Monazite-(Ce)-huttonite solid solutions in granulite-facies metabasites from the Ivrea-Verbano Zone, Italy

Composite populations of monazite-group minerals of both metamorphic and me tasomatic origin have been discovered in thin layers of granulite-facies me tabasites interlayered with metapelites, located in the Val Strona di Omegn a region of the Ivrea-Verbano Zone, Italy. In addition to monazite-(Ce), wh ich is uncommonly poor in Th and is probably formed by incongruent dissolut ion of apatite, these populations include members of the monazite-huttonite series. The latter minerals contain between 13 and 30.1 mol.% ThSiO4 [= hu ttonitic monazite-(Ce)], and are known from only half a dozen other occurre nces worldwide. We propose that breakdown of primary monazite-(Ce) in the m etapelites during granulite-facies metamorphism mobilized Th and the REEs, which were then transported by high-grade metamorphic fluids into the metab asite layers to form the Th-rich minerals of the monazite-huttonite series.