ZIRCONIUM ABUNDANCE IN GRANULITE-FACIES MINERALS, WITH IMPLICATIONS FOR ZIRCON GEOCHRONOLOGY IN HIGH-GRADE ROCKS

Zircon growth in high-grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconiu m (Zr). We have measured the concentration of Zr in the major minerals in granulite-facies rocks of differing bulk composition. Both garnet and hornblende contain tens of parts per million Zr, and no other majo r phase contains significant Zr. Simple calculations show that reactio n of either garnet or hornblende to form non-Zr-bearing phases win rel ease sufficient Zr to account for at least some new zircon growth. U-P b ages from new zircon, grown as a result of either hornblende or garn et breakdown, are not expected to record the time of peak metamorphism , but rather will record the time of particular metamorphic reactions, allowing direct correlation of zircon ages with petrologically derive d pressure-temperature-time paths. This approach offers the potential for more rigorous interpretation of the metamorphic significance of zi rcon ages than has previously been possible.