Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and the Piemontais and Briançonnais Zones

Article Article scientifique Article Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and the Piemontais and Briançonnais Zones eng 2020-10-15 2020-12 author Ricchi Emmanuelle 0000-0003-4953-2430 716833 author Gnos Edwin 704533 Services communs du département des sciences de la terre (DETCO) author Rubatto Daniela 403294 author Whitehouse Martin John author Pettke Thomas 10.1186/s00015-020-00365-3 1661-8726 https://link.springer.com/10.1186/s00015-020-00365-3 Ion probe 208 Pb/ 232 Th fissure monazite ages from the Argentera External Massif and from the high-pressure units of the Western Alps provide new insights on its Cenozoic tectonic evolution. Hydrothermal monazite crystallizes during cooling/exhumation in Alpine fissures, an environment where monazite is highly susceptible to fluid-mediated dissolution-(re)crystallization. Monazite growth domains visualized by BSE imaging all show a negative Eu anomaly, positive correlation of Sr and Ca and increasing cheralite component (Ca + Th replacing 2REE) with decreasing xenotime (Y) component. The huttonite component (Th + Si replacing REE and P) is very low. Growth domains record crystallization following chemical disequilibrium in a fissure environment, and growing evidence indicates that they register tectonic activity. Fissure monazite ages obtained in this study corroborate previous ages, recording crystallization at ~ 36 Ma, ~ 32–30 Ma, and ~ 25–23 Ma in the high-pressure regions of the Western Alps, interpreted to be respectively related to top-NNW, top-WNW and top-SW thrusting in association with strike-slip faulting. During this latter transpressive phase, younger fissure monazite crystallization is recorded between ~ 20.6 and 14 Ma in the Argentera Massif, interpreted to have occurred in association with dextral strike-slip faulting related to anticlockwise rotation of the Corsica-Sardinia Block. This strike-slip activity is predating orogen-parallel dextral strike-slip movements along and through the internal part of all other External Crystalline Massifs (ECM), starting only at ~ 12 Ma. Our combined compositional and age data for hydrothermal monazite track crystallization related to tectonic activity during unroofing of the Western Alps for over more than 20 million years, offering chronologic insights into how different tectonic blocks were exhumed. The data show that fissures in the high-pressure units formed during greenschist to amphibolite facies retrograde deformation, and later in association with strike-slip faulting. Dewey 550 42 Département des sciences de la Terre 154 false Swiss journal of geosciences 113 1 586067e1-d694-4de9-abf4-7358f9044738 Article (Published version) Public Article_scientifique_from_crossref.pdf 4631657 application/pdf CC BY-4.0 d9aec48d454638b983ce0d17388477dee7c4f8e1cd41ef1cbab8de83cb036b96 openAccess Faculté des sciences / Section des sciences de la Terre et de l'Environnement / Département des sciences de la Terre

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Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and the Piemontais and Briançonnais Zones	Swiss journal of geosciences			2020	4	7

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Rubatto, Daniela