Garnet trace element systematics in an amphibolite- to granulite-facies metapelitic sequence (Ivrea-Verbano Zone, Italy)

Abstract

Abstract This study examines the systematic changes in two-dimensional trace element distributions in garnet from metapelites of the Kinzigite Formation (Ivrea-Verbano Zone, north-west Italy) across a continuous metamorphic field gradient from 0.4 to 0.6 GPa, ~ 650–700 °C to 1.0 GPa, ~ 900 °C, plus a garnet formed at ~ 1000 °C in a metapelite enclave within gabbro. Major element zoning is mostly obliterated at these elevated temperatures, except for Ca 2+ , which reflects its slower diffusivity compared to Fe 2+ , Mg 2+ and Mn 2+ . In contrast, garnet-compatible trace elements (V, Cr, Zr and Y + HREE) preserve distinct zoning patterns across all metamorphic grades. At mid-amphibolite facies, garnet cores have low Zr contents (2 µg/g) and complex zoning patterns in Cr and V, which are truncated by a rim with higher Zr contents (4–6 µg/g) and distinctly lower and more homogenous V contents (30 µg/g). This pattern is indicative of a sub-solidus core, followed by interface-coupled dissolution reprecipitation of garnet at the onset of muscovite melting. At upper-amphibolite facies, garnet cores display only a relic patchy zoning and much higher V contents (up to 140 µg/g), while rims are lower in V (~ 60 µg/g), but higher in Zr (8 µg/g). These patterns suggest complete recrystallisation of the subsolidus garnet and new growth in equilibrium with melt. Granulite-facies garnet, in samples that experienced the complete consumption of biotite, displays a core with little zoning and very high V (250 µg/g) and Zr contents (10–20 µg/g). The core is overgrown by a rim where Cr contents increase from 100 to 400 µg/g while Zr increases to 40 µg/g. The resetting of garnet trace elements in the cores suggests a process of melt-mediated garnet recrystallisation followed by new peritectic garnet growth related to biotite melting. Mass balance calculations indicate that V and Cr are redistributed from biotite to garnet and rutile in granulite-facies samples. The mapping of V and Cr in garnet is thus suitable to document peritectic garnet growth during biotite melting. Garnet in the metapelite enclave retains only patchy zoning in Y + Zr and very high Zr contents of up to 300 µg/g, indicative of complete recrystallization at ultra-high temperature conditions. Volume diffusion modelling in granulite-facies garnet formed at ~ 900 °C is consistent with two end-member scenarios for the duration of metamorphism of either 2 Ma at 910 °C or 25 Ma at 800 °C and a slow HREE diffusion mechanism in garnet. Diffusion modelling of the coupled Y-Zr patchy zoning in the UH T garnet (~ 1020 °C) reveals timescales of < 1 Ma for garnet-melt interaction, and Zr diffusivity up to one order of magnitude slower than that of HREE in garnet. The Zr content of garnet consistently increases with metamorphic grade, suggesting the potential use of Zr-in-garnet thermometry for high temperature metapelites.