Preservation of old (prograde metamorphic) U–Th–Pb ages in unshielded monazite from the high-pressure paragneisses of the Variscan Ulten Zone (Italy)

Abstract

Three samples of garnet–kyanite paragneiss from the Variscan Ulten Zone (Northern Italy) were studied in detail for U–Th–Pb monazite dating. Monazite in these gneisses is abundant, shows highly variable grain size and occupies different textural positions: within the matrix, as inclusion in garnet and kyanite, within apatite aggregates. Monazite shows different deformation features as a function of the textural position: enclosed (shielded) monazite is generally more fractured than matrix (unshielded) monazite. The integration of textural information with deformation features and in situ U–Th–Pb analyses by LA–ICP-MS indicates that there is no direct correlation between textural site and monazite ages. Old ages of 351–343Ma, determined on portions of large matrix (unshielded) monazite and on rare domains of monazite shielded by garnet, have been related to a prograde stage of the Variscan metamorphic evolution of the Ulten Zone. Ages of 330–326Ma, which are related to the thermal peak, are recorded by small matrix monazite, external domains of large matrix monazite, and by (domains of) fractured monazite enclosed in garnet and kyanite. Large, old unshielded grains formed as blasts during the prograde metamorphic history and survived the peak metamorphism during which crystallisation/re-crystallisation partially occurred.