Preservation of old U-Th-Pb ages in shielded monazite: example from the Beni Bousera Hercynian kinzigites (Morocco)

Four samples from the metamorphic aureole around the Beni Bousera ultramafi c massif were studied in detail for U-Th-Pb electron microprobe dating on m onazite. The samples include three meta-sedimentary granulites (kinzigites) , collected at variable distance from the peridotites, and one kyanite-bear ing leucosome in the kinzigite. Two types of monazite were identified in th in section, using SEM. The main population consists of interstitial grains, 20-70 mu m in size, while the second population consists of small grains ( < 20 mu m), included in garnet. A total of 64 U-Th-Pb electron microprobe m easurements on 53 monazite crystals were undertaken. Most crystals have a P b content lower than the Pb detection limit, indicating that they crystalli zed, or were reset, during a young event, probably Cainozoic in age. Few cr ystals, all entirely included in garnet, have Hercynian age, the best estim ate of which is 284 +/- 27 Ma. This is a direct demonstration of the shield ing effect of garnet for the U-Th-Pb system in monazite. The grains in incl usion in garnet are not reset by the post-Hercynian events, despite the hig h temperature reached at this time (> 850 degrees C). Thus, the monazite cl osure temperature depends on its textural position in the host rock. The da ta also show that a Hercynian event occurred in the Beni Bousera granulitic metapelites, which equates with a high-P, high-T event. The emplacement of the peridotite in the Cainozoic may be linked a low-P, high-T event, follo wed by a low-P, low-T retrogression. These two events reset the U-Th-Pb sys tem in almost all monazite grains, except for the few crystals shielded by garnet.