Interpretation of zircon corona textures from metapelitic granulites of the Ivrea-Verbano Zone, Northern Italy: Two-stage decomposition of Fe-Ti oxides
Abstract. In this study, we report the occurrence of zircon coronae textures in metapelitic granulites of the Ivrea-Verbano Zone. Unusual zircon textures are spatially associated with Fe-Ti oxides and occur as (1) vermicular-shaped aggregates 50–200 µm long and 5–20 µm thick, and as (2) zircon coronae and fine-grained chains, hundreds of µm long and ≤ 1 µm thick, spatially associated with the larger zircon grains. Formation of such textures is a result of zircon precipitation during cooling after peak metamorphic conditions, which involved: (1) decomposition of Zr-rich ilmenite to Zr-bearing rutile and vermicular-shaped zircon during retrograde metamorphism and hydration; (2) recrystallization of Zr-bearing rutile to Zr-depleted rutile intergrown with quartz and submicron-thick zircon coronae during further exhumation and cooling. We also observed hat-shaped grains that are composed of preexisting zircon overgrown by zircon coronae during stage (2). Formation of vermicular zircon (1) preceded ductile and brittle deformation of the host rock, as vermicular zircon is found both plastically- and cataclastically-deformed. Formation of thin zircon coronae (2) was coeval with, or immediately after brittle deformation, as coronae are found to fill fractures in the host rock. The latter is evidence of local, fluid-aided mobility of Zr. This study demonstrates that metamorphic zircon can nucleate and grow as a result of hydration reactions and mineral breakdown during cooling after granulite-facies metamorphism. Zircon corona textures indicate metamorphic reactions in the host rock, and establishing the direction of the reaction front.