# Application of in-situ Rb-Sr isotopic system on magmatic-hydrothermal systems: example from the Southern Domain of the Armorican Massif (Brittany, France)

Yujin Jegal, Christophe Ballouard, Chantal Peiffert, Philippe Boulvais, Camille Dusséaux, Nathan Cogné, Étienne Deloule, J. Mercadier

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## To cite this version:

Yujin Jegal, Christophe Ballouard, Chantal Peiffert, Philippe Boulvais, Camille Dusséaux, et al. Application of in-situ Rb-Sr isotopic system on magmatic-hydrothermal systems: example from the Southern Domain of the Armorican Massif (Brittany, France). 29 ème édition de la réunion des sciences de la Terre (RST 2025), Société Géologique de France (SGF), Oct 2025, Montpellier, France. pp.659265. insu-05350771

HAL Id: insu-05350771

https://insu.hal.science/insu-05350771v1

Submitted on 6 Nov 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Application of in-situ Rb-Sr isotopic system on
magmatic-hydrothermal systems: example from the
Southern Domain of the Armorican Massif (Brittany,
France)

Yujin Jegal*†<fnref n="1" />, Christophe Ballouard<fnref n="1" />, Chantal Peiffert<fnref n="1" />, Philippe Boulvais<fnref n="2" />, Camille Dusséaux<fnref n="3" />, Nathan Cogné<fnref n="2" />, Etienne Deloule<fnref n="4" />, and Julien Mercadier<fnref n="1" />

Résumé

Rb-Sr geochronology on micas currently represents a valuable approach for understand-
ing low to medium-temperature hydrothermal events and associated conditions of fluid-
rock interactions. In-situ Rb-Sr dating of K-rich or Rb-bearing minerals (e.g., micas and
feldspars) using laser ablation ICP-MS/MS (LA-ICP-MS/MS) indeed offers a powerful and
increasingly common alternative in various geological settings when traditional U-Pb datable
minerals like zircons or monazites are absent and/or affected by secondary processes that
disrupt geochronometers, or to track geological processes not detectable or register by other
geochronometers.

In this study, we applied in-situ Rb-Sr dating by LA-ICP-MS/MS to a representative set of samples from the Quiberon and Guérande peraluminous leucogranites of the South Armorican Massif (France). These two granites were emplaced in a detachment zone during the late-orogenic extension of the Variscan belt. The syntectonic leucogranites and pegmatites were already investigated using U(-Th)-Pb geochronometers and 40Ar/39Ar on micas, along with O and D isotopic analyses for their magmatic-hydrothermal history in relation to U mineralizing events. Therefore, the selected samples represent a unique opportunity to document the behavior of the Rb-Sr system in relation to tectonically assisted magma emplacement and hydrothermal circulations involving different fluid reservoirs of magmatic and meteoric origin. Our primary focus is on muscovite and biotite, with additional analyses of feldspar and apatite, or tourmaline when available.

Our preliminary findings coupled with other radiogenic systems reveal a multi-stage his-
tory of magmatic to hydrothermal fluid activity:

Muscovite from weakly deformed samples, notably those located at significant distance below

<footnote n="1">¹GeoRessources – Université de Lorraine, CNRS – France</footnote>

<footnote n="2">²Géosciences Rennes – CNRS, Université de Rennes – France</footnote>

<footnote n="3">³Laboratoire Chrono-environnement – Université de Franche Comté, CNRS, UMLP – France</footnote>

<footnote n="4">⁴Centre de Recherches Pétrographiques et Géochimiques (CRPG) – Université de Lorraine, CNRS – France</footnote>

<footnote>* Intervenant</footnote>

<footnote>†Auteur correspondant: yujin.jegal@univ-lorraine.fr</footnote>
the detachment zone, yields the timing of leucogranitic magmatism at ~320-310 Ma.

A second significant event, also recorded in muscovite, aligns with a period of sub-synchronous
hydrothermal circulation at ~300 Ma.

The most recent events, recorded in muscovite and biotite from samples deformed at sub-solidus conditions (C/S granites and mylonites), likely represent resetting when meteoric fluids were percolated at ≤~300-270 Ma. This last event was possibly associated with U leaching from the leucogranites.

This research demonstrates the power of in-situ Rb-Sr geochronology on Rb-bearing minerals like micas when coupled to other radiogenic systems to decipher discrete fluid events in polyphased geological processes.

**Mots-Clés:** laser ablation ICP MS/MS, in situ Rb Sr dating, mica, fluid rock interaction, the southern Armorican domain