We report an extensive ®eld-based study of zircon and monazite in the metamorphic sequence of the Reynolds Range (central Australia), where greenschistto 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 dierent behaviours of zircon and monazite during metamorphism. Monazite ®rst 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 (³700°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 inuenced 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 (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.

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A B S T R A C TThe products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the Upper Calcsilicate Unit in the granulite portion of the Late Palaeoproterozoic Reynolds Range Group, northern Arunta Block, central Australia. The zones of retrogression, characterized by minerals such as wollastonite, grossular and clinohumite, local resetting of oxygen isotopic compositions and local major element metasomatism, were channelways for water-rich fluids derived from granulite facies metapelites. U-Th-Pb isotopic ages measured by the SHRIMP ion microprobe on zircon and monazite from a granulite facies semipelite, an early semiconcordant aluminous quartz-rich fluid-flow segregation and a late discordant quartz-rich segregation record some of the extended thermal history of the area. Zircon cores from the semipelite show its likely protolith to be an igneous rock 1812+ 11 Ma old, itself derived from a source containing zircon as old as 2.2 Ga. Low-Th/U overgrowths on the zircon grew during granulite facies metamorphism at 1594 k 6 Ma. Monazite cooled to its blocking temperature at 1576 8 Ma. Zircon cores from the semiconcordant segregation are dominantly > 2.3 Ga old, indicating that the source of the fluids was not the particular metamorphosed semipelite studied. Two generations of low-Th/U overgrowths on the zircon give indistinguishable ages for the older and younger of 1589f8 and 1582+8 Ma, respectively. The monazite age is the same, 1576-t 12 Ma. Zircon from the late discordant segregation gave 1568 + 4 Ma. Fluid flow occurred for at least 18 3 ( 0 ) Ma and ended 26+3 ( 0 ) Ma after the peak of metamorphism, suggesting a very slow cooling rate of z 3 'C Ma-'. The last regional high-grade metamorphism in the Reynolds Range occurred at 1.6 Ga, not z 1.78 Ga as previously thought. The high-grade event at % 1.78 Ga is a separate event that affected only the basement to the Reynolds Range Group.

The formation, age and trace element composition of zircon and monazite were investigated across the prograde, low-pressure metamorphic sequence at Mount Stafford (central Australia). Three pairs of inter-layered metapelites and metapsammites were sampled in migmatites from amphibolite-facies (T 600 C) to granulite-facies conditions (T 800 C). Sensitive high-resolution ion microprobe U-Pb dating on metamorphic zircon rims and on monazite indicates that granulite-facies metamorphism occurred between 1795 and 1805 Ma. The intrusion of an associated granite was coeval with metamorphism at 1802 ± 3 Ma and is unlikely to be the heat source for the prograde metamorphism. Metamorphic growth of zircon started at T 750 C, well above the pelite solidus. Zircon is more abundant in the metapelites, which experienced higher degrees of partial melting compared with the associated metapsammites. In contrast, monazite growth initiated under sub-solidus prograde conditions. At granulite-facies conditions two distinct metamorphic domains were observed in monazite. Textural observations, petrology and the trace element composition of monazite and garnet provide evidence that the first metamorphic monazite domain grew prior to garnet during prograde conditions and the second in equilibrium with garnet and zircon close to the metamorphic peak. Ages from sub-solidus, prograde and peak metamorphic monazite and zircon are not distinguishable within error, indicating that heating took place in less than 20 Myr.

The mid-Miocene structural evolution of the metamorphic complex on Naxos involved deformation in a kilometre-scale low angle ductile shear zone. The structures described are consistent with exhumation of the metamorphic complex during crustal extension after, and possibly during, localized high temperature Barrovian-style metamorphism. Non-coaxial fabrics developed throughout the structural history indicate that upper levels were transported to the north during ductile extension. This shear Sense is opposite that inferred previously for the development ‘Cordilleran-style metamophic core complexes’ in the Cvclades. Ductile deformation was accompanied during its later stages by E-W directed compression.