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high-alumina basalt An aphyric, extrusive igneous rock of basaltic composition containing a high proportion of modal plagioclase, and an alumina content of more than 17 wt.%. Tholeiitic and calc-alkali basalts (see THOLEIITE; and CALCALKALINE) can have alumina contents of more than 17 wt.%, so the term does not refer to a specific basalt type. Rather, the variety of basalt should be specified, e.g. ‘high-alumina basalt of calc-alkali type’.