Abstract
FISHER recently reported1 the results of K/Ar measurements on a basalt from the Nigerian Cainozoic alkaline volcanic province. This basalt is distinguished by its content of sodic plagioclase megacrysts2,3. Fisher discussed the problems raised by the geologically quite unrealistic K/Ar age of 95 m.y. which he obtained for this rock; clearly excess argon became incorporated into the magma and failed to outgas during cooling. A similar high age for the same rock was obtained by N. K. Grant, D. C. Rex and S. J. Freeth (to be published). They attributed the excess argon to the big feldspar crystals, which they believe to be xenocrysts from the underlying metamorphic basement. An almost identical alkali basalt plus sodic plagioclase assemblage has been described from the southern Rocky Mountains by Doe et al.4, who also conclude that the feldspars are of xenocrystal origin.
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