trachybasalt

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trachybasalt The fine-grained, extrusive equivalent of syenogabbro. When undersaturated, feldspathoidal minerals take the place of alkali feldspar to generate feldspathoidal trachybasalts known as tephrites (olivine-free) and basanites (olivine present). Trachybasalts are found on the stable continental crust and on some oceanic islands.