Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia

We report an extensive field-based study of zircon and monazite in the meta morphic sequence of the Reynolds Range (central Australia), where greenschi st-to granulite-facies metamorphism is recorded over a continuous crustal s ection. Detailed cathodoluminescence and back-scattered electron imaging, s upported by SHRIMP U-Pb dating, has revealed the different behaviours of zi rcon and monazite during metamorphism. Monazite first recorded regional met amorphic ages (1576 +/- 5 Ma), at amphibolite-facies grade, at similar to 6 00 degreesC. Abundant monazite yielding similar ages (1557 +/- 2 to 1585 +/ - 3 Ma) is found at granulite-facies conditions in both partial melt segreg ations and restites. New zircon growth occurred between 1562 +/- 4 and 1587 +/- 4 Ma, but, in contrast to monazite, is only recorded in granulite-faci es rocks where melt was present (greater than or equal to 700 degreesC). Ne w zircon appears to form at the expense of pre-existing detrital and inheri ted cores, which are partly resorbed. The amount of metamorphic growth in b oth accessory minerals increases with temperature and metamorphic grade. Ho wever, new zircon growth is influenced by rock composition and driven by pa rtial melting, factors that appear to have little effect on the formation o f metamorphic monazite. The growth of these accessory phases in response to metamorphism extends over the 30 Ma period of melt crystallisation (1557-1 587 Ma) in a stable high geothermal regime. Rare earth element patter ns of zircon overgrowths in leucosome and restite indicate that, during the prot racted metamorphism melt-restite equilibrium was reached. Even in the extre me conditions of long-lasting high temperature (750-800 degreesC) metamorph ism, Pb inheritance is widely preserved in the detrital zircon cores. A tra ce of inheritance is found in monazite, indicating that the closure tempera ture of the U-Pb system in relatively large monazite: crystals can exceed 7 50-800 degreesC.