Temperature and bulk composition control on the growth of monazite and zircon during low-pressure anatexis (Mount Stafford, Central Australia)

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.