Interpretation of isotopic ages in orogenic belts

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Abstract
K-Ar and Rb-Sr methods of age-determination are the most widely applied; geological interpretation of ages so obtained is in some cases facilitated by U-Pb ages on zircons. On a broad continental scale isotopic ages have been used to delineate the time-span of evolution of structural provinces; the spread of ages so obtained is as great as 800 m.y. Within such a spread, interpretation of isotopic ages is discussed with reference to the orogenic cycle; orogenic belts may be multicyclic and involve rocks formed in a previous cycle, and there may also be repetition of phases within a cycle. The interpretation of ages obtained on rocks developed in the syn-, late- and post-orogenic phases of the orogenic cycle is considered. The question of what is being dated with respect to rocks formed in syn- and late-orogenic plutonism is considered in terms of multi-phase events and of cooling of the orogenic belt to temperatures at which radiogenic daughter products become frozen in; post-tectonic igneous activity may provide valuable datum points in the orogenic cycle. It is emphasized that full geological information is essential to any interpretation of detail within the spread of ages obtained from an orogenic cycle.