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Unformatted text preview: Daniela Rubatto Ã¡ Ian S. Williams Ã¡ Ian S. Buick Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia Received: 7 April 2000 /Accepted: 12 August 2000 Abstract We report an extensive Â®eld-based study of zircon and monazite in the metamorphic sequence of the Reynolds Range central Australia), where greenschist- to granulite-facies metamorphism is recorded over a continuous crustal section. Detailed cathodolumines- cence and back-scattered electron imaging, supported by SHRIMP UÂ±Pb dating, has revealed the dierent be- haviours of zircon and monazite during metamorphism. Monazite Â®rst recorded regional metamorphic ages 1576 5 Ma), at amphibolite-facies grade, at 600 Â° C. Abundant monazite yielding similar ages 1557 2 to 1585 3 Ma) is found at granulite-facies conditions in both partial melt segregations and restites. New zircon growth occurred between 1562 4 and 1587 4 Ma, but, in contrast to monazite, is only re- corded in granulite-facies rocks where melt was present Â³ 700 Â° C). New zircon appears to form at the expense of pre-existing detrital and inherited cores, which are partly resorbed. The amount of metamorphic growth in both accessory minerals increases with temperature and metamorphic grade. However, new zircon growth is in- Â¯uenced by rock composition and driven by partial melting, factors that appear to have little eect on the formation of metamorphic monazite. The growth of these accessory phases in response to metamorphism extends over the 30 Ma period of melt crystallisation 1557Â±1587 Ma) in a stable high geothermal regime. Rare earth element patterns of zircon overgrowths in leucosome and restite indicate that, during the pro- tracted metamorphism, melt-restite equilibrium was reached. Even in the extreme conditions of long-lasting high temperature 750Â±800 Â° C) metamorphism, Pb in- heritance is widely preserved in the detrital zircon cores. A trace of inheritance is found in monazite, indicating that the closure temperature of the UÂ±Pb system in relatively large monazite crystals can exceed 750Â±800 Â° C. Introduction The reconstruction of the pressureÂ±temperatureÂ±time PÂ±TÂ±t) evolution of crustal sections is fundamental to understanding many tectonic processes. This task, par- ticularly dicult in the case of polymetamorphic rocks, requires the combination of metamorphic petrology and geochronology of dierent mineral phases that poten- tially can record more than one geological event. Zircon and monazite have been largely used for this role in high-grade terrains because their UÂ±Pb system is able to retain the memory of polyphase evolution even at rela- tively high temperatures. However, the correct inter- pretation of zircon and monazite ages in metamorphic rocks is restricted by our limited knowledge of the be- haviour of these trace minerals during metamorphism....
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