Monazite–allanite phase relations in metapelites

Calculations of the transition from allanite to monazite-bearing assemblages in typical pelitic bulk compositions have been made using thermodynamic data estimated from oxide sums and inferred from natural parageneses. Calculations in the CFASHPCe and MnNCKFMASHPCe systems place the allanite to monazite transition in the middle amphibolite facies (525–600 °C) for a bulk composition similar to Shaw's average pelite. The temperature of the transition is pressure dependent, and strongly dependent on the bulk rock CaO content, consistent with inferences from natural parageneses. The transition is also a function of the bulk Al2O3 content, although the calculated result is opposite to that inferred from natural samples. Comparison with published results in the La–Mg system suggest that the nature of the REE phosphate does not greatly influence the conditions of the transition.

Research Highlights
► The monazite to allanite phase transition has been calculated from newly derived thermodynamic data.
► The monazite to allanite phase transition is a continuous reaction with a positive dP/dT.
► The transition occurs in the amphibolite facies for typical pelitic bulk compositions.
► The transition is a strong function of bulk CaO content and a lesser function of Al2O3 content.