application of in situ zircon geochronology and accessory ... · o. bruguier et al. / chemical...

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Chemical Geology 201 (2003) 319–336

Application of in situ zircon geochronology and accessory

phase chemistry to constraining basin development

during post-collisional extension: a case study

from the French Massif Central

O. Bruguiera,*, J.F. Becq-Giraudonb, M. Champenoisc, E. Deloulec,J. Luddenc, D. Manginc

aService ICP-MS, cc 056, ISTEEM, Universite de Montpellier II, Place Eugene Bataillon, 34095 Montpellier Cedex 5, FrancebBRGM, 3 Avenue C. Guillemin, BP 6009, 45 060 Orleans, France

cCRPG, 15 rue Notre Dame des Pauvres, 54 000 Vandoeuvre-les-Nancy, France

Accepted 7 August 2003

Abstract

A series of five volcanic ash layers interbedded in Late Carboniferous sedimentary basins from the southern part of the French

Massif Central (FMC, France) have been studied by ion-microprobe analyses of zircons in order to constrain the age of basin

formation and sedimentation. Weighted mean 206Pb/238U ages for the five studied tuffs are indistinguishable at the 95%

confidence level and range from 295.5F 5.1 Ma (Graissessac) to 297.9F 5.1 Ma (Roujan–Neffies). These U–Pb ages support

the argument for intense magmatic activity in the southern part of the French Massif Central during the period 295–300 Ma.

Inherited zircons were identified in two out of the five dated tuff horizons and indicate a anatexis of basement source rocks with

ages of ca. 2400 (Jaujac basin), 1900 and 600Ma (Graissessac basin). The Proterozoic components suggest a Gondwanan affinity

for the deep-seated material. Chemical compositions of apatites and of one single zircon grain from the Roujan–Neffies bentonite

further indicate magma generation mainly from anatexis of the continental crust and a rhyolitic affiliation. Conversely, the same

minerals extracted from the Jaujac bentonite indicate involvement of a mantle component in the source of the magmas and a

trachytic affiliation. The 295–300 Ma volcanic episode in the French Massif Central is contemporaneous with volcanic events

identified in other parts of the Variscan Belt which suggests it was triggered by orogen-wide processes. Contemporaneous

eruption of trachytic and rhyolitic magmas may be related to replenishment of magma chambers at depth by influx of mantle-

derived magmas triggering the Late Carboniferous flare-up.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Stephanian basins; Ash-fall tuffs; Zircon; Apatite; French Massif Central; Variscan orogen

0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemgeo.2003.08.005

* Corresponding author.

E-mail address: [email protected] (O. Bruguier).

1. Introduction

Extensional tectonics is preferentially located

along orogenic belts with a thickened crust and is

an important feature of post-collisional orogenic

Fig. 1. (A) Geological sketch map of the French Massif Central in the framework of the European Variscides (after Matte, 1986). (B) Outline of the main Stephanian–Autunian basins

of the French Massif Central. Granites not shown.

O.Bruguier

etal./Chem

icalGeology201(2003)319–336

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O. Bruguier et al. / Chemical Geology 201 (2003) 319–336 321

stages (e.g. Ratschbacher et al., 1989). Implicit to

this is the creation of a pervasive series of continen-

tal basins accompanying extension in the upper crust.

Hence, the formation of sedimentary basins often

mirrors deeper processes and they can be used as

tectonic markers (e.g. Zoback et al., 1993) particu-

larly in cases where sedimentary infilling is tecton-

ically controlled (e.g. Bruguier et al., 1997). A key

issue is therefore to determine precisely the age of

basin formation and the relationships between basin

initiation and tectonic structures such as major fault

systems. These parameters potentially carry impor-

tant information that have implications for under-

standing the tectonic control on the sedimentary

record, and to punctuate the different stages of

extensional tectonics characterising the evolution of

mountain belts.

The French Massif Central (FMC) is one of the

most important exposures of the Internal Zone of

the Variscan Belt which extends along ca. 3000 km

from the Iberian Massif in the West to the Bohe-

mian Massif in the East (Fig. 1A). The Late

Carboniferous–Early Permian time interval is prob-

ably one of the most important period in the

evolution of the belt as it corresponds to the final

assembly and early evolution of the supercontinent

Pangea and also includes global climate changes

from the Late Carboniferous icehouse to the Perm-

ian greenhouse. In the whole belt, this period is

characterised by the development of numerous coal-

bearing intramontane basins containing volcano-sed-

imentary successions (e.g. Faure, 1995; Becq-Gir-

audon et al., 1996). These basins represent isolated

troughs closely associated with fault zones and

filled with coarse, clastic, nonmarine sediments

deposited unconformably on the metamorphic and

igneous basement. As these basins are widely dis-

tributed in the whole Variscan Belt, they can be

used to bracket the phases of extensional tectonics

affecting basement country rocks. Absolute dating

of volcanic tuffs preserved in these basins is thus

expected to help refine and understand crustal scale

processes governing this period of the evolution of

the Variscan Belt.

In this study, we report U–Th–Pb results on

zircons and trace element analyses of selected

apatite concentrates and single zircon grains from

bentonites interbedded within sediments in five Late

Carboniferous basins from the southern part of the

FMC. The aim of this paper is to provide time

constraints on this important volcanic and basin

forming event and to compare these ages with

those from other parts of the Variscan Belt. The

trace element analyses from constituent minerals of

some of the studied bentonites will provide insights

into the sources and origin of magmas erupted at

the end of the Carboniferous period.

2. Analytical techniques

2.1. SIMS analyses

Bentonite samples of ca. 15–25 kg were separated

from the enclosing sediments. They were subsequently

jaw-crushed and screened to < 500 Am. Zircon con-

centrates were extracted by Wifley table, heavy liquids

and magnetic separation following standard techniques

(e.g. Bosch et al., 1996). Zircons from the nonmagnetic

fraction were washed in 6 N HNO3 and hand-picked in

alcohol under a binocular microscope. Grains, together

with chips of standard zircon, were then mounted in

epoxy resin and polished to approximately half their

thickness to expose internal structure. SIMSU–Th–Pb

analyses were performed with a spot size of about 25–

40 Am on the CAMECA IMS 1270 ion microprobe at

the CRPG Nancy (France) following the technique

outlined by Deloule et al. (2001). Isotopic ratios were

measured with a primary O2 beam of 10–15 nA at a

mass resolution of ca. 5000, at which no significant

interferences on the Pb, U and Th isotopes were

detected. Oxygen flooding was used to enhance sensi-

tivity. Under these operating conditions, the sensitivity

for Pb isotopes ranged from 15 to 22 cps/ppm/nA of

primary beam. Pb/U ratios were normalised using

quadratic working curves, to values measured on the

G91500 standard zircon (Wiedenbeck et al., 1995).

Common Pb was corrected using 204Pb and a compo-

sition taken from the model of Stacey and Kramers

(1975). Because of the low abundance of radiogenic

lead in most of the zircon grains analysed, and as

radiogenic 207Pb is about 20 times less abundant than206Pb in Paleozoic zircons, the 207Pb/206Pb corrected

ratios can often give both inaccurate and nonprecise

ages. Thus, only the 206Pb/238U ages are discussed in

this paper. Weighted averages were calculated at the

Table 1

IMS 1270 U–Th–Pb results for zircons extracted from Carboniferous volcanics of the French Massif Central (France)

Grain U Th Pb Th/ 204Pb/ 208Pb/ 206Pb/ F 207Pb/ F Rho 207Pb/ F Apparent age (Ma)

area (ppm) (ppm) (ppm) U 206Pb 206Pb 238U (1rerror)

235U (1rerror)

206Pb (1rerror)

206Pb/238U

F 207Pb206Pb/

F

Bertholene basin (296.2F 7.2 Ma)

Ci5-1 596 609 23 1.02 0.00580 0.321 0.0462 0.0005 0.326 0.028 0.77 0.0513 0.0042 291 3 254 176

Ci5-2 449 112 17 0.25 0.00047 0.063 0.0448 0.0023 0.298 0.015 0.86 0.0482 0.0013 283 14 109 60

Ci5-3 484 130 20 0.27 0.00014 0.074 0.0473 0.0026 0.330 0.019 0.91 0.0507 0.0012 298 17 227 54

Ci5-4 760 319 32 0.42 0.00012 0.132 0.0484 0.0004 0.343 0.004 0.90 0.0513 0.0002 305 3 254 10

Ci5-5 251 173 10 0.69 0.00069 0.207 0.0442 0.0004 0.297 0.013 0.75 0.0488 0.0020 279 3 138 94

Ci5-6 489 183 21 0.37 0.00022 0.101 0.0498 0.0007 0.349 0.005 0.88 0.0508 0.0004 313 5 232 17

Ci5-7 418 159 18 0.38 0.00022 0.100 0.0500 0.0029 0.329 0.022 0.92 0.0477 0.0013 315 18 84 64

Ci5-8 265 181 10 0.68 0.00052 0.176 0.0454 0.0025 0.345 0.021 0.74 0.0551 0.0024 286 16 416 93

Ci5-9 278 195 11 0.70 0.00039 0.162 0.0467 0.0024 0.299 0.022 0.87 0.0464 0.0017 294 15 18 86

Ci5-10 351 265 15 0.76 0.00115 0.237 0.0482 0.0007 0.328 0.008 0.65 0.0494 0.0009 303 5 167 43

Ci5-11 745 252 32 0.34 0.00010 0.096 0.0500 0.0008 0.351 0.006 0.95 0.0509 0.0028 315 5 236 122

Ci5-12 736 317 31 0.43 0.00009 0.133 0.0482 0.0011 0.343 0.008 0.97 0.0516 0.0003 303 7 268 12

Ci5-13 475 162 20 0.34 0.00068 0.099 0.0480 0.0012 0.336 0.009 0.87 0.0508 0.0007 302 8 232 32

Ci5-14 417 114 17 0.27 0.00014 0.080 0.0487 0.0008 0.350 0.007 0.92 0.0521 0.0004 307 8 290 17

Ci5-15 794 378 30 0.48 0.00472 0.103 0.0443 0.0011 0.282 0.009 0.88 0.0473 0.0007 279 7 64 33

Average 501 237 20 0.49 0.139

Roujan–Neffies basin (297.9F 5.1 Ma)

Ci7-1 913 288 44 0.32 0.00002 0.064 0.0560 0.0034 0.421 0.026 0.99 0.0546 0.0005 351 21 394 20

Ci7-2 803 389 32 0.48 0.00004 0.140 0.0464 0.0026 0.337 0.019 0.99 0.0527 0.0001 293 16 314 4

Ci7-3 438 208 18 0.48 0.00006 0.138 0.0488 0.0014 0.356 0.011 0.99 0.0529 0.0002 307 9 326 10

Ci7-4 554 289 22 0.52 0.00020 0.139 0.0462 0.0014 0.322 0.011 0.92 0.0506 0.0007 291 9 223 30

Ci7-5 623 158 24 0.25 0.00005 0.071 0.0449 0.0013 0.323 0.010 0.99 0.0522 0.0002 283 8 296 7

Ci7-6 655 161 26 0.25 0.00004 0.070 0.0464 0.0015 0.336 0.011 0.99 0.0526 0.0002 292 9 312 8

Ci7-7 512 367 26 0.72 0.00009 0.195 0.0600 0.0064 0.433 0.047 0.98 0.0523 0.0011 376 39 299 48

Ci7-8 768 344 57 0.45 0.00012 0.071 0.0870 0.0077 0.643 0.059 0.98 0.0537 0.0011 538 46 356 44

Ci7-9 420 178 17 0.42 0.00024 0.121 0.0480 0.0015 0.346 0.012 0.94 0.0522 0.0006 303 9 296 26

Ci7-10 1092 363 42 0.33 0.00008 0.067 0.0443 0.0013 0.330 0.010 0.99 0.0540 0.0002 279 8 370 8

Ci7-11 571 235 22 0.41 0.00013 0.095 0.0451 0.0010 0.326 0.007 0.98 0.0525 0.0002 284 6 305 10

Ci7-12 275 71 11 0.26 0.00010 0.081 0.0477 0.0003 0.346 0.003 0.61 0.0526 0.0004 300 2 313 17

Average 635 254 29 0.41 0.104

Graissessac basin (295.3F 4.8 Ma)

Ci9-1 141 44 6 0.31 0.00043 0.082 0.0482 0.0007 0.319 0.005 0.78 0.0480 0.0007 303 4 99 34

Ci9-2 980 803 39 0.82 0.00020 0.245 0.0458 0.0006 0.329 0.004 0.92 0.0520 0.0005 289 4 288 21

Ci9-3-1 264 149 11 0.56 0.00027 0.159 0.0466 0.0005 0.334 0.005 0.94 0.0521 0.0006 294 3 289 27

Ci9-3-2 322 131 13 0.41 0.00030 0.111 0.0466 0.0035 0.323 0.004 0.73 0.0503 0.0005 293 22 211 24

Ci9-5 276 88 11 0.32 0.00017 0.086 0.0464 0.0013 0.341 0.005 0.66 0.0533 0.0007 293 8 342 28

Ci9-7 539 196 22 0.36 0.00022 0.102 0.0479 0.0011 0.343 0.003 0.56 0.0519 0.0004 302 7 282 17

Ci9-8 636 204 25 0.32 0.00025 0.083 0.0462 0.0029 0.330 0.004 0.25 0.0518 0.0005 291 18 277 24

Ci9-9 203 63 8 0.31 0.00021 0.076 0.0485 0.0012 0.348 0.006 0.32 0.0520 0.0007 305 7 287 32

Ci9-10 247 84 9 0.34 0.00023 0.094 0.0426 0.0010 0.312 0.005 0.98 0.0531 0.0008 269 6 333 33

Ci9-4 340 137 28 0.40 0.00013 0.105 0.0976 0.0011 0.805 0.008 0.54 0.0598 0.0004 601 6 596 15

Ci9-6 122 84 29 0.69 0.00011 0.211 0.2795 0.0021 4.439 0.050 0.52 0.1152 0.0004 1589 11 1883 6

Average 370 180 18 0.44 0.123

Jaujac basin (296.0F 6.8 Ma)

Ci12-1 177 127 7 0.72 0.00006 0.234 0.0460 0.0007 0.332 0.004 0.45 0.0523 0.0001 290 4 299 4

Ci12-2 252 215 10 0.85 0.00018 0.276 0.0466 0.0017 0.339 0.010 0.42 0.0529 0.0002 293 10 323 10

O. Bruguier et al. / Chemical Geology 201 (2003) 319–336322

Grain U Th Pb Th/ 204Pb/ 208Pb/ 206Pb/ F 207Pb/ F Rho 207Pb/ F Apparent age (Ma)

area (ppm) (ppm) (ppm) U 206Pb 206Pb 238U (1rerror)

235U (1rerror)

206Pb (1rerror)

206Pb/238U

F 207Pb206Pb/

F

Jaujac basin (296.0F 6.8 Ma)

Ci12-3 115 63 5 0.55 0.00016 0.190 0.0479 0.0017 0.357 0.008 0.54 0.0541 0.0002 302 10 375 7

Ci12-4 715 374 28 0.52 0.00197 0.176 0.0462 0.0030 0.376 0.022 0.42 0.0590 0.0005 291 19 567 18

Ci12-5 400 324 16 0.81 0.00129 0.267 0.0453 0.0031 0.318 0.016 0.43 0.0508 0.0004 286 19 234 17

Ci12-6 1405 1629 48 1.16 0.00761 0.382 0.0400 0.0011 0.311 0.025 0.11 0.0563 0.0008 253 7 463 31

Ci12-7-1 278 114 11 0.41 0.00090 0.129 0.0474 0.0015 0.335 0.025 0.15 0.0512 0.0006 299 9 250 26

Ci12-7-2 81 71 22 0.88 0.00008 0.312 0.3185 0.0045 6.610 0.049 0.81 0.1505 0.0009 1782 63 2352 10

Ci12-8 137 96 5 0.70 0.00012 0.245 0.0455 0.0016 0.334 0.010 0.41 0.0532 0.0002 287 10 338 9

Ci12-9 185 104 8 0.56 0.00054 0.200 0.0485 0.0028 0.356 0.022 0.33 0.0533 0.0005 305 17 339 19

Ci12-10 195 103 8 0.53 0.00144 0.157 0.0493 0.0034 0.304 0.052 0.12 0.0448 0.0011 310 21 � 67 60

Ci12-11 145 99 7 0.68 0.00095 0.203 0.0523 0.0049 0.347 0.040 0.28 0.0481 0.0007 328 30 104 36

Ci12-12 202 120 8 0.59 0.00133 0.187 0.0459 0.0043 0.327 0.041 0.24 0.0516 0.0010 289 27 268 44

Ci12-13 293 394 12 1.34 0.00011 0.372 0.0458 0.0016 0.359 0.024 0.52 0.0569 0.0032 289 10 487 120

Ci12-14 125 93 6 0.74 0.00001 0.231 0.0518 0.0016 0.401 0.022 0.55 0.0562 0.0026 326 10 459 98

Ci12-15 128 64 6 0.50 0.00020 0.170 0.0506 0.0027 0.435 0.050 0.47 0.0624 0.0063 318 17 686 202

Average 302 249 13 0.72 0.233

Ales basin (297.4F 4.4 Ma)

Ci13/1/1-1 487 238 23 0.49 0.00003 0.081 0.0504 0.0015 0.354 0.011 0.97 0.0510 0.0003 317 9 240 15

Ci13/1/1-2 115 44 6 0.38 0.00027 0.076 0.0419 0.0012 0.255 0.011 0.68 0.0441 0.0013 265 7 � 103 73

Ci13/1/2-1 227 158 12 0.70 0.00016 0.131 0.0414 0.0012 0.295 0.009 0.89 0.0516 0.0008 262 8 269 33

Ci13/1/2-2 674 342 15 0.51 0.00009 0.146 0.0365 0.0027 0.265 0.020 0.99 0.0526 0.0006 231 17 312 25

Ci13/1/3 221 144 16 0.65 0.00010 0.104 0.0487 0.0017 0.359 0.013 0.95 0.0534 0.0006 307 10 348 26

Ci13/1/4 129 78 10 0.60 0.00011 0.098 0.0492 0.0017 0.355 0.013 0.92 0.0524 0.0008 310 10 302 33

Ci13/1/5 743 409 31 0.55 0.00017 0.087 0.0502 0.0014 0.356 0.010 0.97 0.0515 0.0003 316 8 262 15

Ci13/1/6 508 109 22 0.21 0.00011 0.212 0.0467 0.0016 0.338 0.012 0.99 0.0525 0.0003 294 10 307 13

Ci13/1/7 230 97 10 0.42 0.00022 0.106 0.0492 0.0015 0.355 0.011 0.95 0.0523 0.0005 310 9 299 22

Ci13/1/8 698 158 30 0.23 0.00025 0.089 0.0479 0.0019 0.329 0.013 0.90 0.0498 0.0009 301 11 186 40

Ci13/1/9 183 53 7 0.29 0.00021 0.074 0.0482 0.0014 0.349 0.010 0.97 0.0524 0.0004 304 9 302 17

Ci13/1/10 349 162 14 0.47 0.00023 0.097 0.0446 0.0014 0.329 0.012 0.88 0.0534 0.0009 282 9 347 37

Ci13/2/1 219 37 9 0.17 0.00059 0.042 0.0468 0.0007 0.313 0.008 0.59 0.0485 0.0010 295 5 122 50

Ci13/2/2-1 636 365 26 0.57 0.00140 0.147 0.0470 0.0008 0.362 0.011 0.58 0.0559 0.0014 296 5 449 55

Ci13/2/2-2 651 391 27 0.60 0.00149 0.147 0.0482 0.0008 0.363 0.013 0.45 0.0547 0.0017 303 5 400 69

Ci13/2/3 263 148 11 0.56 0.00284 0.178 0.0469 0.0024 0.423 0.025 0.26 0.0655 0.0126 295 15 790 360

Ci13/2/4 597 261 24 0.44 0.00054 0.133 0.0464 0.0006 0.335 0.008 0.52 0.0523 0.0011 292 4 300 46

Ci13/2/5 568 256 22 0.45 0.00024 0.137 0.0458 0.0008 0.342 0.009 0.70 0.0542 0.0010 289 5 380 42

Ci13/2/6 392 121 16 0.31 0.00005 0.100 0.0470 0.0009 0.342 0.007 0.90 0.0527 0.0005 296 5 317 20

Average 415 188 17 0.45 0.115

Table 1 (continued)


95% confidence level using the Isoplot program

(Ludwig, 1999). Standard decay constants are those

recommended by the IUGS Subcommission on Geo-

chronology (Steiger and Jager, 1977). Analytical un-

certainties are listed as 1r in Table 1.

2.2. ICP-MS analyses

Trace element on accessory minerals (apatite and

zircon) from three bentonite samples were analysed

by conventional, nebulisation, ICP-MS using a VG

Plasmaquad II turbo at the University of Montpel-

lier II. Small samples were weighed on a Cahn

electrobalance. Zircons were dissolved under pres-

sure during 3 days at 195 jC with 10 Al of

suprapure tridistilled HF 48% using micro-capsule

dissolution (Parrish, 1987). Fluorides were subse-

quently converted to chlorides by dissolution over-

night under pressure in 6 N HCl. Apatites were

dissolved on a hot plate at 130 jC using suprapure


tridistilled 6 N HCl. After evaporation to near

dryness, all samples were subjected to three steps

of evaporation with decreasing HNO3 quantities.

Final sample uptake with 2% HNO3 was applied

shortly before analysis. Quantitative determination

of element concentration has been described in

detail in Ionov et al. (1992), which reports results

for the same elements measured in this study and is

given below only in brief. Analyses were performed

Table 2

Trace element contents (ppm) of accessory minerals (apatite and zircon) f

Rock type Fire clay Ci1 (Bosmoreau basin) Fire clay Ci12 (

Sample name Apatite/1 Apatite/2 Apatite/3 Apatite/1 Z

Weight (mg) 0.008 0.150 0.427 0.253 0

Cs – 0.23 0.32 0.85 1

Rb 1.34 1.49 1.34 5.48 1

Sr 139 161 202 405 1

Ba 40.7 30.2 36.3 20.6 4

U 9.24 8.66 11.4 28.6 9

Th 19.7 21.5 21.5 124 5

Pb 23.1 11.3 10.6 11.2 1

Hf – 0.12 0.17 0.77 1

Zr – 2.35 4.12 19.6 6

Ta – 0.16 0.14 0.11 2

Nb – 0.36 0.07 0.75 8

Y 792 747 788 454 4

La 800 862 844 395 4

Ce 1979 2402 2206 877 2

Pr 246 265 259 111 4

Nd 1066 1111 1105 538 3

Sm 202 191 198 124 9

Eu 8.04 8.94 9.70 15.5 4

Gd 201 176 177 109 4

Tb 25.7 22.8 23.3 13.8 1

Dy 146 129 136 79.0 6

Ho 27.1 24.3 24.8 14.2 1

Er 69.8 62.5 63.6 37.0 4

Tm 7.97 7.80 7.69 4.75 8

Yb 43.0 42.8 41.4 27.5 6

Lu 6.0 6.0 5.8 4.24 1

Zr/Hf 19.8 23.8 25.7 4

Y/Ho 29.2 30.7 31.7 32.0 2

Sm/Nd 0.19 0.17 0.18 0.23 2

La/Y 1.01 1.15 1.07 0.87 0

Th/U 2.1 2.5 1.9 4.4 0

Eu/Eu* 0.12 0.15 0.16 0.40 0

SCT s

m

LOD is the limit of detection, calculated as the concentration equivalent

measured on 10 acquisition of a blank prepared using conditions identical

Tree classification of zircons after Belousova et al. (2002).

in pulse counting mode (three points per peak) with

an instrumental sensitivity of ca. 30� 106 counts

per second per ppm of 115In. Concentrations were

determined by external calibration using two multi-

element calibration solutions prepared from 10 Agml� 1 single element solutions. Nb and Ta were

measured by surrogate calibration using Zr and Hf,

respectively, following the method outlined by

Jochum et al. (1990) for Spark Source Mass Spec-

rom Carboniferous volcanics of the French Massif Central (France)

Jaujac basin) Fire clay Ci7 (Roujan–Neffies basin) Limit of

ircon/1 Apatite/1 Zircon/1

.005 0.203 0.015

detection

(LOD)

4.9 0.20 0.65 0.00015

1.5 1.42 6.63 0.00309

1.7 535 5.10 0.00284

4.5 29,938 16.4 0.00629

79 13.3 620 0.00023

93 140 359 0.00019

6.2 4.78 24.3 0.00464

2,988 0.26 12,593 0.00025

38,189 4.02 602,766 0.00255

5.6 0.24 4.85 0.00040

6.4 0.14 78.6 0.00072

594 1412 2409 0.00015

.04 373 6.25 0.00161

5.7 1127 31.4 0.00010

.70 170 7.36 0.00004

6.6 889 49.8 0.00012

4.6 266 42.8 0.00003

0.4 16.0 13.4 0.00005

88 330 97.0 0.00012

02 45.2 29.3 0.00005

64 260 278 0.00022

59 45.6 76.1 0.00002

95 112.5 303 0.00008

9.5 12.8 61.6 0.00007

92 65.9 506 0.00003

43 8.9 99.4 0.00007

9.1 15.8 47.9

8.9 31.0 31.7

.59 0.30 0.86

.001 0.26 0.003

.61 10.5 0.58

.47 0.17 0.62

yenite/

onzonite

granitoid

(70–75%

SiO2)

to three times the standard deviation of average signal intensities

to those applied to the samples. SCT is the result of the Short Cart

Fig. 2. Lithostratigraphic columns of part of the sedimentary sequences accumulated in the Bosmoreau (A), Bertholene (B), Graissessac (C),

Jaujac (D), Ales (E) and Roujan–Neffies (F) basins, with location of the studied bentonites ( ).



trometry and applied to ICP-MS in this study.

Instrumental drift was corrected for by addition of

doping elements, namely In and Bi, at a concen-

tration level of 10 ng ml� 1. Polyatomic interfer-

ences were reduced by optimising the system to an

oxide production level < 1.5% measured on Ce and

corrections were applied using yields for MO+ and

MOH+ determined periodically by running batches

of synthetic solutions containing interfering ele-

ments. The analytical results are listed in Table 2.

Analytical precision of the ICP-MS measurements

is generally better than 5% (1r RSD) except for

low concentration elements which show precision

up to 10%.

3. Geological setting

The ash layers studied here are from Stephanian

(terrestrial Uppermost Carboniferous of northwestern

Europe) basins located in the southern part of the

FMC (Fig. 1B). All of these basins are characterised

Fig. 3. SEM photomicrographs of zircons from bentonites sampled in the studied basins. White ellipses show the approximate location of

the analysed area. Polished surfaces have been HF-etched to highlight internal structures. White patches along crystals or filling fractures

are due to gold remains. (a) Grain Ci5-10, from the Bertholene basin, is a euhedrally zoned zircon with a fine central channel of volcanic

origin. The crystal shows a slight rounding of the concentric oscillatory zoning (marked R) which indicates changes in the growth medium

and a temporary episode of Zr undersaturation with local dissolution. (b) About 350 Am long fragment of oscillatory zoned zircon with a

minimum length to width ratio of ca. 4. The pyramidal form grew assymetrically and shows an inversion of the length of the faces (see

white arrow) in the unbroken termination. The late stage of crystallisation shows a strongly reduced growth rate of the prism (Roujan–

Neffies basin). (c) Fragment of oscillatory zoned zircon showing fluctuation of the prism growth rate. Episodes of strongly inhibited

growth of the prism are related to slow cooling, possibly in the magmatic chamber, before eruption (Roujan Neffies basin). (d) Euhedral,

sector-zoned zircon with superposed oscillatory zoning (Graissessac basin). (e) Faintly zoned shard-like zircon fragment showing a central

gas tube characteristic of a volcanic origin (Jaujac basin). (f) Euhedral oscillatory zoned zircon with length to width ratio of ca. 3. The

crystal shows rounding of the internal zoning suggesting a low Zr undersaturation episode (Cevennes basin). (g) Example of euhedral

oscillatory zoned zircon with a euhedral core. The core is inclusion-rich which gives it a sponge-like texture and is partly resorbed on the

right side of the picture. Accordance of zoning between the core and the overgrowth, and the sponge-like texture suggest the core may

have crystallised in the early history of the magma and that the strongly zoned rim formed later in the magma chamber. The width of the

grain is due to the core, whereas the growth rate of the prism is inhibited in the rim. Volume expansion of the core due to radiation

damage induced radial micro-fracturation of the rim (Roujan–Neffies basin). (h) Euhedrally zoned zircon dated at ca. 290 Ma surrounding

a euhedral, sector-zoned core. Sector zoning of the core indicates an igneous precursor (Graissessac basin). (i) Euhedrally zoned zircon

consisting of a 299F 9 Ma (1r) magmatic overgrowth surrounding a strongly embayed ca. 2.4 Ga old core. Oscillatory zoning of the core

indicates melting of igneous source rocks (Jaujac basin). (j) Euhedrally zoned zircon with a central, strongly embayed, ca. 1.9 Ga old core

(Graissessac basin).


by thick accumulations (ca. 500 to up to 2500 m) of

fluvio-lacustrine sediments (see a review in Faure,

1995). Volcanic intercalations are common and occur

as thin (10–50 cm) irregular ash layers that are

considered to be products of an explosive, possibly

rhyolitic to rhyodacitic, volcanism (Bouroz, 1966).

The thicker bentonites (such as in the Roujan–Neffies

basin, see Fig. 2) are generally coarse-grained, hence


suggesting transportation over shorter distance than

thin, fine-grained volcanics (e.g. Graissessac basin)

although volcanic centres have not been identified.

The occurrence of accretionary lapilli in several tuffs

suggests that at least some of them were transported as

eruption cloud and were subsequently deposited as

fallout particles in the basins. All studied bentonites

consist of an argillaceous matrix and contain phenoc-

rysts of quartz, biotite, feldspar and variable amounts

of zircon and apatite. They are generally well pre-

served within low-energy deposits such as siltstones,

claystones or coals but can also occur intercalated

with sandstones where they are generally only locally

preserved and truncated by erosion surfaces.

4. Results

4.1. U–Th–Pb results

U–Th–Pb results of the five dated ash layers are

listed in Table 1 and presented on concordia plots

(Fig. 4a–e). The internal structures of the grains were

examined under SEM imaging after HF etching (Fig.

3a–j) to highlight zoned and unzoned domains. Se-

lected grains have euhedral shapes with sharp termi-

nations suggesting short sedimentary transport. They

occur either as long prismatic grains or as fragments

that reach 300 Am in length. Crystals are mostly clear

and colourless but sometimes cracked and inclusion

rich (mainly small apatite grains). They are entirely

(Fig. 3a–c) or faintly (Fig. 3e) oscillatory zoned, a

feature commonly interpreted as reflecting growth in a

magma. In addition, many grains contain a central gas

tube or a channel filled with glassy material (Fig. 3a

and e) which is typical of a volcanic origin (Pupin,

1976). Some grains show examples of sector zoning

(Fig. 3d) with superimposed oscillatory zoning. Al-

though first described as an unusual growth phenom-

enon (Hoffmann and Long, 1984), sector zoning has

since been reported for zircons from numerous igne-

ous rocks (Vavra, 1990; Benisek and Finger, 1993)

and is regarded as resulting from the incorporation of

different levels of trace element (mainly REE and Y)

in different portions of the crystal, depending on the

crystallographic orientation of the growing surface.

Although kinetic factors can be invoked, in particular

for rapidly grown volcanic phenocrysts, sector zoning

cannot be taken as a typical volcanic feature as it can

also simply result from slow lattice diffusion (Watson

and Liang, 1995). Lastly, cores of zircon, both oscil-

latory (Fig. 3g and i) and sector-zoned (Fig. 3h)

surrounded by zoned zircon are also present and

may represent either earlier stages of crystallisation

or inheritance from the source region of the magmas.

The euhedral shape of some cores (Fig. 3g and h)

indicates zircon saturation of the melt (Watson and

Harrison, 1983). Conversely, strong embayment of

others suggests resorption/dissolution reaction in a

zirconium undersaturated melt (Fig. 3i and j) subse-

quently followed by plating of new magmatic zircon

on exposed nuclei.

4.1.1. Bertholene basin (Strait of Rodez)

Sample Ci5 was taken close to the Bertholene U

mine and corresponds to a 20-cm-thick grey layer

located at the base of the sedimentary column, about

12 m above the basement represented by the Palanges

orthogneiss (see Fig. 2). Fifteen grains have been

analysed from this sample. They are U-rich (251–

794 ppm, average: 501 ppm) and yield an average Th/

U ratio of 0.49. The latter is in the range of typical

magmatic values (>0.1) as proposed by Williams and

Claesson (1987). The 15 analyses cluster close to

concordia (Fig. 4a) and, when combined, provide a

weighted mean 206Pb/238U age of 296.2F 7.2 Ma

(MSWD = 6.3). No inherited components were

detected in the analysed grains and the 296 Ma age

is thus interpreted as the crystallisation age of the

zircon.

4.1.2. Roujan–Neffies basin (Montagne Noire)

The volcanic horizon is located at the base of the

sedimentary pile, which is exposed in a grapevine

close to the village of Neffies. Sample Ci7 was taken

from the base of a 50-cm-thick pink to reddish colour

layer containing numerous accretionary lapillis and

overlying a coal layer (see Fig. 2). Twelve grains

were analysed which, on average (see Table 1) have

U, Th and Pb content and mean Th/U ratio (0.41)

similar to the Bertholene bentonite. Three grains (#7-

1, #7-7 and #7-8 excluded from Fig. 4b) have ages

older than the remainder of the analyses which could

indicate inheritance, but their high error margins

preclude any interpretations. The nine remaining

analyses plot close to concordia as a coherent group

Fig. 4. Concordia plots showing SIMS zircon analyses. Analytical uncertainty is represented by 2r error ellipses.



and define a mean 206Pb/238U age of 297.9F 5.1 Ma

(MSWD=2.3) which is interpreted as the eruption

age.

4.1.3. Graissessac basin (Montagne Noire)

Sample Ci9 was taken from a 10- to 20-cm-thick

layer of a yellow clay-altered ash intercalated within a

coal seam and outcropping in the Senegra quarry.

Eleven spots have been performed on 10 grains.

Although the U and Th contents of these grains are

generally lower than in the two other bentonites, their

mean Th/U ratio is identical and has a value of 0.44,

once more within the range of magmatic values. Eight

spots show a simple distribution with almost over-

lapping 206Pb/238U ratios (Fig. 4c). A pooled estimate

of the age from these analyses gives 295.3F 4.8 Ma

(MSWD=1.4). Analyses #9-4 and #9-6 have signif-

icantly older ages indicating that these grains contain

a component of inherited radiogenic Pb. Analysis #9-

4 with a 207Pb/206Pb age of 596F 30 Ma is from a

grain which has a euhedral form resembling igneous

zircons. This indicates no resorption or plating of new

magmatic zircon on the original crystal thus suggest-

ing a relatively short residence time in a Zr saturated

melt. This grain is thus interpreted as a xenocryst,

either inherited from basement rocks during ascent of

the magma and mixed with the volcanic ash during

the explosive stage of the eruption, or stripped from

wall rocks in the magma chamber at depth, shortly

before eruption. The oldest value, from analysis #9-6,

yields a 207Pb/206Pb age of 1883F 12 Ma and corre-

sponds to a strongly embayed core from a euhedral,

oscillatory zoned crystal (see Fig. 3j). Since the spot

partly overlaps the surrounding magmatic overgrowth,

the ca. 1.9 Ga age is interpreted as a minimum age for

the inherited component in this grain. The core–rim

relationship, observed in grain #9-6, indicates deriva-

tion of the magma through partial melting of Paleo-

proterozoic deep-seated crustal units. Analysis #9-10

defines a lower Saxonian age (ca. 269 Ma), which is

too young for the chronostratigraphic framework of

the basin. This suggests the grain has undergone

recent isotopic disturbance responsible for some Pb

loss.

4.1.4. Jaujac basin (Tanargue area)

Sample Ci12 is from a 15- to 20-cm-thick layer of

pale green colour. Only the base of the layer was

sampled, as detrital muscovite progressively appeared

upward in the bed. Sixteen spots were measured on 15

grains and, except for two analyses (#6 and #7-2)

discussed below, analyses cluster close to concordia

with overlapping error margins (Fig. 4d). Overall,

zircons from this volcanic ash have lower U content

(mean of 302 ppm), but higher Th content (mean of 249

ppm) and Th/U ratio (mean of 0.72) than the other

studied tuffs. Analysis 6 has the highest U and Th

content (both >1400 ppm) and the highest 204Pb/206Pb

ratio (0.00761) suggesting the beam struck a high U, Th

and Pb inclusion. This grain moreover yields a206Pb/238U Thuringian age (ca. 253 Ma) suggesting

disturbance of its U–Th–Pb system, which was there-

fore discarded from calculation. Thirteen analyses of

euhedrally zoned zircons and one analysis of a zoned

rim around core have concordant and overlapping

analytical points (Fig. 3d) with a weighted 206Pb/238U

age of 296.0F 6.8 Ma (MSWD=1.4). This is adopted

as our best estimate of the age of the magmatic zoned

zircons. Analysis #7-2 is from a strongly embayed,

euhedrally zoned core which yields a minimum appar-

ent 207Pb/206Pb age of 2352F 20 Ma. This core is

surrounded by ca. 299Ma oscillatory zoned zircon (see

Table 1). The zircon morphology and ages indicate that

new (i.e. Stephanian) magmatic zircon grew around

and shielded an older zircon during partial melting and

interaction with deep-seated source rocks. The euhe-

dral shape and zoning of the core is evidence for an

igneous precursor.

4.1.5. Cevennes basin

The Cevennes basin is probably one of the most

important Carboniferous basin of the French Massif

Central. Sample Ci13 is from a 10- to 15-cm-thick

blue-grey coloured layer sampled close to the local-

ity of Portes, i.e. in the middle part of the sedimen-

tary pile accumulated in this basin. A total of 19

spots were measured on 16 grains during two indi-

vidual sessions (see Table 1 and Fig. 4e). Zircons

have U and Th contents consistent with those from

the other studied volcanites and Th/U ratio (mean of

0.45), typical of a magmatic origin. No inherited

component was detected during the two SIMS ses-

sions and 16 analyses out of the 19 data points were

combined to provide a weighted mean 206Pb/238U

age of 297.4F4.4 Ma (MSWD=1.6). This is inter-

preted as the crystallisation and eruption age of the


magma. Three analyses (#1/1-2, #1/2-1 and #1/2-2)

gave younger 206Pb/238U ages of ca. 260–265 Ma

and ca. 230 Ma, interpreted as reflecting recent Pb

losses.

4.2. Trace element analyses of apatites and zircons

Small samples of apatites (weight ranging from

200 to 250 Ag) were analysed by solution ICP-MS

along with two single zircon grains from ash layers

from the Jaujac and Roujan–Neffies basins. In

addition, one single grain (weight of 8 Ag) and two

small fractions of apatite (150–425 Ag) from a

Visean bentonite collected in the Bosmoreau basin

(see Bruguier et al., 1998) were also analysed.

Chemical analyses of trace elements are reported in

Table 2 and shown in the chondrite-normalized

diagram of Fig. 5.

Apatites from bentonite Ci1 (Bosmoreau basin),

Ci7 (Roujan–Neffies basin) and Ci12 (Jaujac basin)

have chondrite-normalised LREE-enriched patterns.

One single apatite grain from sample Ci1 has trace

element concentrations and pattern similar to the two

apatite concentrates from the same rock. This indi-

cates that apatite crystallised from a magma which

remained homogeneous over the entire crystallisation

interval of this mineral in the magma chamber or

Fig. 5. Chondrite-normalized rare earth element patterns for apatites and

from McDonough and Sun (1995).

during ascent of the magma. All apatite REE-normal-

ised patterns show pronounced negative Eu anomalies

which are not typical of magmas of crustal origin as

this has been observed for apatites from undifferenti-

ated material such as the Acapulco meteorite (Zipfel et

al., 1995). On the other hand, the magnitude of the Eu

depletion is related to the oxygen fugacity of the

magma and crystal chemistry of apatite (Sha and

Chappell, 1999). Apatites from sample Ci1 and Ci7

have similar Eu anomalies with Eu/Eu* ranging from

0.12 to 0.17 whereas apatite from sample Ci12 has a

less pronounced Eu anomaly (Eu/Eu* = 0.40). Apatite

favors Eu3 + vs. Eu2 + in its structure. Eu3 + is less

abundant in S-type and felsic I-type magmas which

have a lower oxygen fugacity than mafic I-type

magmas thus leading to a pronounced Eu anomaly

in the formers. The greater Eu depletion in apatites

from Ci1 and Ci7 may thus be taken as indicating a

more reduced and peraluminous character of the

parent magmas, although this can also result from

plagioclase crystallisation. Some element ratios can be

used to characterise the different types of magmas

from which apatite crystallised such as the La/Y and

Sm/Nd ratios. From this standpoint (see Fig. 6),

apatites from samples Ci1 and Ci12 share similar

characteristics with La/Y and Sm/Nd ratios clearly in

the range of mafic I-type magmas (>0.2–3.25 and

zircons from ash-fall tuffs Ci1, Ci7 and Ci12. Chondrite values are

Fig. 6. Sm/Nd vs. La/Y ratios diagram for apatites from the ash-fall tuffs Ci1, Ci7 and Ci12. Fields of mafic I-type and S- and felsic I-type

granitoids are from Sha and Chappell (1999). Mafic I-type granitoids correspond to magmas with a range in SiO2 content of 57–70% (andesite,

dacite and trachyte volcanic equivalents), while felsic I-type termed magmas have SiO2 content >70% (rhyolite and low-alkali dacite volcanic

equivalents).


0.12–0.26, respectively, after Sha and Chappell,

1999). On the contrary, apatites from sample Ci7 plot

in the field of S- and felsic I-type magmas. In

addition, they have a flat LREE pattern characteristic

of peraluminous granitoids (Fig. 5). This supports

derivation of the parent magma by partial melting of

crustal material.

Two single zircon grains were hand-picked under a

binocular microscope from the least magnetic fraction

of the zircon concentrate recovered from samples Ci7

and Ci12. Zircon REE patterns show a typical enrich-

ment in HREE and low level of LREE ranging from

10 and 102 times the chondrite abundance (Fig. 5).

However, both zircons do not yield the significant

positive Ce anomalies which commonly reflect the

preferential uptake of Ce4 + from the melt and the

magmatic oxidation state (Ballard et al., 2002). This

may be related to apatite inclusions that can bias the

LREE concentrations in zircons. Belousova et al.

(2002) recently proposed to use other trace element

composition as an indicator of source rock type and

defined a classification ‘‘tree’’ to recognize zircons

from distinctive rock types. Using the ‘‘short’’ CART

tree of Belousova et al. (2002), the single grains from

the Roujan–Neffies and Jaujac bentonites fall in the

field of high SiO2 (70–75% SiO2) granitoids and

syenite/monzonite, respectively.

5. Discussion

5.1. Constraints on basin formation

Concordant clusters of results of zircon U–Pb

analyses from the five investigated volcanic tuffs fall

within the age interval of 295–300 Ma, i.e. in the

Gzelian stage of the Stephanian series according to

Odin (1994). Because ash clouds are rapidly deposit-

ed, they instantaneously date the sedimentation of

adjacent strata. All five individual U–Pb ages are

indistinguishable at the 2r level, and it is considered

that the time of eruption and sedimentation of the

volcanic ash in the five basins is essentially coeval.

Age bias due to reworking of older volcanoclastic

material, or even to incorporation of detrital material,

is unlikely given the zircon morphology and occur-

rence of accretionary lapillis in some of the layers

dated. The later formed during the flight of the ash

cloud, and are too fragile to be reworked or trans-

ported even over short distances. Moreover, the ex-

cellent consistency of the present data set argues

against such an hypothesis. Although the error mar-

gins are too large to be used as precise markers in the

Carboniferous time scale, these ages are consistent

with the stratigraphic position of the volcanic layers

dated. This is important to note, as some Stephanian


basins in the FMC (Bruguier et al., 1998) and also

within other parts of the Variscan Belt (Schaltegger

and Corfu, 1995; Von Raumer, 1998) are clearly

successors of older basins, that indicate an earlier

phase of extensional tectonics. All spot analyses have

been combined (see Fig. 7) to give a weighted mean206Pb/238U age of 297.9F 2.1 Ma (95% confidence

level) which is interpreted as bracketing the range of

the Stephanian volcanic activity in the southern part of

the FMC. Further studies must reveal whether this is

real or rather an artifact due to the limited number of

samples studied. This 295–300 Ma time interval is in

good agreement with K–Ar dating of clay particles

from the Bosmoreau basin in the northwestern part of

the FMC which gave a mean deposition age of

296.5F 3.5 Ma (Bruguier et al., in press). This

similarity suggests that basin formation during the

Stephanian was synchronous through the entire FMC.

The upper Stephanian volcanic and basin forming

event in the FMC is contemporaneous with volcanic

events identified in other parts of the Variscan Belt

which yield ages broadly ranging from 295 to 300

Ma, although slightly older ages (300–305 Ma) have

been also obtained (see Schaltegger and Corfu, 1995;

Fig. 7. Frequency histogram showing the distribution of the 206Pb/238U

Breitkreuz and Kennedy, 1999; Koninger et al., 2002).

Since the different basins throughout the Variscan Belt

may have different geodynamic positions, they do not

need to be strictly contemporaneous but clustering of

ages in the 295–300 Ma time range suggests that this

period may be the climax of a short-lived pulse of

explosive volcanism close to the Carboniferous/Perm-

ian boundary.

5.2. Constraints on source material and origin of the

magmas

Inherited cores have been identified in some of the

sudied bentonites (see Fig. 3i and j) and are taken as

evidence for melting of crustal material involved in

magma genesis. Analyses indicate melting of a Pre-

cambrian basement (ca. 600, 2000 and 2400 Ma).

Although the age of the crust below the southern part

of the FMC and Montagne Noire is not known, the

Pan-African, Eburnean and Early Proterozoic/Late

Archean ages of these inherited components point to

a Gondwanan affinity (West African Craton) for deep-

seated basement components beneath the FMC. This

observation is in good agreement with studies of

ages of SIMS zircon analyses from the investigated ash-fall tuffs.


lower-crustal granulitic xenoliths (Ben Othman et al.,

1984; Downes et al., 1990) from the FMC, as well as

from volcanic products from the Montagne Noire area

(Simien et al., 1999) which have Nd model ages

broadly ranging from 1.2 to 2.0 Ga. Remnants of

volcanic edifices do not exist to help establishing

regional correlations with possible source areas for

the volcanic ash and add geochemical constraints on

magma genesis. As most of the original volcanic

material has been altered, tuffs now consist predom-

inantly of clay minerals, and only phenocrysts of

accessory minerals resistant to alteration can be used

to shed some light on the source of the magmas. From

this standpoint, REE chemistry of zircons from a wide

variety of rock types gave remarkably similar results

(Hoskin and Ireland, 2000) and only a combination of

trace element abundances (Belousova et al., 2002)

indicates that zircon can be sensitive to its crystal-

lisation environment. The classification mentioned

above (Belousova et al., 2002) indicates that the two

single zircon grains from the Jaujac and Roujan–

Neffies basins crystallised from different types of

source magma (see Table 2) and shows that these

bentonites can be related to trachytic/andesitic and

rhyolitic volcanism, respectively.

Apatite has been shown to be sensitive to changes

in the concentrations of the REE during igneous

processes (Gromet and Silver, 1983) and can even

record the different stages of differentiation of one

single magma (Buhn et al., 2001). Apatites from the

upper Visean ash-fall tuff Ci1 sampled in the Bos-

moreau basin (Bruguier et al., 1998) have a chemical

signature indicating growth in a magma akin to those

producing mafic I-type granitoids and a likely dacitic

to trachytic affiliation. The most likely source could

then be found in the rhyodacitic to trachytic upper

Visean «tuffs anthraciferes» located ca. 60 km east of

the Bosmoreau basin in the Roanne area of the FMC

(Scott et al., 1984). Chemical composition of the

apatite concentrate from the Stephanian Jaujac basin

has a similar affiliation. This is supported both by the

REE pattern in Fig. 5 and by the inter-element

correlation diagram of Fig. 6. Combined with the

observation given by the zircons analyses, this sug-

gests for the volcanic ash layer Ci12 an origin of the

magma by partial melting of crustal units (including a

ca. 2.4 Ga source component) with a contribution of a

mantle-derived component to explain both the apatite

and zircon trace element signature. In contrast, apatite

from the Roujan–Neffies tuff has a flat LREE pattern

and plots in the field of S- and felsic I-type granitoids

suggesting a rhyolitic affiliation and magma genera-

tion mainly through anatexis of continental crust

again supported by the trace element signature of

the single zircon grain. Eruption can result from two

different processes, one of which is the injection of

mafic magmas from a deep source into a crustal

reservoir (Eichelberger, 1980). This represents a high

heat influx, a significant volume increment and can

generate vapor overpressure responsible for fractura-

tion and opening of a conduit. A second possible

mechanism is related to crystallisation in the mag-

matic reservoir which results in a volume expansion

and significant overpressure (Tait et al., 1989). In

addition, the remaining melts are more silicic and thus

more buoyant. Crystallisation alone can be responsi-

ble for some individual eruptions but, it is very

unlikely that, at orogenic belt scale, all crustal reser-

voirs had reached at the same time an overpressure

level driving magmas to the surface. The volcanic

activity prevailing at the end of the Carboniferous in

the whole Variscan Belt, on the contrary, pleads for

large-scale processes. The observed rhyolitic and

trachytic parentage of volcanic products from the

Roujan–Neffies and Jaujac basins, respectively, is

consistent with injection of mafic magmas which,

while adding heat to an already thickened crust,

may have triggered eruptions. This is broadly similar

to the South Bohemian Massif and Central Iberian

Zone, wherein granitoids emplaced at ca. 305 Ma

have characteristics pointing to a genesis by melting

of the lower crust with varying degrees of involve-

ment of a mantle component (see references in Gerdes

et al., 2000; Fernandez-Suarez et al., 2000, respec-

tively). Involvement of mantle sources does not rule

out a model of crustal anatexis by accumulation of

radioactive heat as proposed by Gerdes et al. (2000),

but simply provides extra heat to an already hot and

softened continental crust. This can explain both the

long-lived Carboniferous magmatic activity (see re-

view of ages in Ledru et al., 1994 for the FMC) and

episodicity of volcanic pulses at orogenic belt scale.

Lithospheric delamination has been invoked to ex-

plain the origin of these pulses (Pin and Duthou,

1990; Schaltegger, 1997; Fernandez-Suarez et al.,

2000) but is a long-lived phenomenon with a charac-


teristic wavelength of ca. 60 Ma (Nelson, 1992). The

Stephanian volcanic flare-up requires an additional

phenomenon which may tentatively be found in large

strike-slip faults cutting accross Central Europe and

Northern Africa (Bard, 1997). These faults are asso-

ciated with the Late Carboniferous–Early Permian

clockwise rotation of Gondwana (Matte, 2001) and

may have intersected parts of the sinking slab. As a

result of slab break-off, it is expected that the orogen

uplifted and extended due to gravitational instabil-

ities. Since the lower crust was hot and already

softened by about 30 Ma of heat advection and/or

production, it was able to flow rapidly. Mechanical

extension was thus predominant over erosion as

suggested by the preservation of the Stephanian intra-

montane basins.

Acknowledgements

Samples were collected by the two first authors

during the course of the GeoFrance3D program and

was supported by the BRGM. Preparation of the

mounts were performed by Christophe Nevado from

the «Service Commun de Litholamellage» and SEM

imaging by Claude Grill from the «Service Commun

de Microscopie Electronique» from the University of

Montpellier II. We thank J. Fernandez-Suarez, R.

Rudnick and U. Schaltegger for helpful and con-

structive reviews. [RR]

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