An alternate mechanism for the production of garnet zoning: a case study from the contact aureole of the Western Adamello Tonalite, northern Italy

Correct geological interpretation of garnet compositional zoning relies on recognising the mechanisms responsible for producing the zonation. Two generations of garnet are observed in the Western Adamello Tonalite contact aureole. A Variscan generation of garnet predates the intrusion while a second generation is associated with contact metamorphism. We show how the second generation of garnet forms via replacement of the first generation of garnet through interface-coupled dissolution-reprecipitation. While compositional zoning profiles in garnet could be mistaken for diffusional relaxation of an initially sharp epitaxial overgrowth, misfit between the compositional zoning and the unreasonable timescales produced by multicomponent diffusion modelling illustrates that this is not the case. Further, we demonstrate through reaction-transport modelling that the reaction from old to new garnet composition must be dissolution rather than transport-limited. The compositional zoning across the old/new garnet interface is produced by a density gradient in micro-fractures at the replacement interface rather than diffusional transport through a pore-fluid. Through the modelling of dissolution rates, we estimate that the replacement of the precursor garnet by the new garnet generation would have occurred on a timescale of hundreds to thousands of years. This is significantly faster than the estimated more than 250 kyrs required to produce the observed compositional profiles via volume diffusion in the solid garnet. We argue that the recognition of the proper mechanism of garnet growth is important for accurate inversion of timescales of metamorphic processes from garnet compositional profiles.