CONSTRAINING THE TIMING OF MONAZITE GROWTH IN METAPELITIC LEUCOSOMES BY MONITORING MELT SATURATION PROXIES IN PERITECTIC PHASES

Metapelitic rocks that have undergone partial melting to generate granitic leucosomes typically contain multiple generations of monazite. Recent theoretical studies (Kelsey et al., 2008) have provided a framework for understanding the dissolution and crystallization of monazite as a rock passes through the melt regime and have shown that bulk rock LREE content should control whether ‘inherited’ prograde monazite will survive dissolution in aluminous melts. It is now possible to use rapid LA-ICP-MS trace element mapping to document zoning patterns and quantify concentrations in both monazite and garnet. This approach reveals if compositional domains in monazite and garnet exist that have concentrations consistent with equilibrium partitioning with LREE, P, and Zr-saturated aluminous melt. This approach is demonstrated for a garnet-orthopyroxene-cordierite granulite that contains K-feldspar-plagioclase-quartz leucosomes with accessory monazite, zircon, and apatite. A combination of LA-ICP-MS trace element mapping and conventional trace-element quantification shows that outermost portions of garnet in leucosomes display elevated P, Sm, and Zr and have concentrations for these elements consistent with (poorly) known garnet/melt distribution coefficient (K_D) values. This suggests that the anatectic melt became saturated in these components while garnet growth continued. P-T pseudosection modeling suggests that garnet growth during melting is restricted to a narrow temperature interval during which biotite dehydrates to generate garnet-orthopyroxene-liquid assemblages. This suggests that monazite (and zircon) also crystallized along the prograde path. This hypothesis is supported by the identical age of monazite and zircon in leucosomes (2635 ± 5 Ma). Anatectic monazite domains within leucosomes display lower U, MREE, and HREE compared to inherited prograde cores. LA-ICP-MS mapping also reveals systematic changes in Eu/Eu* in monazite, potentially reflecting the onset of feldspar crystallization. Combining trace element concentration and zoning information for monazite and garnet, in-situ U-Pb geochronology, and P-T pseudosection modelling provides a framework for rigorously interpreting the timing of monazite growth in leucosomes relative to peak temperature conditions.