocks in DVDP-1 drilled at McMurdo Station on the Hut Point Peninsula range widely on the total alkali-silica diagram from basanite (Bn), to tephrite (Tp), to phonotephrite (Ptp), and to tephriphonolite (Tpp). Therefore, the rocks in this core (201.5 m) are almost as diversified as the McMurdo Volcanics in the entire Erebus volcanic province in Fig. 16.4. Samples of DVDP-2 and 3 are not shown but range just as widely as the rocks of DVDP-1 (Data from Kyle (1981a))...

The total The total alkalis- ilica diagram is one of the most useful classification schemes alkalis-silica available for volcanic rocks. Chemical data — the sum of the Na20 and K2O diagram (TAS) content (total alkalis, TA) and the S1O2 content (S) — are taken directly from a rock analysis as wt% oxides and plotted onto the classification diagram. 

Fig. 3. Total alkalis versus silica (TAS) and (b) AFM plot of Irvine and Baragar (1971), (c) modified Zr/Ti02-Nb/Y plot (Pearce, 1996) of Winchester and Floyd (1977), (d) Rock/chondrite-normalized REE diagram for rocks of amli-llica pluton and (e) rock/MORB-normalized spidergrams, (f) Th/Yb vsTa/Yb diagram.

Fig. 1.2. Total alkali vs. silica (TAS) classification diagram of Le Maitre (1989) for representative Italian Plio-Quatemary volcanic rocks. The thick dashed line is the divide between the subalkaline and the alkaline fields of Irvine and Baragar (1971).

Fig. 4.4. Total alkali vs. silica (TAS) diagram for Vulsini volcanic rocks.

The chemical classification and nomenclature of volcanic rocks usmg the total alkalis versus silica (TAS) diagram of Le Maitrc et al, (1989). Q. = normative quartz 01 = normative olivine. 

Fig. 16.4 The silica and total-alkali concentrations of Cenozoic lavas of the McMurdo Volcanics in the Erebus volcanic province range widely and are classifiable as basanite (Bn), tephrite (Tp), phonotephrite (Ftp), tephriphonolite (Tpp), phonolite (P), and benmoreite (Bm). All of the Cenozoic lavas in the diagram are alkali-rich and undersaturated with respect to silica. The alkali-rich Cenozoic lavas are associated with the West Antarctic rift system and occur not only in the Transantarctic Mountains but also in Marie Byrd Land and Ellsworth Land in West Antarctica (Data from Goldich et al. (1975) and Stuckless et al. (1981))...

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