Three metamorphic monazite generations in a high-pressure rock from the Bohemian Massif and the potentially important role of apatite in stimulating polyphase monazite growth along a PT loop

Based on microstructural and compositional criteria, three generations of monazite can be identified in a peraluminous, garnet-rich, high-pressure metamorphic rock from the Bohemian Massif. The first monazite generation (M1) formed probably on the prograde part of the clockwise PT loop, a second (M2) during the high-pressure stage (26 ± 3 kbar and 830 ± 30 °C) and a third generation (M3) during subsequent near-isothermal decompression of the rock to 8 ± 2 kbar. The high-pressure, M2 monazites are volumetrically dominant and form 100–300 μm grains. They are characterised by an unusual Sr–brabantite component (SrO up to 2 wt.%) and grew together with accessory Sr-rich fluorapatites (SrO up to 33 wt.%). The Sr contents of both minerals are indicative of high-pressure conditions and result from the breakdown of plagioclase and the release of Sr that occurred during this process.