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 (Pr approximate to 9-1.0 GPa, T >= 850-900 degrees 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 similar 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. (C) 2012 Elsevier B.V. All rights reserved.