Micro-drilling ID-TIMS U-Pb dating of single monazites: A new method to unravel complex poly-metamorphic evolutions. Application to the UHT granulites of Andriamena (North-Central Madagascar).

Abstract

In terranes subject to complex metamorphic evolution, zircon and monazite U-Pb system can record successive growth and/or resetting episodes. Conven- tional isotope dilution-thermal ionisation mass spec- trometry (ID-TIMS) dating of fractions or single grains from conventionally separated zircon and monazite populations may produce inaccurate ages. A new tech- nique combining textural analysis and ID-TIMS dating on single monazite crystals is proposed. This method is applied directly on thin sections with three successive steps comprising (1) a characterisation of textural rela- tionships using electron microprobe analyses and ima- ges, (2) extraction by micro-drilling of selected monazites and (3) U-Pb dating of each extracted grain by ID-TIMS. The potential for this ''in situ'' dating technique are tested on Ultra High Temperature (UHT) granulites from North-Central Madagascar. Four dis- tinct events on a single thin section were distinguished. The UHT metamorphism was dated at 2.5 Ga, while two retrogression events were dated at 790 and 500 Ma. Finally, a single crystal shielded in quartz seems to preserve a pre-metamorphic age around 2.7 Ga.