Subalkaline basalts

Fig. 12.10 The lava flows on Tobin Mesa are subalkaline basaltic andesites and even stray into the andesite field in this alkali-silica diagram (Wilson 1989). The phenocryst assemblage of plagioclase, pyroxene, and magnetite identifies these rocks as tholeiitic basalt in spite of their high silica concentrations (Data from Siders and Elliot 1985)...

The amli pluton is subalkaline and calc-alkaline, and mainly diorite porphyry, granodiorite, quartz monzonite, and granite-porphyry in composition (Fig. 3). The metadiabase is alkali basalt in composition,... 

Sardinia (5.3 to 0.1) Capo Ferrato (—5), Montiferro (4-2), Orosei-Dorgali (4-2), Monte Ar-ci (3.7-2.3), Logudoro (3-0.1) - Stratovolcanoes, basaltic plateaux and monogenetic centres composed of subalkaline and Na-alkaline rocks, sometimes with a K-affinity. 

Monti Ernici about 0.7 to 0.1 - HKS 0.7 to 0.2 -KS 0.2 to 0.1 - About twenty monogenetic cones formed of strombolian scoriae, hydrovolcanic surge and flow deposits and some lava flows displaying a mafic composition and a subalkaline (LKB) to alkaline potassic (KS) and ultrapotassic (HKS) petrochemical affinity (basalt, K-trachybasalt, tephrite, phonotephrite, leucit-ite). 

Alkali basalts are here defined as nepheline- and olivine-normative basaltic rocks with moderate enrichment in alkalis, which fall in the basalt field on the TAS diagram above the divide between alkaline and subalkaline fields of Irvine and Baragar (1971). 

The Ustica rocks range from mildly Na-alkaline basalt to trachyte on the TAS classification diagram, with a gap in intermediate compositions between about 55-60 wt % SiC>2 (Fig. 8.19). The mafic rocks straddle the boundary between the subalkaline and the alkaline fields, displaying a... 

Figure 6 Major element compositions of alkali basalts and continental flood basalts (a) total alkali elements versus wt.% Si02- Dashed line separating alkaline from subalkaline (tholeiitic) basalts from Irvine and Baragar (1971) and (b) wt.% Ti02 versus wt.% MgO. Data from Tables 2 and 3.

Volcanic rocks of the subalkaline series have been further subdivided on the basis of their concentrations of K2O and SiO (Peccerillo and Taylor, 1976). Le Maitre et al. (1989) propose a division of subalkaline rocks into low-K, medium-K and high-K types and suggest that these terms may be used to qualify the names basalt,... 

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