Reactive and resilient: the contrasting behaviour of monazite and titanite during deformation (the Forno-Rosarolo shear zone; Ivrea-Verbano Zone)

Abstract Dating the shear zone activity remains challenging and depends on geochronometer reactivity. We investigate the Forno-Rosarolo Shear Zone (Ivrea-Verbano Zone, Italy), developed in the intermediate-low continental crust under amphibolite-facies conditions. Sheared paragneisses and calc-silicates were dated using in situ U–(Th–)Pb monazite and titanite geochronology. Three monazite generations (MNZI-III) were identified based on microstructural position, internal features, chemical zoning (Th, Y) and isotopic data. Deformation was mainly recorded by MNZII, with high-Y domains yielding Triassic dates (average ages of: 238±8 and 222±8 Ma). Rare, highly fractured or porous MNZIII grains provided younger dates (202±8 to 184±6 Ma). MNZI, abundant in protomylonites, retains regional metamorphism, linking monazite U–Th–Pb data to fabric evolution. Titanite shows different zoning features and chemistry as a function of the surrounding mineral assemblage: (i) strongly zoned grains are mostly associated with silicate-rich layers; (ii) homogeneous grains are generally within the silicate-poor layers. Both types show a decoupling between chemistry, almost completely related to the peak metamorphism, and U–Pb isotopes. Deformation microstructures promoted a total reset of the U–Pb dataset at the beginning of deformation and a subsequent volume diffusion through the grains: the innermost domains of both titanite types provide a Triassic lower intercept age (240±5 Ma) while the rims/tips, locally coinciding with high strained portions, define an alignment of isotopic data with a Jurassic lower intercept age (186±6 Ma). This study highlights how combining monazite and titanite geochronology refines the timing and duration of deformation, particularly in large-scale shear zones involving different lithologies.