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). |
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| Authors: | Email author" target="_blank">Jean-Louis?PaquetteEmail author Philippe?Goncalves Bertrand?Devouard Christian?Nicollet |
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| Institution: | (1) Laboratoire Magmas et Volcans, Université Blaise Pascal-CNRS, 5 rue Kessler, 63 038 Clermont-Ferrand, France;(2) Geosciences Department, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003-9297, USA |
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| Abstract: | In terranes subject to complex metamorphic evolution, zircon and monazite U-Pb system can record successive growth and/or resetting episodes. Conventional isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) dating of fractions or single grains from conventionally separated zircon and monazite populations may produce inaccurate ages. A new technique 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 relationships using electron microprobe analyses and images, (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 distinct 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.Editorial responsibility: J. Hoefs |
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