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

N.M. Kelly, S.L. Harley, A. Möller

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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≈9-1.0GPa, T≥850-900°C) conditions during the Rayner Structural Episode between 930 and 890Ma. High-Th rims (≤22wt.% ThO ) 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±18Ma) within error of core domains (903±14Ma). 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 ~500Ma, 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.
Original languageEnglish
Pages (from-to)192-208
Number of pages17
JournalChemical Geology
Publication statusPublished - 5 Sep 2012


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