The geological significance of radiometric age studies on volcanic and hypabyssal rocks

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Abstract
Volcanic and minor intrusive igneous rocks often have more closely defined geological ages than rocks of the plutonic association. Isotopic ages from these rocks are thus important in geochronology. Isotope loss and other factors often result in low ages being obtained, rather than the true age of primary crystallization of an igneous rock. In favourable circumstances, however, the date of a subsequent cleavage, metamorphism, or mineralizing event can be determined. Detailed geological and petrological analysis are essential if the isotopic data are to be correctly interpreted.