Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and the Piemontais and
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Swiss Journal of Geosciences
Open Access
ORIGINAL PAPER
Ion microprobe dating of fissure monazite in the Western Alps: insights from the Argentera Massif and the Piemontais and Briançonnais Zones Emmanuelle Ricchi1* , Edwin Gnos2, Daniela Rubatto3,4, Martin John Whitehouse5 and Thomas Pettke3
Abstract Ion probe 208Pb/232Th 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. Keywords: 208Pb/232Th fissure monazite age, Western Alps, High pressure, Argentera Massif, Tectonic activity, Hydrothermal monazite chemistry
Editorial handling: Paola Manzotti. *Correspondence: [email protected] 1 Department of Earth Sciences, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland Full list of author information is available at the end of the article
1 Introduction Thorium-Pb dating of hydrothermal monazite-(Ce), hereafter called monazite, offers the possibility to constrain deformation along the retrograde path of a cooling orogen. Recent studies conducted in several parts of the Alpine chain and in the Mexican orogen have acquired numerous Th-Pb
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