Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia |
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| Authors: | Daniela Rubatto Ian S Williams Ian S Buick |
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| Institution: | (1) Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia e-mail: daniela.rubatto@anu.edu.au, AU;(2) School of Earth Sciences and VIEPS, La Trobe University, Bundoora, Victoria 3038, Australia, AU |
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| Abstract: | We report an extensive field-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 cathodoluminescence
and back-scattered electron imaging, supported by SHRIMP U–Pb dating, has revealed the different behaviours of zircon and
monazite during metamorphism. Monazite first recorded regional metamorphic ages (1576 ± 5 Ma), at amphibolite-facies grade,
at ∼600 °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 recorded in granulite-facies rocks where melt was present (≥700 °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 influenced by rock composition and driven
by partial melting, factors that appear to have little effect 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 (1557–1587 Ma) in a stable
high geothermal regime. Rare earth element patterns of zircon overgrowths in leucosome and restite indicate that, during the
protracted metamorphism, melt-restite equilibrium was reached. Even in the extreme conditions of long-lasting high temperature
(750–800 °C) metamorphism, Pb inheritance is widely preserved in the detrital zircon cores. A trace of inheritance is found
in monazite, indicating that the closure temperature of the U–Pb system in relatively large monazite crystals can exceed 750–800 °C.
Received: 7 April 2000 / Accepted: 12 August 2000 |
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