DIVERGENT BEHAVIOR OF TH AND U IN MIGMATITES

Mobilization and migration of the heat producing elements (HPE) during anatexis is a critical process in the development of orogenic systems and in models of crustal evolution and stabilization of cratons. In many crustal rocks accessory minerals are the dominant hosts of Th and U, and their behavior during partial melting controls the concentrations of these elements in the residual crust. We use phase equilibrium modeling to evaluate if melt saturated in the essential structural constituents of accessory minerals can explain the concentrations of Th and U in residual metasedimentary migmatites and granulites along two well-characterized crustal transects in the Ivrea Zone, Italy and at Mt Stafford, Australia. While an equilibrium model of accessory mineral breakdown and melt loss approximates the depletion of U in the residual crust along both transects, it does not explain the slight enrichment of Th. To explain the enrichment of Th in residual crust we propose either inhibition of monazite dissolution by kinetic factors or near-peak growth of high Th rims on undissolved monazite. Retention of the HPE in the middle and deep crust may be partly responsible for the slow cooling of many granulite facies terrains, and may also play a role in reworking of middle and lower crust during periods of later orogenesis.