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Part 1: Introduction |
Department of Geology, Birkbeck College, Malet Street, London, W.C.I
Department of Geodesy and Geophysics, Madingley Rise, Madingley Road, Cambridge
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.
A general review of published work on bentonites, tuffs, lavas, and related sub-volcanic intrusions reveals the potentialities and limitations of age studies on this category of rocks. Young volcanic and the unfolded eruptive rocks of post-tectonic environments and of the stable zones of ancient shields provide the best material for initial age studies. The significance of isotopic age studies on basic igneous rocks in the dating of orogenic and mineralizing events is discussed. Determinations of the age of certain Caledonian and Variscan events in Britain are quoted to illustrate the methods used.
