Preservation of old U–Th–Pb ages in shielded monazite: example from the Beni Bousera Hercynian kinzigites (Morocco) |
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| Authors: | Montel Kornprobst & Vielzeuf |
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| Institution: | CNRS-UMR 6524 'Magmas et Volcans', OPGC, UniversitéBlaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand Cedex, France (;) |
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| Abstract: | Four samples from the metamorphic aureole around the Beni Bousera ultramafic massif were studied in detail for U–Th–Pb electron microprobe dating on monazite. The samples include three meta-sedimentary granulites (kinzigites), collected at variable distance from the peridotites, and one kyanite-bearing leucosome in the kinzigite. Two types of monazite were identified in thin section, using SEM. The main population consists of interstitial grains, 20–70 μ m in size, while the second population consists of small grains (<20 μ m), included in garnet. A total of 64 U–Th–Pb electron microprobe measurements on 53 monazite crystals were undertaken. Most crystals have a Pb content lower than the Pb detection limit, indicating that they crystallized, or were reset, during a young event, probably Cainozoic in age. Few crystals, all entirely included in garnet, have Hercynian age, the best estimate of which is 284±27 Ma. This is a direct demonstration of the shielding effect of garnet for the U–Th–Pb system in monazite. The grains in inclusion in garnet are not reset by the post-Hercynian events, despite the high temperature reached at this time (>850 °C). Thus, the monazite closure temperature depends on its textural position in the host rock. The data 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, followed by a low- P , low- T retrogression. These two events reset the U–Th–Pb system in almost all monazite grains, except for the few crystals shielded by garnet. |
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| Keywords: | Beni Bousera closure temperature electron microprobe monazite U–Th–Pb dating |
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