Location, Location, Location: Monazite Behaviour During UHT Metamorphism and Melt Crystallization

ABSTRACT Monazite is a robust mineral for recording the suprasolidus evolution of migmatites and granulites. However, monazite commonly has diverse compositions and yields variable dates in a single sample; understanding the controls on monazite behaviour and composition during partial melting and melt crystallization are not always straightforward. Here, we integrate in situ monazite petrochronology from sapphirine–quartz granulites in the Arequipa Massif in Peru with an equilibrium model of monazite behaviour. Monazite from leucocratic microdomains—inferred to represent crystallized remnants of melt—is generally older and enriched in Th relative to monazite in melanosome (i.e., residual) microdomains. Enrichment and depletion of Y are unrelated to the presence or absence of garnet in these samples. An equilibrium model of monazite crystallization accounts for the decrease in Th content in monazite with decreasing date but cannot reproduce the wide range of measured Y concentrations and europium anomalies in monazite in the sapphirine–quartz granulites. We suggest that the microstructural setting of monazite is an important control on trace element composition and that whole‐rock equilibration of Eu/Eu* and Y (and the HREE) is unlikely. Monazite proximal to the principal sources and sinks of Y (garnet) and Eu (feldspar) may serve as a monitor of the behaviour of these minerals whereas monazite distal from these minerals are a monitor of evolving melt composition during crystallization. Monazite have similar dates to zircon from the same rocks, but monazite in melanocratic domains can be younger than zircon and this monazite may have been affected by late fluid ingress whereas monazite in leucosome was not. We emphasize the importance of in situ analysis of monazite and illustrate the limitations of a whole‐rock equilibrium approach to understanding accessory mineral growth and compositions in anatectic systems.