The giant monazite crystals from Manangotry (Madagascar)
Résumé
The Manangotry area in the South-East of Madagascar is famous for its giant monazite crystals. The studied occurrence is an outcrop near the Manangotry pass, were giant, cm-sized crystals have been found. The crystals are embedded in biotitite levels, also containing cm-sized xenomorphic ilmenite, and are associated with a 1 m thick layer of apatite. Besides giant crystals, monazite is also present as abundant small (mm-sized) grains. The surrounding rocks are leucogranites and charnockites. The monazite giant crystals are chemically homogeneous and thorium-rich, whereas small crystals tend to be heterogeneous. Both biotite and apatite are F-rich. Temperature estimates from biotite and apatite yielded 750 to 800 °C. Despite their high radioactivity, the giant crystals preserve a high degree of crystallinity. Electron microprobe U-Th-Pb dating produced ages of 537 ± 14, 534 ± 10 and 530 ± 10 Ma, considered to be the age of the main monazite crystallization episode but some minor domains in small grains yielded 482 ± 12 Ma. The enclosing biotites yielded 453 ± 6 Ma Ar-Ar ages. More than 98 % of radiogenic helium have been retained in the studied monazite samples, and kinetic experiments indicates the intrinsic diffusion parameters for high-temperature to be Ea = 53 kcal/mol and D0 = 3085 cm2/s. The paragenesis (monazite-apatite-ilmenite-biotite) is different from any other occurrences of larges monazite elsewhere. Based on the current data and literature, the preferred genetic model for these giant monazites is crystallization from a high-temperature fluorine-rich fluid, possibly originating from surrounding charnockites.