ARTHUR HOLMES AND THE EVOLUTION OF THE CONTINENTAL CRUST

The continental crust is the key archive of Earth history. The development of precise radiometric age dating is the basis for determining the ages of events in the geological record. Fundamental questions remain over how, when, where and why continental crust was generated, and the extent to which it provides a representative record. Much of the discussion is about how best to interrogate the geological record, and dramatic advances have resulted from the development of in situ analytical techniques. Zircons offer robust records of the magmatic and crust-forming events preserved in the continental crust, and perhaps unexpectedly the continental crust is characterised by peaks in the distribution of U-Pb crystallization ages, and in the ages of rocks that reflect new continental crust. Such peaks may reflect periods of high magmatic activity, and as such they might be due to magmatism associated with deep-seated mantle plumes. However, the bulk composition of the crust is similar to that generated in destructive plate margin settings, and the peaks in ages also mark the times of supercontinent formation. This contribution explores new ways in which U-Pb, Hf and O isotopes in zircon can be used to constrain the bulk composition of new crust, and to evaluate how and when the continental crust was generated, and the tectonic setting in which detrital zircons were deposited. It argues that the apparent peaks of magmatic activity reflect the preservation potential of magmatism in different tectonic settings, rather than fundamental pulses of magmatic activity. It explores links between the rates at which new crust is generated and the rates of crustal growth, and why the inferred rates of crustal growth in the Archaean may have been similar to the rates at which new crust has been generated in the recent geological past.