Complexity in the behavior and recrystallization of monazite during high-T metamorphism and fluid infiltration

A detailed study of monazite grains in silica-undersaturated and quartz-bearing Mg–Al metapelite from the Oygarden Group of islands, east Antarctica, reveals a complex history of growth and recrystallization during two separate events in the Neoproterozoic and earliest Cambrian. Monazite grains from garnet-poor and garnet-rich metapelite preserve core domains that have ages corresponding to growth and/or recrystallization at granulite facies (P ≈ 9–1.0 GPa, T ≥ 850–900 °C) conditions during the Rayner Structural Episode between 930 and 890 Ma. High-Th rims (≤ 22 wt.% ThO2) that are in textural equilibrium with sapphirine–orthopyroxene symplectites formed after garnet during late-Rayner decompression, occur on monazite grains in garnet-rich assemblages, and give electron microprobe ages (883 ± 18 Ma) within error of core domains (903 ± 14 Ma). These high-Th rim domains are interpreted to have formed through recrystallization of liberated inclusions via a process dominated by coupled dissolution–reprecipitation reactions facilitated by transient fluid films on the large surface areas in the symplectite and not by new growth. In garnet-poor metapelite, monazite grains that have grain boundaries in textural equilibrium with the granulite-facies assemblage also show alteration to higher-Th compositions on rims and along vein-like fractures. This compositional shift is accompanied by partial- to complete-resetting of ages to ~ 500 Ma, along with minor modification of core domain ages. Textures and patterns of chemical age resetting are interpreted to also be the result of a coupled dissolution–reprecipitation reaction process, but in contrast to garnet-bearing assemblages, this alteration occurred during a fluid influx at lower temperatures during regional ‘Pan African’ tectonism. This study highlights the susceptibility of monazite to resetting, with fluid-present conditions dramatically increasing the potential for recrystallization.

► Monazite shows effects of recrystallization under two contrasting conditions.
► High-Th rims developed under high-T, fluid-poor conditions during grain-boundary readjustment.
► High-Th monazite also formed in a separate event at lower-T, likely in presence of alkali-rich fluid.
► Recrystallization involved coupled dissolution–reprecipitation reactions causing age resetting.
► Monazite may be a sensitive recorder of fluid events.