RECRYSTALLIZATION OF OSCILLATORY ZONED ZIRCON - SOME GEOCHRONOLOGICAL AND PETROLOGICAL IMPLICATIONS

Oscillatory zoning is a common feature in zircons from acid igneous rocks and is believed to form during crystallisation of zircons from a magma by a mechanism which is not yet understood. Many zircons with oscillatory zoning also show a patchwork replacement of zoned by unzoned zircon. The unzoned zircon occurs as rounded, transgressive patches distributed throughout the zoned zircon and as areas of transitional replacement where zoned zircon is progressively replaced by unzoned zircon such that only faint traces of original zones remain. This structure is interpreted as a progressive recrystallisation of the oscillatory zoned zircon made unstable by the incorporation of high concentrations of contaminant elements during magmatic crystallisation. Recrystallisation overprints oscillatory zones and appears to have occurred after completion of primary crystallisation. It is accompanied by loss of U, Th and Pb and the removal of oscillatory zones. The recrystallised unzoned zircon is extremely stable with respect to later Pb loss and tends to retain a concordant or slightly discordant U-Pb age. Recrystallisation provides a mechanism for resetting zircon U-Pb ages which is independent of the degree of radiation damage of the zircon lattice. This differs from other models of discordance which involve a leaching of radiogenic Pb as a consequence of a progressive breakdown of the zircon structure through time-integrated radiation damage further enhanced by high concentrations of trace-element contaminants. The U-Pb age of the unzoned zircon may date the recrystallisation event, which may be close to the age of primary crystallisation or reflect a later metamorphism.