# Dating C/S Structures in Syntectonic Granites by In Situ Rb-Sr Analyses on Muscovite

Philippe Boulvais, Romain Tartèse

<figure><img src="image_1.png" /></figure>

To cite this version:

Philippe Boulvais, Romain Tartèse. Dating C/S Structures in Syntectonic Granites by In Situ Rb-Sr Analyses on Muscovite. Terra Nova, In press, 10.1111/ter.70022. insu-05419340

HAL Id: insu-05419340

https://insu.hal.science/insu-05419340v1

Submitted on 16 Dec 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_3.png" /></figure>

RESEARCH ARTICLE

OPEN ACCESS

# Dating C/S Structures in Syntectonic Granites by In Situ Rb-Sr Analyses on Muscovite

Philippe Boulvais<fnref n="1" /> | Romain Tartèse²

**Correspondence:** Romain Tartèse (romain.tartese@manchester.ac.uk)

**Received:** 4 September 2025 | **Revised:** 3 November 2025 | **Accepted:** 13 November 2025

**Keywords:** deformation | fluids | magmatism | Rb-Sr dating | syntectonic granite

## ABSTRACT

The interplay of crystallisation, deformation and fluid-rock interactions makes deciphering geochronological information complex in syntectonic granites. In this study, we apply in situ Rb-Sr analyses to selected samples from the Questembert Variscan syntectonic granite in Western France, which was emplaced along the South Armorican Shear Zone as indicated by widespread C/S structures. The results show that the Rb-Sr system in muscovite records about 25–30 Ma of geological activity, from crystallisation of the granite at ca. 320–315 Ma, recorded by muscovite porphyroclasts, to late-stage permeation of small quantities of fluids along pre-existing shear planes at 290 Ma that triggered localised recrystallization of fine-grained muscovite during the waning stages of regional deformation. These results, thus, show that microscale in situ Rb-Sr analyses allow recognising discrete events long after the main phase of crystallisation of the granite, emphasising the need for careful petrographic analysis to inform interpretation of geochronological data.

## 1 | Introduction

The widespread occurrence of C/S structures in granites is commonly used to argue for their syntectonic crystallisation (e.g., Berthé et al. 1979). Strain localization in shear bands ('C' stands for 'Cisaillement' in French) is accompanied by deformation, recrystallisation and/or crystallisation of new muscovite and quartz, whereas larger muscovite porphyroclasts, often referred to as 'micas fish' (e.g., Ribeiro et al. 2023), align with feldspar grains during deformation, defining the schistosity planes ('S' stands for 'Schistosité' in French). Thus, two types of muscovite grains coexist in syntectonic granites. This concept of C/S structures has been coined by studying synkinematic granites emplaced along the South Armorican Shear Zone (SASZ) (Berthé et al. 1979; Gapais 1989), a major lithospheric-scale dextral structure in the West European Variscan orogeny (Jégouzo 1980; Gumiaux et al. 2004; Tartèse et al. 2012)

(Figure 1A). Notably, superb C/S structures can be observed in the Variscan Questembert granite (Figure 1B), a highly differentiated granite emplaced at shallow depth (ca. 1–2 kbar; Tartèse and Boulvais 2010). Except for undeformed samples from its eastern tip, most samples from the granite have been pervasively affected by fluid-rock-deformation interactions. Indeed, petrographic features, whole rock and mineral chemistry, and oxygen isotopes suggest several stages of hydrothermal alteration (Tartèse and Boulvais 2010). Zircon U–Pb dating and ⁴⁰Ar/³⁹Ar dating of muscovite from an undeformed sample indicate a crystallisation age of ~315–320 Ma for the granite (Table S1). Muscovite ⁴⁰Ar/³⁹Ar dating of further samples collected throughout the pluton yielded dates between 317 and 305 Ma interpreted as representing long-lasting fluid-rock interactions recorded locally at different times, as suggested by the progressive overprint of the magmatic chemical characteristics of muscovite grains (Tartèse et al. 2011).

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *Terra Nova* published by John Wiley & Sons Ltd.

<footnote n="1">¹Géosciences Rennes, UMR 6118, CNRS Université de Rennes, Rennes, France | ²Department of Earth and Environmental Sciences, The University of Manchester, Manchester, Manchester, UK</footnote>
<figure><img src="image_4.png" /><figcaption>**FIGURE 1** | (A) Simplified geologic map of the area of interest around the Questembert (Q) granite. The numbers next to the sampling sites refer to those in sample names. SASZ(N) and SASZ(S) stand for northern and southern branches of the South Armorican Shear Zone, respectively, and L for the Lizio granite. Inset shows the location of the studied area within Western France. (B) Photograph of typical C/S structures in sample QRT67 from the Lescastel quarry. [Colour figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

Here, we focus on this Questembert granite and apply in situ Rb-Sr analyses on muscovite grains from both shear bands and schistosity planes to better understand the significance of C/S structures in syntectonic granites, in terms of relationships between magmatism, deformation and fluid circulations. For that purpose, we selected four samples from different sites in the granite (Figure 1A), which represent different amounts of finite strain, from isotropic structure to highly strained C/S granites, and for which we already obtained chemical and isotopic data (Tartèse and Boulvais 2010; Tartèse et al. 2011). We also carried out U–Pb dating on apatite in these samples as apatite is likely to react with magmatic–metamorphic fluids, providing independent information on the fluid regime in the system. The main take away is that while attempting to date syn-magmatic deformation through targeting C plane muscovite, one actually gets information on post-magmatic, localised synkinematic circulation of hydrothermal fluids along these C planes.

## 2 | Development of the Rb-Sr In Situ Systematics

The Rb-Sr dating system is based on the radioactive beta decay of ⁸⁷Rb in ⁸⁷Sr, with a half-life of ca. 49.6 Ga (corresponding to a decay constant of 1.3972×10⁻¹¹ a⁻¹; Villa et al. 2015). Because of the general abundance of Rb in K-bearing phases such as K-feldspar, biotite, and muscovite, the Rb-Sr system has been extensively used for geochronological studies, notably for dating granitic magmatism in the 70's and 80's (e.g., Hamet and Allègre 1976; Jahn et al. 1976; Bernard-Griffiths et al. 1985). Historically, Rb-Sr dating has been applied to bulk rocks and mineral separates. However, it has been shown that this system can be affected by post-crystallisation disturbance causing partial to total resetting of the Rb-Sr chronometer, such as regional metamorphism at moderate temperatures (e.g., Black 1988; Evans 1989; Patel et al. 1999) and hydrothermal alteration

(e.g., Schaltegger 1990; Simon 1990). Therefore, the loss of petrographic context when analysing bulk rock and mineral fractions is prejudicial to properly interpret Rb-Sr dates in complex settings.

In situ Rb-Sr analyses using secondary ion mass spectrometry (SIMS) and laser ablation—inductively coupled plasma mass spectrometry (LA-ICP-MS) have been hampered by the isobaric interference between ⁸⁷Rb (isotopic mass of ~86.90918 Da) and ⁸⁷Sr (isotopic mass of ~86.90888), requiring very high mass resolution typically unreachable for most instruments. The development of triple quadrupole ICP-MS (ICP-MS/MS) instruments in the last decade has allowed overcoming these analytical challenges, spurring a revival of the Rb-Sr dating system (e.g., Zack and Hogmalm 2016; Hogmalm et al. 2017; Gorojovsky and Alard 2020; Rösel and Zack 2022). By introducing a reactive gas such as N₂O in the collision cell located between the two quadrupoles in an ICP-MS/MS, it is possible to separate ⁸⁷Sr from ⁸⁷Rb as the former efficiently reacts with O while the latter does not. Thus, Sr isotopes can be measured as their mass-shifted ⁸SrO reaction products (where x stands for 86, 87, and 88) while Rb isotopes are measured on-mass. This now allows in situ Rb-Sr dating, preserving key petrographic context for the phases dated, as recently applied to dating regional deformation and associated fluid circulations (e.g., Olierook et al. 2020; Tillberg et al. 2020; Redaa et al. 2022; Ribeiro et al. 2023; Kellett et al. 2024; Boschetti et al. 2025). In this study, we use in situ Rb-Sr dating to better understand C/S structures, which are typical of syntectonic granitic magmatism.

## 3 | Strategy and Methods

We have selected four variably deformed samples from the Questembert granite (Figure 1; Figure S1) that cover the bulk
muscovite <sup>40</sup>Ar/<sup>39</sup>Ar date range obtained by Tartèse et al. (2011) (see Table S1). The youngest date of 305 Ma was obtained on sample QRT02, which is highly strained and in which muscovite displays a 'hydrothermal' composition (Tartèse and Boulvais 2010). C/S structures are well developed in samples QRT67 (Figure 2) and QRT09, which yielded partially altered muscovite compositions and intermediate <sup>40</sup>Ar/<sup>39</sup>Ar dates of 309–313 Ma. The nearly isotropic facies (QRT01) provided 'magmatic' muscovite compositions and the oldest <sup>40</sup>Ar/<sup>39</sup>Ar date of 321 Ma, which was interpreted as recording the emplacement age of the granitic magma.

We acquired backscattered electron (BSE) images and X-ray chemical maps of polished thick sections of these samples using scanning electron microscopy (SEM) at the University of Manchester. We then carried out LA-ICP-MS analyses at the University of Manchester for determining apatite U–Pb dates and rare earth elements (REE) abundances, and for Rb-Sr analysis of a range of phases in the selected granite samples (plagioclase, K-feldspar, and muscovite). Detailed methodological information for these two analytical protocols, as well as all results obtained on quality control reference materials and granite

<figure><img src="image_5.png" /><figcaption>**FIGURE 2** | False-colour composite energy dispersive spectroscopy X-ray map of the studied polished section of QRT67, where quartz (Qz) appears blue, plagioclase (Pl) purple, K-feldspar (Kfs) dark green, apatite (Ap) yellow, muscovite (Ms) light green, and biotite red. 'Schistosite' planes ('S') are marked by muscovite porphyroclasts (Ms-I) and feldspar, 'Cisaillement' planes ('C') are marked by recrystallised muscovite (Ms-II), K-feldspar, and quartz. The yellow box highlights an area where the C plane cut through the schistosity. The thin section was prepared on a horizontal plane perpendicular to both 'S' and 'C' planes, parallel to the stretching lineation. [Colour figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

samples, can be found in the **Supporting Information**. All uncertainties quoted in the text are given at the 2σ level.

## 4 | Results

The undeformed sample QRT01 yielded a plagioclase–K-feldspar–muscovite Rb-Sr isochron date of 317 ± 3 Ma (Figure 3A). In samples QRT09 and QRT67, we obtained two isochron dates depending on whether we included larger muscovite porphyroclast (Ms-I) analyses or those from finer muscovite in shear bands (Ms-II) with plagioclase and K-feldspar analyses, with older dates of 306 ± 5 Ma and 316 ± 5 Ma for plagioclase–K-feldspar–Ms-I isochrons in QRT09 and QRT67, respectively, and an identical plagioclase–K-feldspar–Ms-II date of 290 ± 4 Ma in both samples (Figure 3B,C). Finally, in sample QRT02 both muscovite populations, together with plagioclase and K-feldspar, define a single isochron date of 306 ± 4 Ma (Figure 3D).

Samples QRT01, QRT09, QRT67, and QRT02 yielded apatite U–Pb dates of 312 ± 4 Ma, 305 ± 4 Ma, 301 ± 5 Ma, and 311 ± 6 Ma, respectively (Figure S2). These apatite grains are characterised by Th/U ratios ranging between ca. 0.01 and 0.5 (Figure S3). Their REE patterns are more homogenous for sample QRT01 than for the other samples. The spectra are characterised by a large range of Eu/Eu* values between ~0.05 and 5, and all display a tetrad effect, with TE<sub>1,3</sub> values higher than 1.1 (tetrad effect parameter TE<sub>1,3</sub> calculated after Irber (1999)) (Figure S3).

## 5 | Discussion

### 5.1 | Accuracy of Rb-Sr Dates

Some of the in situ Rb-Sr results are fully consistent with the known timing of magmatic and hydrothermal events in the Questembert granite. The isotropic sample QRT01 yielded a Rb-Sr date of 317 ± 3 Ma, within error of the previously determined muscovite <sup>40</sup>Ar/<sup>39</sup>Ar date of 320.8 ± 1.3 Ma, which we interpret as the emplacement age of the Questembert granite. It is also consistent with a whole rock Rb-Sr isochron date of 323 ± 6 Ma constructed from analyses of 9 samples from Vidal (1980) and Tartèse and Boulvais (2010) (Figure S4). In situ Rb-Sr analyses in QRT01 also yielded an initial (<sup>87</sup>Sr/<sup>86</sup>Sr)<sub>0</sub> ratio of 0.7118 ± 0.0022 (Figure 3A), identical to the (<sup>87</sup>Sr/<sup>86</sup>Sr)<sub>0</sub> of 0.7115 ± 0.0012 defined by whole rock analyses (Figure S4).

### 5.2 | Identification of an Early Permian Event

In C/S structure samples QRT67 and QRT09, in situ Rb-Sr analyses provide new information. Indeed, these in situ analyses record a younger date for muscovite in shear bands (290 ± 4 Ma in both samples) compared to dates recorded by muscovite porphyroclasts (306 ± 5 Ma and 316 ± 5 Ma for QRT09 and QRT67, respectively) (Figure 2B,C). These older dates are consistent with known dates for magmatic emplacement and hydrothermal activity in the Questembert granite (Table S1). The Early Permian date suggests that hydrothermal activity persisted for
<figure><img src="image_6.png" /><figcaption>**FIGURE 3** | Rb-Sr isochron plots for the four studied samples from the Questembert granite. Blue and red ellipses correspond to analyses of muscovite porphyroclasts (Ms-I) and muscovite in shear bands (Ms-II), respectively. Ellipses with dashed outlines correspond to analyses excluded from isochron calculations. All ellipses correspond to 2 standard error uncertainties. [Colour figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

millions of years after the main phase of Carboniferous magmatic activity along the SASZ. Interestingly, this Permian date is recorded in the two samples closest to the SASZ, consistent with the fact that this structure acted as a major channel for late fluid circulations (e.g., Lemarchand et al. 2012; Tartèse et al. 2012). Additionally, Permian events are known in the Western Europe Variscan orogen, related to both large-scale magmatic activity such as in the Cornubian batholith (e.g., Searle et al. 2024), and fluid circulations and accompanying mineralization (i.e., U deposits in the Armorican Massif; Ballouard et al. 2017). These new results illustrate the usefulness of in situ analyses for chronological studies. Indeed, bulk muscovite ⁴⁰Ar/³⁹Ar dating did not identify this Permian date; this is likely related to the fact that larger muscovite grains were hand-picked for bulk analyses, and, thus, we missed the younger event recorded by (re)crystallised small muscovite in shear bands.

## 5.3 | Behaviour of the Rb-Sr System During Fluid-Rock Interactions

Our in situ analyses illustrate the complex behaviour of the Rb-Sr isotopic system in muscovite at the millimetric scale. In

two out of the three deformed samples, the Rb-Sr system records significantly older dates for muscovite porphyroclasts in schistosity planes that correspond to magmatic crystallisation and minor magmatic-hydrothermal alteration events, compared to small muscovite grains in shear bands that record dates that are about 25–30 Ma younger. It is unlikely that this Permian date corresponds to a late metamorphic, retrograde age, as it is thought that Sr thermal diffusion in muscovite ends at fairly high temperatures > 500°C (e.g., Kellett et al. 2024 and references therein). Thus, it likely dates deformation, fluid circulations, or fluid-assisted deformation, which promoted dynamic (re)crystallisation of muscovite in localised shear bands. Indeed, evidence for the involvement of fluids in the studied system is numerous. For instance, Tartèse et al. (2011) argue for long-lasting fluid-rock interactions to explain the variable chemical composition of muscovite grains, ranging from magmatic in sample QRT01 to largely hydrothermal in QRT02. The fact that the Rb-Sr system in muscovite porphyroclasts located a few millimetres away from the shear bands remained unaffected by the Early Permian event recorded by recrystallised muscovite in shear bands argues for fluid-rock interaction limited in space and time at 290 Ma with low fluid/rock ratios.
## 5.4 | Implication for Understanding C/S Structures

To fully understand the significance of the measured Rb-Sr dates, one has to decipher if the younger dates at 290 Ma correspond to a late phase of fluid-assisted deformation along the SASZ that created the shear bands or to discrete fluid circulations that exploited these fine-grained permeable pre-existing structures. We argue for the second option. From a regional perspective, it is obvious that the Questembert and Lizio granites, for example, acquired their cartographic asymmetric shape during dextral synkinematic emplacement (Figure 1A). The fact that the Questembert granite is more elongated than the Lizio granite relates to the larger amount of displacement along the southern branch of the SASZ (~150–200 km) than along the northern branch (~40 km) (Jégouzo and Rossello 1988). Consistently, C/S structures are more widespread in the Questembert granite than in the Lizio one, where they are limited to the southern part of the granite close to the SASZ.

As argued by Gapais (1989), the C/S structures likely formed during synkinematic emplacement and cooling of the granite (Figure 4), and regional tectonic stress in the area persisted for several million years after its crystallisation (e.g., Gumiaux et al. 2004). At 290 Ma, a short-lived event allowed limited

<figure><img src="image_7.png" /><figcaption>**FIGURE 4** | Schematic drawing representing three conditions of fluid-rock-deformation relationships for syntectonic granites. The ages indicated correspond to the Questembert example. In A, only fluids exsolved from the magmas are involved; no deformation accompanies crystallisation. In B, crystallisation occurs during deformation leading to the formation of C/S structures; fluids exsolved from the magma at greater depth pervasively invade the volume of rock considered. In C, late permeation of hydrothermal fluids is localised along C planes, possibly triggering a resurgence of deformation along these planes. [Colour figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

quantities of fluids to permeate the deformed granite along C planes (Figure 4). We propose that these fluids induced recrystallization and/or neocrystallization of muscovite in these C planes, leading to weakening that allowed the resurgence of some localised deformation. This is suggested by observation of some C planes locally cutting through the schistosity (Figure 2). This fluid-induced deformation event at 290 Ma, thus, reused the C planes developed ca. 25 Ma earlier during magma emplacement and crystallisation.

Altogether, these new results show that in situ Rb-Sr analysis provides a detailed record of the magmatic-hydrothermal-deformation history of synkinematic granites. Magmatic crystallisation is recorded in muscovite porphyroclasts, while younger dates correspond to localised hydrothermal events associated with the continuing tectonic evolution of the area. It remains that in such complex geological contexts, independent considerations like regional geological constraints or the use of other complementary geochronometers are crucial for a comprehensive interpretation of the processes involved.

### Acknowledgements

This work greatly benefited from discussions with Yannick Branquet. We thank an anonymous reviewer and Olivier Vanderhaeghe for their constructive suggestions. R.T. acknowledges funding from the University of Manchester and the UK Science and Technology Facilities Council (STFC) (grant \#ST/S002170/1) for funding the LA-ICP-MS facility in Manchester, and the Royal Society for funding the benchtop SEM in Manchester.

### Funding

This work was supported by the University of Manchester and the UK Science and Technology Facilities Council (STFC) (\#ST/S002170/1).

### Conflicts of Interest

The authors declare no conflicts of interest.

### Data Availability Statement

The data that supports the findings of this study is available in the Supporting Information of this article.

### References

Ballouard, C., M. Poujol, P. Boulvais, et al. 2017. "Magmatic and Hydrothermal Behavior of Uranium in Syntectonic Leucogranites: The Uranium Mineralization Associated With the Hercynian Guérande Granite (Armorican Massif, France)." *Ore Geology Reviews* 80: 309-331. https://doi.org/10.1016/j.oregeorev.2016.06.034.

Bernard-Griffiths, J., J. J. Peucat, S. Sheppard, and P. Vidal. 1985. "Petrogenesis of Hercynian Leucogranites From the Southern Armorican Massif: Contribution of REE and Isotopic (Sr, Nd, Pb and O) Geochemical Data to the Study of Source Rock Characteristics and Ages." *Earth and Planetary Science Letters* 74: 235-250. https://doi.org/10.1016/0012-821X(85)90024-X.

Berthé, D., P. Choukroune, and P. Jégouzo. 1979. "Orthogneiss, Mylonite and Non Coaxial Deformation of Granites: Example of the South Armorican Shear Zone." *Journal of Structural Geology* 1: 31-42. https://doi.org/10.1016/0191-8141(79)90019-1.

Black, L. P. 1988. "Isotopic Resetting of U-Pb Zircon and Rb-Sr and Sm-Nd Whole-Rock Systems in Enderby Land, Antarctica: Implications for
the Interpretation of Isotopic Data From Polymetamorphic and Multiply Deformed Terrains." *Precambrian Research* 38: 355-365. https://doi.org/10.1016/0301-9268(88)90033-2.

Boschetti, L., C. Boullerne, Y. Rolland, et al. 2025. "Shear Zone Memory Revealed by In-Situ Rb-Sr and $^{40}$Ar/$^{39}$Ar Dating of Pyrenean and Alpine Tectonic Phases in the External Alps." *Lithos* 514-515: 108168. https://doi.org/10.1016/j.lithos.2025.108168.

Evans, J. A. 1989. "Resetting of the Rb-Sr Whole-Rock Isotope System of an Ordovician Microgranite During Devonian Low-Grade Metamorphism." *Geological Magazine* 126: 675-679. https://doi.org/10.1017/S0016756800006968.

Gapais, D. 1989. "Shear Structures Within Deformed Granites: Mechanical and Thermal Indicators." *Geology* 17: 1144-1147. https://doi.org/10.1130/0091-7613(1989)017<1144:SSWDGM>2.3.CO;2.

Gorozovsky, L., and O. Alard. 2020. "Optimisation of Laser and Mass Spectrometer Parameters for the *In Situ* Analysis of Rb/Sr Ratios by LA-ICP-MS/MS." *Journal of Analytical Atomic Spectrometry* 35: 2322-2336. https://doi.org/10.1039/D0JA00308E.

Gumiaux, C., D. Gapais, J.-P. Brun, J. Chantraine, and G. Ruffet. 2004. "Tectonic History of the Hercynian Armorican Shear Belt (Brittany, France)." *Geodinamica Acta* 17: 289-307. https://doi.org/10.3166/ga.17.289-307.

Hamet, J., and C. J. Allègre. 1976. "Rb-Sr Systematics in Granite From Central Nepal (Manaslu): Significance of the Oligocene Age and High $^{87}$Sr/$^{86}$Sr Ratio in Himalayan Orogeny." *Geology* 4: 470-472. https://doi.org/10.1130/0091-7613(1976)4<470:RSIGFC>2.0.CO;2.

Hogmalm, K. J., T. Zack, A. K. O. Karlsson, A. S. L. Sjöqvist, and D. Garbe-Schönberg. 2017. "*In Situ* Rb-Sr and K-Ca Dating by LA-ICP-MS/MS: An Evaluation of N₂O and SF₆ as Reaction Gases." *Journal of Analytical Atomic Spectrometry* 32: 305-313. https://doi.org/10.1039/C6JA00362A.

Irber, W. 1999. "The Lanthanide Tetrad Effect and Its Correlation With K/Rb, Eu/Eu*, Sr/Eu, Y/ho, and Zr/Hf of Evolving Peraluminous Granite Suites." *Geochimica et Cosmochimica Acta* 63: 489-508. https://doi.org/10.1016/S0016-7037(99)00027-7.

Jahn, B. M., P. Y. Chen, and T. P. Yen. 1976. "Rb-Sr Ages of Granitic Rocks in Southeastern China and Their Tectonic Significance." *GSA Bulletin* 87: 763-776. https://doi.org/10.1130/0016-7606(1976)87<763:RAOGRI>2.0.CO;2.

Jégouzo, P. 1980. "The South Armorican Shear Zone." *Journal of Structural Geology* 2: 39-47. https://doi.org/10.1016/0191-8141(80)90032-2.

Jégouzo, P., and E. A. Rossello. 1988. "La Branche Nord du Cisaillement Sud-Armoricain (France): Un essai d'évaluation du déplacement par l'analyse des mylonites." *Comptes Rendus de l'Académie des Sciences* 307: 1825-1831.

Kellett, D. A., K. P. Larson, and D. R. Skipton. 2024. "Integration of White Mica In Situ $^{87}$Rb/$^{87}$Sr With In Situ and Step-Heat $^{40}$Ar/$^{39}$Ar Dates in Orogenic Settings." *Lithos* 482: 107687. https://doi.org/10.1016/j.lithos.2024.107687.

Lemarchand, J., P. Boulvais, M. Gaboriau, et al. 2012. "Giant Quartz Vein Formation and High Elevation Meteoric Fluid Infiltration Into the South Armorican Shear Zone: Geological, Fluid Inclusion and Stable Isotope Evidence." *Journal of the Geological Society* 169: 17-27. https://doi.org/10.1144/0016-76492010-186.

Olierook, H. K., K. Rankenburg, S. Ulrich, et al. 2020. "Resolving Multiple Geological Events Using In Situ Rb-Sr Geochronology: Implications for Metallogenesis at Tropicana, Western Australia." *Geochronology* 2: 283-303. https://doi.org/10.5194/gchron-2-283-2020.

Patel, S. C., C. D. Frost, and B. R. Frost. 1999. "Contrasting Responses of Rb-Sr Systematics to Regional and Contact Metamorphism, Laramie Mountains, Wyoming, USA." *Journal of Metamorphic Geology* 17: 259-269. https://doi.org/10.1046/j.1525-1314.1999.00411.x.

Redaa, A., J. Farkaš, A. Hassan, A. S. Collins, S. Gilbert, and S. C. Löhr. 2022. "Constraints From *In-Situ* Rb-Sr Dating on the Timing of Tectono-Thermal Events in the umm Farwah Shear Zone and Associated cu-Au Mineralisation in the Southern Arabian Shield, Saudi Arabia." *Journal of Asian Earth Sciences* 224: 105037. https://doi.org/10.1016/j.jseaes.2021.105037.

Ribeiro, B. V., C. L. Kirkland, M. A. Finch, et al. 2023. "Microstructures, Geochemistry, and Geochronology of Mica Fish: Review and Advances." *Journal of Structural Geology* 175: 104947.

Rösel, D., and T. Zack. 2022. "LA-ICP-MS/MS Single-Spot Rb-Sr Dating." *Geostandards and Geoanalytical Research* 46: 143-168. https://doi.org/10.1111/ggr.12414.

Schaltegger, U. 1990. "Post-Magmatic Resetting of Rb-Sr Whole Rock Ages — A Study in the Central Aar Granite (Central Alps, Switzerland)." *Geologische Rundschau* 79: 709-724. https://doi.org/10.1007/BF01879210.

Searle, M. P., R. K. Shail, J. M. Pownall, C. Jurkowski, A. B. Watts, and L. J. Robb. 2024. "The Permian Cornubian Granite Batholith, SW England; Part 1: Field, Structural, and Petrological Constraints." *Geological Society of America Bulletin* 136: 4301-4320. https://doi.org/10.1130/B37457.1.

Simon, K. 1990. "Hydrothermal Alteration of Variscan Granites, Southern Schwarzwald, Federal Republic of Germany." *Contributions to Mineralogy and Petrology* 105: 177-196. https://doi.org/10.1007/BF00678985.

Tartèse, R., and P. Boulvais. 2010. "Differentiation of Peraluminous Leucogranites 'en Route' to the Surface." *Lithos* 114: 353-368. https://doi.org/10.1016/j.lithos.2009.09.011.

Tartèse, R., P. Boulvais, M. Poujol, et al. 2012. "Mylonites of the South Armorican Shear Zone: Insights for Crustal-Scale Fluid Flow and Water-Rock Interaction Processes." *Journal of Geodynamics* 56: 86-107. https://doi.org/10.1016/j.jog.2011.05.003.

Tartèse, R., G. Ruffet, M. Poujol, P. Boulvais, and T. R. Ireland. 2011. "Simultaneous Resetting of the Muscovite K-Ar and Monazite U-Pb Geochronometers: A Story of Fluids." *Terra Nova* 23: 390-398. https://doi.org/10.1111/j.1365-3121.2011.01024.x.

Tillberg, M., H. Drake, T. Zack, E. Kooijman, M. J. Whitehouse, and M. E. Åström. 2020. "In Situ Rb-Sr Dating of Slickenfibres in Deep Crystalline Basement Faults." *Scientific Reports* 10: 562. https://doi.org/10.1038/s41598-019-57262-5.

Vidal, P. 1980. *L'évolution Polyorogénique du Massif Armoricain: Apport de la Géochronologie et de la Géochimie du Strontium*. Vol. 21, 162. Mémoires de la Société Géologique et Minéralogique de Bretagne.

Villa, I. M., P. De Bièvre, N. E. Holden, and P. R. Renne. 2015. "IUPAC-IUGS Recommendation on the Half Life of $^{87}$Rb." *Geochimica et Cosmochimica Acta* 164: 382-385. https://doi.org/10.1016/j.gca.2015.05.025.

Zack, T., and K. J. Hogmalm. 2016. "Laser Ablation Rb/Sr Dating by Online Chemical Separation of Rb and Sr in an Oxygen-Filled Reaction Cell." *Chemical Geology* 437: 120-133. https://doi.org/10.1016/j.chemg.eo.2016.05.027.

## Supporting Information

Additional supporting information can be found online in the Supporting Information section. **Data S1:** ter70022-sup-0001-TableS1-S3-FigureS1-S7.pdf. **Table S2:** LA-ICP-MS protocol for apatite U-Pb dating and trace element analysis. **Table S3:** Apatite U-Pb dating and trace element analysis results. **Table S4:** LA-ICP-MS/MS protocol for Rb/Sr dating. **Table S5:** In situ Rb/Sr. analysis results.