# Insights on the Permian tuff beds from the Saint-Affrique Basin (Massif Central, France): an integrated geochemical and geochronological study

Marc Poujol, Mathilde Mercuzot, Michel Lopez, Sylvie Bourquin, Olivier Bruguier, Erwan Hallot, Laurent Beccaletto

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## To cite this version:

Marc Poujol, Mathilde Mercuzot, Michel Lopez, Sylvie Bourquin, Olivier Bruguier, et al. Insights on the Permian tuff beds from the Saint-Affrique Basin (Massif Central, France): an integrated geochemical and geochronological study. Comptes Rendus. Géoscience, 2023, 355 (S2), pp.137-161. 10.5802/crgeos.184. insu-03918292

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Distributed under a Creative Commons Attribution 4.0 International License
# Comptes Rendus
## Géoscience
### Sciences de la Planète

Marc Poujol, Mathilde Mercuzot, Michel Lopez, Sylvie Bourquin, Olivier Bruguier, Erwan Hallot and Laurent Beccaletto

**Insights on the Permian tuff beds from the Saint-Affrique Basin (Massif Central, France): an integrated geochemical and geochronological study**

Volume 1, issue 0 (0000), p. 000-000

<https://doi.org/10.5802/crgeos.184>

© Académie des sciences, Paris and the authors, 2022.

*Some rights reserved.*

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This article is licensed under the
CREATIVE COMMONS ATTRIBUTION 4.0 INTERNATIONAL LICENSE.
http://creativecommons.org/licenses/by/4.0/
Tribute to Jean Dercourt / Hommage à Jean Dercourt

# Insights on the Permian tuff beds from the Saint-Affrique Basin (Massif Central, France): an integrated geochemical and geochronological study

Marc Poujol<sup>a, *, 1</sup>, Mathilde Mercuzot<sup>a</sup>, Michel Lopez<sup>b, 2</sup>, Sylvie Bourquin<sup>a</sup>, Olivier Bruguier<sup>b, 3</sup>, Erwan Hallot<sup>a</sup> and Laurent Beccaletto<sup>c</sup>

<sup>a</sup> Univ. Rennes, CNRS, Géosciences Rennes, UMR6118, F-35000 Rennes, France

<sup>b</sup> Géosciences Montpellier, Université de Montpellier, CNRS, F-34000 Montpellier, France

<sup>c</sup> BRGM, F-45060 Orléans, France

E-mails: marc.poujol@univ-rennes1.fr (M. Poujol), mathilde.mercuzot@outlook.com (M. Mercuzot), michel.lopez@umontpellier.fr (M. Lopez), sylvie.bourquin@univ-rennes1.fr (S. Bourquin), Olivier.Bruguier@gm.univ-montp2.fr (O. Bruguier), erwan.hallot@univ-rennes1.fr (E. Hallot), l.beccaletto@brgm.fr (L. Beccaletto)

**Abstract.** The Permian marks the transition between the end of the accretion of the supercontinent Pangea and the beginning of its dislocation. In the Eastern Pangea intertropical domain (i.e. the present-day Western Europe), the late-Paleozoic (i.e. uppermost Carboniferous-Permian) history remains poorly constrained due to the lack of precise radiometric data. This is particularly true for Permian basins from the southern part of the French Massif Central, making it difficult to determine correlations between basins and therefore robust timings and constraints on the environmental and climate events described in these basins, and to compare them with the larger-scale settings. This article focuses on the Saint-Affrique Basin, via an integrated petrological, geochemical and geochronological study of eight of the volcanic-ash levels interbedded in the sedimentary succession. It highlights the existence of two different groups of felsic volcaniclastic rocks. The first group, located at the base of the basin and attributed to the Stephanian continental stage, is related to a late-orogenic volcanic setting and corresponds to calc-alkaline trachy-andesitic tuffites that could not be dated due to the lack of volcanic zircon. The second group, located towards the top of the succession, is composed of calc-alkaline dacitic ash beds and tuffites yielding Kungurian ages, i.e. late early Permian (Cisuralian, 283.5 ± 0.6 to 273.01 ± 0.14 Ma), and are attributed to a post-orogenic deposition setting. These ages show that the sedimentary filling of the basin is younger than hitherto expected (i.e. Artinskian, 290.1 ± 0.26 to 283.5 ± 0.6 Ma). The elemental geochemistry, the presence of inherited detrital zircons and the Hf signatures of the volcanic ones indicate the involvement of an old (Proterozoic

<footnote>* Corresponding author.</footnote>
and older) basement in the magma genesis; this crustal contribution becomes more prominent to-
wards the top of the sedimentary succession.

**Keywords.** Continental Permian basin, Zircon, LA-ICP-MS dating, Volcanism, French Massif Central.

**1. Introduction**

The Permian marks the transition between the end of the accretion of the supercontinent Pangea and the beginning of its dislocation [e.g. Scotese and Langford, 1995, Stampfli and Kozur, 2006, Stampfli et al., 2013, Domeier and Torsvik, 2014]. From the end of the Carboniferous onwards, the Variscan belt, still in erosion, experienced a gravitational collapse [Ménard and Molnar, 1988, Malavieille et al., 1990, Van Den Driessche and Brun, 1992, Burg et al., 1994, Faure, 1995]. The eastern Pangea intra-mountain domain then underwent a generalized extension that resulted in the formation of extensive structures and rifts forming narrow continental basins at the end of the Carboniferous, which widened during the Permian [Ménard and Molnar, 1988, Vallé et al., 1988, Van Den Driessche and Brun, 1989, 1992, Burg et al., 1990, Malavieille et al., 1990, Faure and Becq-Giraudon, 1993, Faure, 1995, Becq-Giraudon et al., 1996, Genna et al., 1998, Choulet et al., 2012, Becq-caletto et al., 2015]. This late-to-post-Variscan transition was widely accompanied by aerial volcanism which has been well recorded in numerous continental siliciclastic sedimentary basins, either as a volcanogenic fraction in the sediments, or as interlayered volcanic ash beds that are still mostly undated [Odin and Conrad, 1987, Châteauneuf and Farjanel, 1989, Nmila, 1995, Timmerman, 2004, Pellenard et al., 2017, Ducassou et al., 2019].

Due to the lack of precise radiometric data and marine biostratigraphic markers, the timings of the late Paleozoic (i.e. uppermost Carboniferous–Permian) history remains poorly constrained in the Eastern Pangea intertropical domain (i.e. the present-day western European basins). This is therefore problematic to accurately reconstruct the paleogeography and the evolution of the depositional environments. The current stratigraphic framework implies a nomenclature specific to European continental basins, with a terminology based on lithology as well as on the floristic and ichnological content [Izart et al., 1998, Lucas and Shen, 2018, Schneider et al., 2020]. Thus, the most recent late-Paleozoic paleogeographic reconstructions incorporate the

Middle to Upper Permian series without distinction. In addition, it is not currently possible to provide a paleogeographic outline of the Lower Permian basins for the terrestrial domain [e.g. Bourquin et al., 2011]. Moreover, new age constraints on the sedimentary successions will allow to accurately link the observed environmental and climate perturbations recorded in the sedimentary successions [e.g. Mercuzot et al., 2021b] to the global climate dynamics.

Significant advances have been recently obtained in terms of temporal calibration (dating of volcanogenic levels) and depositional environments of the Carboniferous–Permian finite age series of the northern Massif Central [Pellenard et al., 2017, Ducassou et al., 2019, Mercuzot. et al., 2021a, Mercuzot et al., 2021b, 2022].

In contrast, little is known about the age and nature of the volcanism in the Carboniferous–Permian basins from south of the Massif Central (Saint-Affrique, Lodève-Graissessac and Gabian-Neffiès basins), where only a few volcanogenic layers in the Lodève-Graissessac Basin have been dated so far [e.g. Bruguier et al., 2003, Michel et al., 2015]. However, numerous volcanic ash beds have been described [Goloubinoff, 1979, Rolando, 1988], in particular thanks to the discovery of uranium mineralization in 1957 in the Lodève Basin during a radio-prospecting campaign performed by the French atomic energy commission (i.e. CEA, Commissariat à l'Energie Atomique). Nearly 240 wells were then drilled and twenty-four different ash beds were identified and numbered using Roman numerals (I to XXIV), following the order of discovery and consequently not the stratigraphic order. Once these benchmark levels had been described and identified in the Lodève Basin, uranium prospectors then mapped the Saint-Affrique Basin (Figure 1A) in order to look for potential sister uranium deposits. There, they also identified several volcanic ash beds and established correlations with those described in the Lodève Basin based on their facies architecture [e.g. Hübner et al., 2011]. These volcanic ash beds have not been dated yet, nor characterized geochemically. Therefore, the aim of this study is to provide new information on the age and origin of these volcanic ash beds in order
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**Figure 1.** (A) Simplified geological map of the Saint-Affrique Basin [modified after Guérangé-Lozes and Guérangé, 1991, Guérangé-Lozes et al., 1995 and Guérangé-Lozes and Alabouvette, 1999]; and (B) NWSE cross section of the Saint-Affrique Basin showing the successive back-faulting and southward migrating depocenter controlled by the rejuvenation and inversion of major Variscan thrusts [modified after Legrand, 1990].

to shed light on the chronology of the Saint-Affrique Basin development and on the geodynamical setting prevailing south of the Massif Central at the end of the Variscan orogeny.

## 2. Geological setting

The Saint-Affrique Basin (Figure 1A) is one of the major intramountain Carboniferous–Permian continental basin from southwestern France [e.g. Rolando,

1988, Rolando et al., 1988]. It is considered as an overall southward-dipping half-graben basin, controlled by repeated back faulting along major Variscan thrusts that were successively inverted during the late Carboniferous to Permian crustal thinning and collapse of the Variscan-related Montagne Noire thrust belt [Figure 1B; Legrand, 1990]. Most of the late Carboniferous to Permian deposits unconformably onlap onto the Precambrian to Cambro-Ordovician feldspathic sandstones and black schists of the Saint-Sernin-sur-Rance nappe, with a major northward
<figure><img src="image_5.png" /><figcaption>**Figure 2.** General description of the Saint-Affrique Basin sedimentary succession [based on Guérangé-Lozes et al., 1995] with the stratigraphic location of the selected volcanic ash bed samples, chronostratigraphic and lithostratigraphic subdivisions [after Goloubinoff, 1979, Rolando et al., 1988, Gand, 1993, Gand et al., 1996], and depositional environment evolution [after Goloubinoff, 1979, Rolando et al., 1988, Gand et al., 1996 and Hübner et al., 2011].</figcaption></figure>

detrital input coming from the erosion of the up-
lifted northern limb of the Montagne Noire Massif.
The latter includes Cambrian to Ordovician sand-
stone, shale and carbonate deposits of the metased-
imentary cover, as well as gneisses, migmatites and
biotite and garnet granites of the axial core com-
plex. The tilted northern border of the basin is in
turn fed by calc-alkaline meta-monzogranites and
metasyenogranites fringing the leptyno-amphibolite
group of the Lévezou.

Syn-rift faulting controlled the asymmetric geom-
etry, depositional partitioning and architecture of an
overall alluvial to lacustrine sedimentation [Rolando,
1988, Legrand et al., 1994]. The stratigraphic range
of the basin infill is assumed to cover the late

Carboniferous, with a low angular unconformity
in the northern part of the basin, to the middle
Permian [Figure 2; Rolando et al., 1988, Gand, 1993].

It has been reported that the late Carboniferous
corresponds to the Stephanian continental subdivi-
sion and includes three main formations (fms): at
the base, the *Mine* Formation (Fm) is composed of a
coarse conglomerate and breccia package and is on-
lapped unconformably by volcaniclastic deposits, in-
cluding fine-grained ash layers and volcanic breccias
belonging to the *Cinéritique* Fm (Figure 2). The latter
passes transitionally to the *Vigne* Fm, mainly com-
posed of volcanic breccias alternating with coarse
arkosic sandstones [David, 1967, Goloubinoff, 1979,
Rolando, 1988; Figure 2].
The Permian deposits correspond to five forma-
tions accumulated during two sedimentary cycles.
Cycle 1 corresponds to the F1, F2 and F3 fms and
Cycle 2 to the F4 and F5 fms [Figure 2; Rolando
et al., 1988]. At the base, the F1 Fm, named the
*Gorb and Canabols* Fm (Figure 1A and B), is mainly
composed of stacked conglomerates considered as
streamflow and debris flow deposits [Hübner et al.,
2011], passing laterally to fluvio-lacustrine deposits
[Rolando, 1988]. The F2 Fm, named the *Saint-Rome-
de-Tarn* Fm [Rolando, 1988], conformably overlays
the F1 Fm and is divided into two lithologic units:
the F2a Unit, described as lacustrine [Rolando, 1988,
Hübner et al., 2011], and the F2b Unit, considered
as fluvio-lacustrine to lacustrine [Rolando, 1988],
evolving upward into sheetflood to alluvial plain de-
posits [Hübner et al., 2011]. This formation is over-
lain by the fluvial F3 Fm, named the *Dourdou* Fm
[Rolando, 1988], interpreted as a low sinuosity me-
andering river system [Hübner et al., 2011]. After a
regional erosive event marked by a low angular un-
conformity, the F4 Fm, named the *Belmont* Fm, is
attributed to the emplacement of alluvial-fan de-
posits during a major tectonic rejuvenation event
from the southern border of the basin. The thick
massive conglomerate package passes upwards and
laterally to playa-lake deposits identified as the F5
Fm, named the *Saint-Pierre* Fm [Rolando, 1988; Fig-
ure 1A and B]. Hübner et al. [2011] consider that
part of the F4 and F5 fms, denoted as F4/F5, i.e.
the *Saint-Pierre transitional facies* Fm, is transitional
and marks the lateral and vertical evolution from al-
luvial fan to meandering and alluvial plain deposits
(Figure 2).

The stratigraphic setting of this sedimentary suc-
cession was firstly based on palynological analyses,
i.e. defining continental floristic stages (Figure 2).
Based on the microflora composition, i.e. spores and
pollens, sediments of Cycle 1 were attributed to late
Autunian to early Thuringian subdivisions [Rolando
et al., 1988]. However, using fauna biostratigraphy
and in particular freshwater jellyfish, in comparison
with German basins, Gand et al. [1996] proposed
an early Asselian age for the F2b Unit, in which the
species *Medusina atava* was identified, but they did
not exclude that a simple paleoenvironment control
could explain this fossil distribution in the stratigra-
phy. In turn, the upper part of the F3 Fm provided
the *Medusina limnica* species which unfortunately

straddles a large stratigraphical repartition from the
Sakmarian to the late Permian. By using climate-
relevant geochemical, petrological and sedimento-
logical proxies, Hübner et al. [2011] correlated the
climate cycles from the Saint-Affrique and Lodève
basins and proposed an Asselian to Artinskian age for
Cycle 1 (F1, F2 and F3 fms), and an Artinskian age
for the base of Cycle 2 (F4 and the base of F5 fms,
Figure 2). As these stratigraphic interpretations based
on fauna and flora depend on environmental and cli-
mate conditions, they lead to large age uncertainties
thereby highlighting the need for absolute dating.

Several volcanic ash layers, named under the gen-
eral term of cinerites, were described throughout the
Carboniferous to Permian sedimentary succession
[Goloubinoff, 1979, Rolando, 1988]. Some of these
layers are reported as equivalent to those found in the
Lodève Basin [Laversanne, 1976, Odin and Conrad,
1987]. Given their instantaneous deposition, these
tuff beds are the best candidates to provide accurate
absolute dating. In outcrops, they are easily identifi-
able by their pale-light colour (from white to salmon),
their regular thickness and their hardness contrast-
ing with the surrounding rocks. The major key tuff
beds were identified in the *Cinéritique* Fm at the base
of F1 Fm and in the F2 and F3 fms [Figure 2; e.g.
Rolando, 1988]. In order to constrain the age of the
sedimentary successions, six different volcanic ash
and tuffitic beds have been collected throughout the
stratigraphic column of the Saint-Affrique Basin (see
Figure 1A).

**3. Samples and methods**

3.1. *Sampling*

Eight samples have been collected in the entire basin
(Figures 1A and 2 and Table 1). Three of them (Per21-
3, Per21-4 and Can21-5) have been sampled in the so-
called *Cinéritique* Fm belonging to the Stephanian
subdivision, but due to lack of continuous outcrops,
their precise stratigraphic location in this Fm is not
known (Figures 1A and 2). One sample (Dev21-1) was
collected in the *Saint-Rome-de-Tarn* Fm (F2), in the
Grey Autunian subdivision (Figures 1A, B and 2). An-
other one (Lat20-1) is located within the *Dourdou*
Fm (F3; Figures 1A, B, 2 and 3A) and corresponds
to Rolando's cinerite III (1988). The remaining three
samples (Gal21-6, Gal21-7 and Cam20-3) were col-
lected within the *Saint-Pierre transitional facies* Fm
<table><caption>**Table 1.** GPS coordinates of the selected samples</caption><thead><tr><th>Samples</th><th>Locations</th><th>Lat/long coordinates</th></tr></thead><tbody><tr><td>Per21-4</td><td>Along the road between the farm of Le Cluzel and the hamlet of Peret</td><td>44° 0' 58.76" N<br>2° 34' 59.44" E</td></tr><tr><td>Per21-3</td><td>Near the hamlet of Peret</td><td>44° 1' 08" N<br>2° 35' 13" E</td></tr><tr><td>Can21-5</td><td>Along the road to the north-east of the Cansac hamlet</td><td>44° 1' 23" N<br>2° 40' 11" E</td></tr><tr><td>Dev21-1</td><td>Left bank of the Jauret creek</td><td>44° 00' 41" N<br>2° 35' 55" E</td></tr><tr><td>Lat20-1</td><td>Just outside the village of Latour-sur-Sorgues along the D7 road</td><td>43° 47' 39" N<br>2° 50' 26" E</td></tr><tr><td>Gal21-7</td><td>Near the farm of Galamans along the road to the hamlet of Verrières, above the Riaudou creek</td><td>43° 50' 50.36" N<br>2° 49' 12.88" E</td></tr><tr><td>Cam20-3</td><td>Along the D999 road</td><td>43° 50' 47" N<br>2° 50' 5" E</td></tr><tr><td>Gal21-6</td><td>Same layer as sample Cam 20-3</td><td>43° 50' 47" N<br>2° 49' 02" E</td></tr></tbody></table>

(F4/F5), close to the top of the red Autunian subdivision (Figures 1A, B, 2 and 3B for sample Cam20-3).

## 3.2. Whole-rock geochemical analyses

Samples were crushed at Thin Section Lab (TSL, Toul, France) following a standard protocol to obtain adequate powder fractions using agate mortars. Chemical analyses (Supplementary Table 1) were performed by the Service d'Analyse des Roches et des Minéraux (SARM; CRPG-CNRS, Nancy, France) using an ICP-AES (inductively coupled plasma-atomic emission spectroscopy) for major-elements and an ICP-MS (inductively coupled plasma-mass spectrometry) for trace-elements, following the techniques described in Carignan et al. [2001]. All geochemical classification and tectonic discrimination diagrams used in this study were drawn using the GCDkit software [Janousek et al., 2006].

## 3.3. Zircon U-Pb dating

A classic mineral separation procedure has been applied to concentrate zircon grains suitable for U-Pb dating using the facilities available at TSL. Zircon grains were imaged by cathode luminescence (CL)

using a Reliotron CL system equipped with a digital colour camera available at the GeOHeLiS platform (University of Rennes 1, France).

U-Pb geochronology of zircon was conducted by in-situ LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) at the GeOHeLiS analytical platform using an ESI NWR193UC Excimer laser (193 nm wavelength), coupled to a quadripole Agilent 7700x ICP-MS equipped with a dual pumping system to enhance sensitivity [Paquette et al., 2014]. The methodology used to perform the analyses can be found in Nosenzo et al. [2022] as well as in Supplementary Table 2. Concordia diagrams have been generated using IsoplotR [Vermeesch, 2018].

## 3.4. Zircon Hf analyses

Lu-Hf isotopes were measured at the Géosciences Montpellier Laboratory, University of Montpellier (AETE-ISO regional facility of the OSU OREME) using a ThermoFinnigan Neptune+ MC-ICP-MS (multicollector-inductively coupled plasma-mass spectrometer) coupled with a Photon-Machine Analyte G2 Excimer laser (193 nm wavelength). Ablation was performed using a 50 µm spot size. The laser frequency was 5 Hz and the energy density of the
<figure><img src="image_6.png" /></figure>

**Figure 3.** (A) Detailed sedimentary section of the outcrop of Latour-sur-Sorgues located along the D7 road, where sample Lat20-1 was collected; (B) Photography of the base of the section; (C) Photography of the cineritic bed providing the Lat20-1 sample; (D) Detailed sedimentary section of the outcrop of Le Mas de Jean located near the Camarès village along the D999 road, where sample Cam20-3 was collected; (E) Photograph of the outcrop with the stratigraphic location of the cinerite providing the Cam20-3 sample.

laser beam was c. 6 J/cm². A typical analysis was 80 s, including a 40 s background measurement and a 40 s ablation period. The correction for the interferences and mass bias followed the procedure outlined in previous reports [e.g. Bruguier et al., 2020]. The correction for the isobaric interference of Yb and Lu on ¹⁷⁶Hf was made following a method detailed in Fisher et al. [2011]. For Yb, the interference-free ¹⁷¹Yb was corrected for mass bias effects using an exponential

law and ¹⁷³Yb/¹⁷¹Yb = 1.130172 [Segal et al., 2003]. The mass bias-corrected ¹⁷¹Yb was monitored during the run and the magnitude of the ¹⁷⁶Yb interference on ¹⁷⁶Hf was calculated using ¹⁷⁶Yb/¹⁷¹Yb = 0.897145 [Segal et al., 2003]. For Lu, the interference-free ¹⁷⁵Lu was corrected for mass bias effects assuming βLu = βYb and using an exponential law. The mass bias-corrected ¹⁷⁵Lu was monitored during the run and the magnitude of the ¹⁷⁶Lu interference on ¹⁷⁶Hf
was calculated using ¹⁷⁶Lu/¹⁷⁵Lu = 0.02655 [Vervoort et al., 2004]. Interference-corrected ¹⁷⁶Hf/¹⁷⁷Hf were corrected for mass bias using an exponential law and ¹⁷⁹Hf/¹⁷⁷Hf = 0.7325 [Patchett et al., 1981]. Initial Hf isotope ratios and εHf values were calculated using the decay constant for ¹⁷⁶Lu of 1.867 × 10⁻¹¹ yr⁻¹ [Söderlund et al., 2004] and the CHUR values of 0.282785 and 0.0336 for ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf [Bouvier et al., 2008]. The accuracy and long-term reproducibility of the measurements were gauged by analysing three zircon reference standards [91,500, GJ1 and Plešovice with reference values taken from Blichert-Toft, 2008, Morel et al., 2008, Sláma et al., 2008, respectively] and all values were found to be in agreement with the reference values: 91,500 (¹⁷⁶Hf/¹⁷⁷Hf = 0.282298 ± 24, *n* = 30), Plešovice (¹⁷⁶Hf/¹⁷⁷Hf = 0.282473 ± 19, *n* = 13) and GJ1 (¹⁷⁶Hf/¹⁷⁷Hf = 0.282007 ± 32, *n* = 15) (all errors at 2 s.d. level).

# 4. Results

## 4.1. Petrology

Sample Per21-4, from the *Cinéritique* Fm (Figures 1A and 2), is a tuffite characterized by a centimetric-scale layering related to grain size alternation. In thin section, it is dominated by a very fine-grained matrix including floating flat parallel unsorted silt to rare coarse sand-size clasts responsible for the laminated fabric. The clast fraction is mainly composed of large elongated quartz grains including micro bubble-like cavities compatible with ancient glass shards, K-feldspars and abundant muscovite flakes (Figure 4A). Secondary carbonates can also be observed.

Also from the *Cinéritique* Fm (Figure 2), the matrix of sample Can21-5 was collected in a 20 cm thick light-beige layer containing numerous plant remnants. It is extremely fine-grained and contains a dispersed very-fine silt-size fraction, including resorbed quartz crystal clasts (Figure 4B) and K-feldspar clasts, consistent with a pyroclastic origin, as well as terrigenous muscovite flakes.

Sample Dev21-1 corresponds to a 5 cm thick very fine-grained light-grey layer belonging to the *Saint-Rome-de-Tarn* Fm (Figures 1A and 2). In thin section (Figure 4C), it is composed of an extremely fine-grained matrix-supported silt- to sand-size unsorted clastic fraction, including larger angular to subangular quartz fragments of possible altered glass shards,

K-feldspar clasts, as well as rare tiny terrigenous muscovite flakes, indicating the lack of hydrodynamical sorting (Figure 4C).

Sample Lat20-1 belongs to the *Dourdou* Fm (F3 Unit, Figures 1A, 2 and 3A). It is a pinkish to light-beige fine-grained tuff with an eutaxitic-like fabric marked by black flame-like structures, suggesting a pyroclastic flow deposit, rather than a fall deposit. In thin section (Figure 4D), the sample is highly recrystallized and displays an anhedral quartz-rich mosaic invading the residual cryptocrystalline matrix with large dispersed carbonate patches and remaining K-feldspar and plagioclase clasts. A striking feature is the strong late carbonatation and recrystallization that this tuff suffered, possibly during the diagenetic processes.

Sample Gal21-7 belongs to the *Saint-Pierre* Fm (F5 Fm, Figure 2). In thin section, it is characterized by a dominant cryptocrystalline matrix, probably at least partly vitreous, containing millimetre thick bands enriched with fine sand-size to silt-size quartz and K-feldspar debris, together with smaller biotite and detrital muscovite grains. As shown on Figure 4E, some of the quartz clasts show resorption gulfs, which is consistent with their interpretation as rhyolitic quartz that underwent fast decompression in volcanic conduits. Lithic fragments, among which angular polycrystalline quartz clasts, can also be found.

Samples Cam20-3 and Gal21-6 were sampled in the same 20 cm thick tuffitic layer of the *Saint-Pierre* Fm (F5, Figures 1A, 2 and 3B). Sample Cam20-3 is a light-grey tuffite with a centimetric scale layering. In thin section, it contains a fair number of lithic fragments and angular quartz clasts (Figure 4F) compatible with a pyroclastic origin, as well as K-feldspar and plagioclase clasts together with chlorite after biotite, muscovite, accessory tourmaline and opaque minerals into a cryptocrystalline matrix. Muscovite and tourmaline indicate a mixing of the pyroclastic content with a terrigenous input. As shown on Figure 4F, this sample also reveals the presence of secondary carbonate minerals. Sample Gal21-6 is a very fine-grained tuffite similar to sample Gal21-7, but with a much smaller grain size with a clear planar microfabric marked by the alignment of the quartz clasts and muscovite microdebris (Figure 4G). Some late goethite veins can be found in this sample (Figure 4G).
<figure><img src="image_7.png" /></figure>

**Figure 4.** Optical cross-polarized light microphotographs for the samples from the Saint-Affrique Basin. (A) Sample Per21-4; note the layering marked by the variations of granulometry. (B) Sample Can21-5; white arrows show resorbed quartz grains. (C) Sample Dev21-1. (D) Sample Lat20-1; note the strong carbonatation. (E) Sample Gal21-7; white arrows show a decompression-related resorbed quartz crystal. (F) Sample Cam20-3; note the presence of angulous quartz fragments. (G) Sample Gal21-6: note the presence of late goethite veins. Mineral abbreviations: Cb carbonate minerals; Gth goethite; Kfs K-feldspar; Ms muscovite; Pl plagioclase; Qz; quartz.
## 4.2. Whole-Rock geochemistry

Eight samples have been selected for major and trace element analyses (Supplementary Table 1). As illustrated in the previous section, all samples consist of ash tuffs and tuffites in which the ter- rigenous input is variable but obviously difficult to accurately quantify for each of the samples, a fea- ture that may disturb their volcanic geochemical sig- natures. In addition, some samples underwent late (i.e. post-deposition) diagenetic modifications, such as carbonatation, sometimes fairly important, as for sample Lat20-1 that contains more than 10 wt% CaO. This increase in CaO (and Sr) content consequently dilutes the other elements. These characteristics can limit the usefulness of geochemical diagrams, which therefore need to be used with caution, especially those based on major element concentrations. In the conventional total alkali-silica (TAS) diagram (Figure 5A), sample Lat20-1 plots in the field of basaltic trachy-andesite, while all the others plot in the rhyolite field.

Whole-rock immobile trace and REE elements are often used in the case of volcanic ashes studies as they are usually less sensitive to post deposition pro- cesses (such as diagenesis). In the Zr/TiO₂ versus Nb/Y diagram of Winchester and Floyd [1977], two distinct groups appear (Figure 5B). A first group, comprising Per21-4, Per21-3, Can21-5 and Dev21-1 plots in the field of trachyte/trachy-andesite whereas a second group (Gal21-6, Gal21-7, Cam20-3 and Lat20-1) plots in the rhyodacite/dacite field. It should be noticed that yttrium can be lost during the alter- ation of volcanic rocks [Hill et al., 2000] and there- fore the alkalinity attributed to the samples should be considered as a maximum estimate.

In a Primitive Mantle normalized multi-elements diagram [McDonough and Sun, 1995, Figure 6a], all the studied samples yield trends that are comparable with the Upper Continental Crust [UCC, Taylor and McLennan, 1985]. Overall, the samples show a strong enrichment in LREE (Figure 5B), and, with the excep- tion of sample Dev21-1, a negative Eu/Eu* anomaly, often found in felsic volcanic rocks [Wray, 1999]. In further detail, it is noteworthy that samples belong- ing to the two groups defined above present two dis- tinct trends overall (Supplementary Table 1 and Fig- ure 6B). Samples from the first group (Per21-4, Per21- 3, Can21-5 and Dev21-1) display a low REE content

<figure><img src="image_8.png" /></figure>

**Figure 5.** Geochemical classification diagrams for the samples collected within the Saint-Affrique Basin. (A) Total alkali-silica diagram [TAS, Le Maitre et al., 1989]; (B) Zr/TiO₂ versus Nb/Y diagram [Winchester and Floyd, 1977]. The yellow ellipse corresponds to the first group of samples; the orange one to the second group of samples.

(22 to 98 ppm), a less pronounced negative Eu/Eu* anomaly (0.65 to 0.98) and strong HREE depletions. In contrast, samples from the second group (Gal21-6, Gal21-7, Cam20-3 and Lat20-1) present a higher REE content (143 to 158 ppm), a more pronounced Eu/Eu* anomaly (between 0.17 to 0.69) and flat HREE patterns. If the REE contents are normalized to that
of the Cody shale [Figure 6C; Jarvis and Jarvis, 1985], most samples show a negative Eu anomaly, are depleted in LREE and are either flat (second group) or depleted (first group) in HREE, which are all typical features for volcanic ashes [Wray and Wood, 1998, Ducassou et al., 2019]. The fact that trend for Dev21-1 is fairly flat (Figure 6C) is often characteristic of a non-negligible detrital input [Wray and Wood, 1998, Ducassou et al., 2019].

In the tectonic discrimination diagram (Figure 7A) presented by Schandl and Gorton [2002], the first group of previously defined (see above) samples plots in the “Within Plate Volcanic Zones” field while the second group plots in the “Active Continental Margin” field. In the tectonic diagram of Wood [1980], based on Hf/3–Th–Ta (Figure 7B), the second group of samples is characterized by a calc-alkaline chemistry, while the first group displays a higher Ta content relative to Th and Hf.

### 4.3. U–Pb dating of zircon

Unfortunately, it was not possible to find any minerals suitable for U–Pb dating in samples Per21-3 and Dev21-1.

Very few zircon grains were present in sample Per21-4 (Cinéritique Fm). Nine grains with various sizes were analyzed (Supplementary Table 3). They yielded variable U and Pb contents and fairly consistent Th/U ratios. With the exception of three discordant analyses (Figure 8A), three age groups can be defined: a first one around 2.0 Ga, a second one around 1.0 Ga and the last one around 0.6 Ga.

Again, very few zircon grains were found in sample Can21-5 (Cinéritique Fm). Thirteen grains with variable sizes were analyzed. They yield very high U (up to 3220 ppm) and Pb (up to 7648 ppm) contents (Supplementary Table 3) together with very high common Pb contents (f206c up to 39%). Their Th/U ratios are also highly variable (from 0.03 up to 0.53). Reported in a Concordia diagram (Figure 8B) they plot mostly in a discordant position with the exception of two grains (Zr12 and 13) which are concordant around 2.0 Ga.

Twenty-four analyses out of sixteen different zircon grains were acquired for sample Lat20-1 (Figure 3A; Supplementary Table 3). All zircon grains were prismatic, ranging in size between 70 and 200 µm and with simple concentric zoning (Figure 9).

<figure><img src="image_9.png" /></figure>

**Figure 6.** (A) Multi-element diagram normalized to Primitive Mantle [Sun and McDonough, 1989]. UCC = upper continental crust [Taylor and McLennan, 1985]; (B,C) Rare earth element plots for the tuff samples collected within the Saint-Affrique Basin. REE data are normalized (B) to the CI-chondrite [McDonough and Sun, 1995] and (C) to the Cody Shale [Jarvis and Jarvis, 1985]. Yellow lines correspond to the first group of samples; the orange line to the second group of samples.
<figure><img src="image_10.png" /></figure>

**Figure 7.** (A) Th/Yb versus Ta/Yb tectonic discrimination diagrams [after Schandl and Goraton, 2002]. ACM = active continental margin, MORB = mid-ocean ridge basalts, WPB = within plate basalts, WPVZ = within plate volcanic zones; (B) Hf/3–Th–Ta triangular diagram [after Wood, 1980]. WPA = within plate alkaline basalts and differentiates, WPT = within plate tholeiitic basalts and differentiates. The yellow ellipse corresponds to the first group of samples; the orange one to the second group of samples.

They yield variable U (236 to 9651 ppm) and Pb (50 to 908 ppm) contents with Th/U ratios consistent with a magmatic origin [0.1 to 0.26; see Witt et al., 2017 and references therein]. Eighteen analyses plot in a concordant position (Figure 8C) and yield a concordia date of 280.3 ± 2.6 Ma (MSWD = 1.2). Two analyses (Zr 1.1 and Zr 11.2, Supplementary Figure 1) were acquired on grains that contain a non-negligible common Pb content (f206c of 1.56 and 6.38% respectively) and that probably belong to the same age group. Two analyses acquired on the same grain (Zr2) plot in a concordant position with apparent <sup>206</sup>Pb/<sup>238</sup>U ages of around 320 Ma (Supplementary Figure 1), whereas a last analysis (Zr12, containing a non-negligible amount of common Pb; f206c% = 6.4), is highly discordant (52%; Supplementary Figure 1).

Thirty-one analyses (Supplementary Table 3) out of thirty-one different zircon crystals were acquired for sample Gal21-7 (Figures 1A and 2). Most crystals are rather elongated (up to 250 µm) and most of them display simple concentric magmatic zoning (Figure 9). Here again, the U (108 to 1886 ppm) and Pb (35 to 875 ppm) contents are highly variable. All the Th/U ratios, however, are compatible with a magmatic origin (0.13 to 0.48). A group of twenty-nine analyses (Figure 8D) define a concordia date of 278.4 ± 2.3 Ma (MSWD = 1.6). One grain (Zr5), characterized by a non-negligible common Pb content (f206c = 1.1%), plots to the right of the main batch of analyses and probably belongs to the same age group. The last analysis (Zr15) yields a concordant date around 470 Ma.

Thirty-two analyses (Supplementary Table 3) were acquired out of thirty-one zircon grains for sample Cam20-3 (Figure 3B). Zircon grains are generally prismatic with sizes ranging from 50 to 200 µm and display a magmatic zoning when imaged by cathode luminescence (Figure 9). They present variable U (212 to 1757 ppm) and Pb (41 to 1162 ppm) contents, with Th/U ratios ranging from “typical” metamorphic (0.03 to 0.09) to magmatic (0.16 to 0.62). Four age groups can be identified (Figure 8E and Supplementary Figure 1). The oldest zircon (Zr 27) yields an apparent <sup>207</sup>Pb/<sup>206</sup>Pb age of ca. 2646 Ma (Supplementary Figure 1). One grain (Zr 25) yields a concordant date of ca. 940 Ma (Supplementary Figure 1). A third group of six zircon grains (Zr 1, 7, 8, 11, 17 and 20) returns apparent ages around 450–460 Ma
<figure><img src="image_11.png" /></figure>

**Figure 8.** (A) Wetherill concordia diagram for sample Per21-4; (B) Wetherill concordia diagram for sample Can21-5; (C) Wetherill concordia diagram for sample Lat20-1; (D) Wetherill concordia diagram for sample Gal21-7; (E) Tera-Wasserburg concordia diagram for sample Cam20-3; (F) Wetherill concordia diagram for sample Gal21-6. MSWD = Mean Square Weighted Deviation. *n* = number of analyses used to calculate the U–Pb dates. All MSWD are calculated for concordance + equivalence. Grey ellipses were not used for age calculation.
<figure><img src="image_12.png" /><figcaption>**Figure 9.** Cathode-luminescence imaging of some of the analyzed zircon grains. The white bar represents 200 µm.</figcaption></figure>

(Supplementary Figure 1), yielding the lowest (meta-
morphic?) Th/U ratios. The youngest group com-
posed of twenty-three different zircon grains (Fig-
ure 8E) yields a lower intercept date of 278.7 ± 2.4 Ma
(MSWD = 0.95).

Ten zircon grains were found in sample Gal21-
6 (Figure 1A and 2; Supplementary Table 3). All the
grains were prismatic with concentric magmatic zon-
ing (not shown). They are rich in U (up to 2558 ppm)
and Pb (up to 1393 ppm), with fairly consistent Th/U
ratios characteristics of magmatic sources (0.21 to
0.66). With the exception of one grain (Zr1) with a
concordant date around 340 Ma (Figure 8F), the re-
maining five concordant analyses yield a concordia
date of 266 ± 7 Ma (MSWD = 0.7, n = 5), whereas
the oldest concordant zircon from this group has a
²⁰⁶Pb/²³⁸U apparent age of 277 ± 15 Ma. The four

other analyses are discordant with variable amount
of common Pb (f206c up to 2.78%).

4.4. Zircon Hf isotope composition

Three samples were selected for zircon Hf isotope
measurements (Lat20-1, Cam20-3 and Gal21-7, Sup-
plementary Table 3).

For each sample, ten zircon grains (eleven for
sample Cam20-3), previously analysed for U–Pb,
were targeted for Hf isotopes. Hf analyses were done
as close as possible to the U–Pb ablation pits. The
results are reported in Figure 10.

For sample Lat20-1, the analyzed zircon grains
display very low ¹⁷⁶Lu/¹⁷⁷Hf (0.0009 ± 1) and ini-
tial (calculated at 280 Ma) ¹⁷⁶Hf/¹⁷⁷Hf ranging from
0.282600 to 0.282630, with nearly chondritic εHf(t)
<figure><img src="image_13.png" /></figure>

**Figure 10.** εHf(t) versus $^{206}$Pb/$^{238}$U age of some selected zircon grains from samples Lat201, Gal21-7 and Cam20-3. Depleted Mantle and New Crust evolutions after Dhuime et al. [2011].

values (Figure 10) ranging from 0.8 to -0.3 (mean 0.1±0.2). T<sub>DM</sub> model ages are broadly similar to those measured from the two other samples and display a mean value of 1269 ± 13 Ma.

Zircons from sample Gal21-7 (Supplementary Table 3) yield low $^{176}$Lu/$^{177}$Hf (0.0019 ± 3) and a mean $^{176}$Hf/$^{177}$Hfi of 0.282576 ± 12 (calculated at 278 Ma). εHf(t) are slightly sub-chondritic (Figure 10), ranging from -0.3 to -2.1 (mean = -1.2±0.4). The T<sub>DM</sub> mean value is 1352 ± 27 Ma.

For sample Cam20-3, the analyzed zircon grains present low $^{176}$Lu/$^{177}$Hf ratios (0.0024 ± 6) and homogeneous $^{176}$Hf/$^{177}$Hfᵢ ratios (0.282507 ± 15) calculated at 279 Ma. The corresponding initial εHf are subchondritic, with values ranging from -4.8 to -2.4, and a mean εHf(t) of -3.6 ± 0.5 (Figure 10). Their T<sub>DM</sub> yield a mean value of 1507 ± 33 Ma. One xenocryst, dated at 454 Ma, was also analyzed and provides a distinct, slightly supra-chondritic εHf(t) of 1.0.

**5. Discussion**

**5.1. Age and nature of the volcanism in the Saint-Affrique Basin**

Two groups of samples can be distinguished among the studied samples.

The first group of volcaniclastic beds (Can21-5, Per21-3, Per21-4 and Dev21-1) belonging to the

*Cinéritique* Fm (Figures 1A and 2), with a proposed Stephanie age (i.e. late Carboniferous), contains clasts (quartz and K-feldspars) that are consistent with a volcanic origin, and a fairly large detrital component. In the Zr/TiO₂ versus Nb/Y diagram (Figure 5B, yellow ellipse) they plot in the field of trachyte/trachy-andesite, and they yield trace-element contents compatible with a felsic composition. They all have a calc-alkaline chemistry and exhibit “Within Plate Volcanic Zone”-like characteristics in the volcano-tectonic diagram of Wood [1980]; these features are compatible with a late to post-collisional volcanism. Sample Dev21-1, belonging to the *Saint-Rome-de-Tarn* Fm (i.e. proposed lower Cisuralian in age), exhibits a flat REE pattern when normalized to shale (Figure 6C), but also displays a “Within Plate Volcanic Zone” geochemical composition (Figure 7B). Unfortunately, it was not possible to date these samples because of the absence of volcanic zircon grains. The detrital zircon grains found in two of these samples demonstrates however the existence of a complex basement underneath and/or aside the Saint-Affrique Basin with ages ranging from the Paleo- to the Neo-Proterozoic (Figure 8A and B).

The second group of samples belong either to the *Dourdou* Fm (Lat20-1) or to the *Saint-Pierre* Fm (Gal21-6 and 21-7 and Cam20-3). These formations were so far attributed to the Sakmarian and Artinskian, respectively [Gand, 1993]. Although these samples have a fair amount of muscovite indicating a terrigenous contribution, and can be sometimes affected by a strong carbonation (Lat20-1), they all contain clasts of feldspars and quartz demonstrating the presence of a rather important volcanic content. These samples can be classified as dacites/rhyodacites, have a strong negative Eu/Eu* anomaly, a significant LREE enrichment and a fairly flat HREE spectrum; these features are all in a good agreement with their felsic nature, and can be defined as calc-alkaline rocks. All of these features are in a good agreement with a post-orogenic deposition setting. All of these samples provided volcanic zircon grains. The stratigraphically oldest sample from this group (Lat20-1) records a volcanic episode during the Kungurian. The obtained date of 280 ± 2.6 Ma for this sample is interpreted as the age of the deposition of the bed, assuming that the dated zircon grains were not reworked after the volcanic eruption [e.g Rossignol et al., 2019]. Applying the same assumption
tion, the samples at the top of the sedimentary suc-
cession were then deposited at 278.4 ± 2.3 Ma (Gal21-
7) and 278.7 ± 2.4 Ma (Cam20-3) respectively, i.e. also
during the Kungurian. The date of 266 ± 7 Ma found
for sample Gal21-6 may be related to slight Pb losses
enhanced by the zircon's high U content (>1500 ppm)
and has to be considered as a minimum age of de-
position, as this sample belongs to the same ash bed
as Cam20-3. Therefore, we propose that the date of
277 ± 15 Ma found for the oldest concordant zircon
from this sample is the best estimate for the deposi-
tion age of this volcanic ash bed.

The mean εHf(t) calculated for the zircon grains
from the three dated volcanic ash beds Lat20-1,
Gal21-7 and Cam20-3 provides chondritic to sub-
chondritic values of 0.1 ± 0.2; -1.2 ± 0.4 and -3.6 ±
0.5, respectively. The corresponding Hf model ages
of 1.3–1.5 Ga represent an average of the sources in-
volved in magma genesis or assimilated during the
travel path upward to the surface of the magmas. In
any case, this indicates a significant degree of recy-
cling of Proterozoic or older material, which is also
consistent with the occurrence (in the three volcanic
levels) of zircon grains with apparent ages ranging
from the Archean to the Neoproterozoic. It is how-
ever difficult to evaluate whether the Hf isotope sig-
nature of the analysed zircon grains reflects either a
source signature or an open system evolution, such
as assimilation of an older crust by juvenile, mantle-
derived, magmas. Clarification of this point is key
to the interpretation of the geochemical data, as it
could indicate crustal reworking processes, or new
crustal growth by addition of material issued from
the mantle during the post-collisional Variscan pe-
riod. Although sampled at different stratigraphical
levels, samples Gal21-7 and Cam20-3 belong to the
same F4/F5 formation and display remarkably simi-
lar U–Pb ages (278.4 and 278.7 Ma, respectively) and
trace- and major-element chemistry. Both samples
display subchondritic, but substantially different, Hf
isotope signatures (εHft = -1.2 ± 0.4 and -3.6 ± 0.5
respectively). This difference indicates that magma
genesis involved partial melting of a composition-
ally heterogeneous crustal section. The mean age for
this crustal section is estimated between 1.3–1.6 Ga,
assuming felsic precursors (with a ¹⁷⁶Lu/¹⁷⁷Hf ra-
tio of 0.015), or 1.6–2.0 Ga, assuming mafic precur-
sors (with a ¹⁷⁶Lu/¹⁷⁷Hf ratio of 0.021). Sample Lat20-
1 yields a chondritic value that can be explained

in two ways. The first is a mixing (at c. 280 Ma)
between a suprachondritic mantle-derived magma
and subchondritic Paleoproterozoic or older crustal
sources in proportions that coincidentally produced
a εHf chondritic value. An alternative possibility is a
remelting (at c. 280 Ma) of a 1.3 Ga old or 1.6 Ga old
Paleoproterozoic source, which would have evolved
with a calculated ¹⁷⁶Lu/¹⁷⁷Hf ratio of, respectively,
0.015 assuming a felsic precursor, or a ratio of 0.021,
typical of the time-integrated evolution of a mafic
(lower) crust (Figure 9). It is worth noting that the
vertical scattering observed for all analyses in Fig-
ure 9 (from 0.8 to -4.8) may reflect such a mixing
trend between mantle- and crustally-derived compo-
nents. Whichever hypothesis is retained, it requires
a heat source that can be found in mantle-derived
magmas present at the base of the continental crust,
as mentioned in many previously published stud-
ies [e.g. Breitkreuz and Kennedy, 1999]. Therefore,
although our results indicate intra-crustal remelting
and recycling of an old Paleoproterozoic to Archean
basement, the addition of juvenile magmas trigger-
ing partial melting of the lower crust or mixed with
crustally-derived magmas cannot be ruled out.

Although this needs further investigation, the Hf
analyses tend to indicate an increase of crustal input
in the production of the magmas, with εHf chondritic
values for the oldest sample (Lat20-1) becoming in-
creasingly negative towards the top of the basin. In-
terestingly, there is also a correlation between εHf(t)
and Eu/Eu*, which may indicate that the magmas
producing the stratigraphically most elevated vol-
canic ash beds Gal21-7 and Cam20-3 underwent
fractional crystallization processes involving plagio-
clase, more pronounced than the other samples, and
that they possibly had more time to interact with, and
assimilate crustal material.

The first group of samples consists of trachy-
andesitic lavas, which are characterized by a greater
HREE depletion and smaller Eu/Eu* anomalies than
the rhyodacitic samples of group 2 (including the
dated samples Lat20-1, Gal21-7 and Cam21-3). The
greater HREE depletion of group 1 samples may in-
dicate that the magmas were generated at a greater
depth than samples from group 2. Lastly, the identi-
fication of Neoproterozoic to Archean zircon grains
in the volcanic ashes substantiates the occurrence of
a complex and old basement both underneath and
alongside the basin.
The different geochemical signatures of the two
groups mark a significant change in the petrological
processes producing the magmas, especially an evo-
lution of the main magmatic sources involved, as the
geodynamical conditions evolved from a late- to a
post-orogenic setting with time. The “Within Plate”-
versus “Active Margin”-like signatures, coupled with
our Hf data interpretations in the younger group, re-
flect that the crustal contribution increased as time
proceeded. In any late- to post-orogenic setting, this
has to be controlled by substantial modifications of
the geothermal gradient, and a significant increase of
the heat flux, which can hardly be conceived without
a simultaneous increase of the production of mantle
melts triggering crustal melting. Because the produc-
tion of mantle magmas at high rates may induce sig-
nificant crustal melting, it is likely that from the base
to the top of the sedimentary succession, the Saint-
Affrique pyroclastic layers recorded the transition
from a late-orogenic extension to a post-orogenic ex-
tension at much higher rates as the basin enlarged.

Our new constraints on the age and nature of the
volcanism recorded in the sedimentary succession
also questions the location of the emission centres of
the volcanic ashes, in the absence of known volcanic
edifices near the Saint-Affrique Basin, feature which
is a recurrent issue in Western Europe Carbonifer-
ous and Permian continental basins [e.g. Koniger
and Stollhofen, 2001, Pellenard et al., 2017]. Explos-
sive subaerial calc-alkaline volcanic activity with Per-
mian ages is known in the Central Pyrenees (about
200 km from the Saint-Affrique Basin), where it is
associated with high-intensity explosive magmatic
eruptions of rhyodacitic to rhyolitic lavas and as-
sociated widespread pyroclastic flows and overriding
ash clouds [Marti, 1996, Pereira et al., 2014], and
transported several hundreds of kilometres under the
dominant wind. On the other side, Permian subaerial
calc-alkaline volcanic episodes, including andesitic,
dacitic and rhyolitic ignimbrite lavas are known in
northwestern Corsica and Sardinia [Cabanis et al.,
1990, Timmerman, 2004, and references therein], and
in the Esterel Massif [Zheng et al., 1992, Nmila, 1995],
and could also represent good candidates for the vol-
canic ash input. Nonetheless, the possible pyroclas-
tic flow nature of the sampled layer from the *Dour-
dou* Fm (sample Lat20-1) suggests that edifices that
were more proximal to the basin still have to be iden-
tified.

5.2. *Implications for the deposition of the Saint-Affrique Basin*

The new radiochronological ages presented in this
study refine the chronological setting of the Saint-
Affrique Basin (Figure 11), the sedimentary succes-
sions of which were considered as deposited from
the early to the middle Permian [Rolando et al.,
1988, Figures 2 and 11], and then during the early
Permian only [Gand, 1993, Figures 2 and 11]. Our
new ages encompass the Kungurian Stage (upper-
most Cisuralian, Figure 11). The oldest age (Lat20-1,
280.3 ± 2.6 Ma), i.e. in the lower part of the *Dourdou*
Fm, is, within errors, very close to the lower limit of
the Kungurian, given that the Artinskian–Kungurian
boundary is set at 283.5 ± 0.6 Ma [Cohen et al., 2013,
updated; Figure 11]. No age could be determined
at the base of the sedimentary succession, but it is
likely that Asselian to Sakmarian (lower Cisuralian)
deposits are preserved.

The top of the sedimentary succession of the Saint-Affrique Basin is not dated, but considering the youngest age found in this study (Cam20-3, 278.7 ± 2.4 Ma), located less than 100 m below the youngest preserved deposits, as well as the absence of significant erosional surfaces or discontinuity in the sedimentation above this volcanic level, it is highly probable that the top of the sedimentary succession was also deposited during the Kungurian, the upper limit of which is set at 273.01 ± 0.14 Ma [Cohen et al., 2013, updated].

The maximum duration of the sedimentary depo-
sition between the stratigraphically lowest and high-
est dated volcanic ash beds levels, i.e. Lat20-1 (280.3±
2.6 Ma) and Cam20-3 (278.7±2.4 Ma), is 6.6 Ma. Dur-
ing this timeframe, the sediment thickness estimated
by coupling borehole data and spatial correlations
[based on the 1/50,000 geological map of Guérangé-
Lozes et al., 1995] is about 1100 to 1500 m. Con-
sidering this maximum duration, the sedimentation
rates (compacted sedimentary succession), appear to
be low, ranging from 0.160 mm/yr to 0.230 mm/yr.
It is worth noting the mean sedimentation rate es-
timated for other late-Carboniferous–early Permian
basins presenting a similar geological setting: in the
northeastern Massif Central Autun Basin [Mercuzot,
2020], sedimentation rates based on precise radio-
metric ages were estimated at 0.45 mm/yr, taking
the compacted sedimentary succession into account,
<figure><img src="image_14.png" /><figcaption>**Figure 11.** Refined chronostratigraphic setting for the Saint-Affrique Basin sedimentary succession based on the new ages obtained in this study.</figcaption></figure>
and at 1.1 mm/yr when estimating a decompacted
succession thickness. In the Saar-Nahe Basin (west-
ern Germany) the sedimentation rate is also higher,
i.e. 0.3 mm/yr (compacted sedimentary succession).
Some modern lacustrine systems, e.g. the Lake Titi-
caca (Altiplano, late-orogenic setting) and the North-
American Great Lakes (Superior, Erie and Huron,
glacial lakes) exhibit sedimentation rates between 0.1
and 7.4 mm/yr [Kemp and Harper, 1976, Kemp et al.,
1977, 1978, Durham and Joshi, 1980, Lojka et al.,
2009] that are consistent with our calculated rates. It
also demonstrates that the sedimentation rates cal-
culated in this study might be underestimated be-
cause of the lack of precise petrophysical and burial
condition data. Consequently, the discontinuity be-
tween F3 and F4 fms (Figures 2 and 11) should not
represent a substantial hiatus.

These new chronostratigraphical data clearly in-
dicate the lack of the Middle (Guadalupian) and Up-
per (Lopingian) Permian deposits in the present-day
sedimentary succession. Moreover, if we consider
a middle-Anisian age for the basal Triassic sand-
stone deposits, as in the Lodève Basin [Lopez and
Mader, 1985], the angular unconformity between the
youngest Permian and the Triassic deposits repre-
sents a major hiatus of about 23 Myrs, during which
the residual Variscan reliefs and the upper part of
the Permian sedimentary succession of the Lodève
and Saint-Affrique basins were eroded and pened-
planed. In the Lodève Basin, the overall thickness of
the eroded sedimentary succession between the top
of the Permian deposits and the middle-Anisian un-
conformity is estimated at 1000 to 1500 m [Lopez
et al., 2008, Figure 2].

In Europe, increasing correlations are tenta-
tively made between Carboniferous–Permian basins,
based on both radiometric ages and biostratigraphic
data [e.g. Schneider and Scholze, 2018, Pellenard
et al., 2017, Ducassou et al., 2019, Schneider et al.,
2020]. The Lodève Basin is one of the most used
basin to establish correlations in Western Europe,
mostly because of its almost complete sedimentary
succession from the late Carboniferous to the late
Guadalupian [i.e. late middle Permian; Schneider
et al., 2020]. However, the present re-evaluation of
the ages of the Saint-Affrique Basin, located in the
direct vicinity and formed in the same geodynamic
setting, calls for new radiometric age constraints in
the Lodève Basin.

**6. Conclusion**

Our study focuses on several volcanic ash beds en-
countered within the sedimentary successions of the
Saint-Affrique Basin. Two different groups of volcani-
clastic rocks are defined: (i) the first group, located
at the base of the sedimentary succession and con-
sidered as Carboniferous in age, could not be dated
due to a lack of volcanic zircon grains in the studied
samples, and hence requires further analyses to re-
fine the age of these ante-Artinskian deposits. These
volcaniclastic beds include a significant terrigenous
input and yield mostly trachytic to trachy-andesitic
felsic compositions. They can be defined as calc-
alkaline rocks that reproduce, in a late orogenic set-
ting, a “Within Plate Volcanic Zone” signature; (ii) the
second group consists of dacitic to rhyodacitic ash
beds with a calc-alkaline affinity, and a geochemi-
cal signature resembling to that of “Active Continen-
tal Margin” rocks. Their deposition ages are younger
than previously estimated, i.e. Kungurian instead of
Artinskian.

The geochemical analyses (elemental and iso-
topic) of samples from both groups attest to a crustal
recycling of older material (Proterozoic and older).
This crustal component becomes more prominent
toward the top of the Permian successions, pos-
sibly due to an increase of a mantle contribution
triggering crustal melting. Overall, their petrological
and geochemical features are compatible with vol-
canic activities that took place during late- to post-
collisional regional extension. These types of vol-
canic centres are known during the late Carbonifer-
ous and Permian, both in the Central Pyrenees and in
the Provence–Corsica–Esterel domain. The presence
of these different volcaniclastic ash beds within the
basin demonstrates the persistence and recurrence
of the volcanic activity in the region during the Per-
mian. Lastly, although preliminary, these new ages al-
low to calculate rather low minimum sedimentation
rates (around 0.2 mm/yr) during the Kungurian in the
Saint-Affrique Basin.

The dating the volcanic ash bed from the outcrop
of Latour-sur-Sorgues (Lat20-1 sample, evidenced as
a key-bed in 70's mining reports in drilled wells lo-
cated along the southeastern border of the basin)
would constitute a new robust anchor point to pre-
cisely correlate the sedimentary successions at basin-
scale. Furthermore, several other volcanic ash beds,
which unfortunately were not recognized in the field
during this work, were also identified during the 70's
at the same stratigraphical position on the differ-
ent logs in the *Saint-Rome-de-Tarn* Fm (F2). Identify-
ing these discrete volcanic levels in the *Saint-Rome-
de-Tarn* Fm cropping out in the hanging wall of the
southern bordering fault would make it easier to date
the oldest Permian deposits of the basin.

While contributing to the refinement of the strati-
graphic setting of the Saint-Affrique Basin, this
study also demonstrates the need for additional pre-
cise dating in the neighbouring basins (e.g. Lodève
Basin), as well as in other late Paleozoic basins from
Western Europe related to the late Variscan dynam-
ics. This would provide some important contribu-
tions for the recalibration of the Western Europe
continental biostratigraphic scales. In addition, es-
tablishing correlations between these basins is para-
mount to better constrain the precise and accurate
timing of their filling history, the relationships be-
tween sedimentation and tectonic and geodynamic
processes, and to assess the various climate events
that prevailed during the Permian in the Eastern
Pangea intertropical domain.

Conflicts of interest

Authors have no conflict of interest to declare.

Acknowledgements

This study is dedicated to the memory of Jean-
Louis Paquette who greatly contributed, among other
things, to our understanding of the French Mas-
sif Central. Xavier Le Coz (Géosciences Rennes) is
thanked for the making of the thin sections, and
Cindy Maliverney (TSL) for sample preparations.
Jean Poujol is thanked for his hospitality during the
two sampling fieldtrips and Sara Mullin for correct-
ing the English content. Finally, the authors want to
thank J. Barbarand, V. Bosse and S. Duchêne for their
insightful comments on the previous version of this
manuscript.

**Supplementary data**

Supporting information for this article is available on
the journal's website under https://doi.org/10.5802/
crgeos.184 or from the author.

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