A MODEL FOR MONAZITE/MELT EQUILIBRIUM AND APPLICATION TO THE GENERATION OF GRANITIC MAGMAS

In order to model the behaviour of Zr, Hf, Ti, P, U, Th, Y and the rare-earth elements (REE) in the continental crust, the parameters which control the equilibria between accessory minerals and granitic melts must be determined. This paper proposes equations for the equilibrium between monazite and Ca-poor felsic melts. In a first step, all the light REE are considered as a single component and an equation describing monazite solubility is deduced from the available experimental data. In a second step, the fractionation of REE by monazite is investigated on the basis of a natural example of coexisting monazite and obsidian from Macusani, Peru. The solubility of monazite can be used as a thermometer, based on the total REE content of natural magmatic rocks. Application of REE thermometry to various peraluminous rock series suggests that it is a reliable tool for determining temperatures of magmatic processes. The equation governing REE fractionation by monazite is used to discuss genetic relationships between a melt and a presumed parental rock, using the composition of the monazite in the parental rocks and the REE pattern of the daughter rock. This method has been applied successfully to High Himalayan Manaslu and Tibetan Slab granites. It is suggested that detailed petrographical work on monazite including electron microprobe analyses and SEM imaging of its internal structures, combined with the model presented here, can be a useful tool for deciphering the genesis and the differentiation of peraluminous granitic magmas in the continental crust.