EPSL ELSEVIER Earth and Planetary Science Letters 144 (1996) 403-419
Precise U-Pb chronometry of 345-340 Ma old magmatism related to syn-convergence extension in the Southern Vosges
( Central Variscan Belt)
Urs Schaltegger a* * , Jean-Luc Schneider b, Jean-Christophe Maurin ‘, Fernando Corfu a
’ Deparrment of Geology Royal Onturio Museum 100 Queen’s Park. Toronto M5S 2C6, Canada h Dt!parremenr des Sciences de la Terre, Unil~ersit~ de Lille. 59655 Villeneuve d’Ascq, France
’ Ecole et Obseruatoire de Physique du Globe, URA 323 CNRS, 5. rue Descartes. 67084 Strasbourg, France
Received 9 January 1996; revised 18 September 1996; accepted 23 September 1996
Abstract
The Southern Vosges host a large volcano-sedimentary basin which contains elastic sediments of Famennian (Late Devonian; ca. 365 Ma) to Late Visean (ca. 340-335 Ma) age. The basin is bordered by an exhumed high-grade basement
complex and intruded by several granitoid plutons. A chronology for basin evolution and magmatism was established dating volcanic rocks of the basin as well as cross-cutting granitoid rocks by the U-PI, technique applied to zircon and titanite: The
basin evolution started with olistolithic deposits and overlying marine shales that have a Famennian fauna (ca. 365 Ma) and contain detrital zircons with an age of 386 Ma. The lowermost rhyolite flows interlayered with the basin sediments and tholeiitic basalts yielded an age of 345 + 2 Ma. Following a tectonic event at the bottom of the Upper Visean. the rates of
sedimentation and volcanism were strongly enhanced, leading to basin emersion, and were concluded by the extrusion of the uppermost ignimbrite (Molkenrain rhyolite) emplaced at 340 & 2 Ma. The whole volcano-sedimentary association was
subsequently intruded by melts of the Ballons and C&es granites as well as by small monzodioritic and dioritic satellite intrusions along the border of the Ballons granite within 3 f 2 myr (342 + 1 to 339.5 + 2.5 Ma). The zircons of most of the volcanic and plutonic rocks investigated contain inherited lead of 0.6 and 2.1 Ga age. The differences in initial Nd isotopic
composition of the Ballons and C&es granites can be explained by differential uptake of crustal material during magma genesis.
The precise age data prove that magmatism and basin formation in the Southern Vosges were extremely short-lived and coeval to the exhumation of adjacent high-grade gneiss terrains, revealing an episode of extension at the end of the Lower Carboniferous between 345 and 340 Ma (Visean). The tectonic regime in the internal part of the Variscan orogen is that of extension and strike-slip, while convergent tectonics were still ongoing in the external part of the Variscan orogen.
Keywords: Carboniferous; Variscides; granites; sedimentary basins; extension tectonics; U/Pb
* Corresponding author’s present address: Institut ftir Isotopengeologie und Mineralische Rohstoffe, ETH-Zentrum, Sonnegstrasse 5,
8092 Ziirich. Switzerland. Tel: +41 1 632 66 16. Fax: +41 1 632 1 I 79. E-mail: [email protected]
0012-821X/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SOO12-821X(96)00187-2
404 U. Schaltegger et al./ Earth md Planetug Science Letters 144 (ICJW 403-419
1. Introduction
Late erogenic processes, including extension, basin subsidence, strike-slip tectonics and basement
exhumation, are known to occur at the rate of plate
convergence (i.e. at IO-50 km/myr). Temporal res- olution of short-lived tectonic and magmatic events
in ancient erogenic belts requires high-precision
geochronology, which can only be achieved by using the conventional U-Pb dating technique for zircon. Ancient erogenic belts are well suited for the study
of the post-thickening tectonic stage of an orogeny.
in contrast to modern orogens such as the Himalaya, where this stage has not yet been reached.
The Variscan Belt of Central Europe hosts excel- lent examples of Lower Carboniferous volcano-sedi-
Colmar n
1Okm
Fig. I. Structural map of the Vosges. eastern France. after [ 11 and [36]. 1 = Moldanubian high-K I-type granites; 2 = Saxothuringian I-type
granites to diorites; 3 = leucogranites; 4 = Lower Paleozoic sedimentary series; 5 = lower metamorphic unit of the Moldanubian nappe;
6 = parautochtonous migmatites; 7 = upper metamorphic unit (granulitic) of the Moldanubian nappe; 8 = Late Carboniferous-Early Permian volcanic% 9 = Southern Vosges Late Devonian-Lower Carboniferous volcano-sedimentary basin: 10 = main Moldanubian nappe
contact; LLSZ = Lalaye-Lubine Shear zone.
U. Schalregger et al/Earth and Plunetap Science Letters I44 (1996) 403-419 405
mentary basins, formed by extension and filled with
marine sediments, which are in contact with contem-
poraneously exhumed deep crustal rocks [Il. Exten- sional tectonics within the Variscan orogen were
active in the Lower Carboniferous (355-325 Ma).
although convergence persisted until the Westphalian (ca. 315-305 Ma) [2], as documented by nappe transport in the northern and southern domains of the
belt dated at ca. 310 Ma [3,4]. One well documented
example of Late Variscan extension is the Late Devonian and Carboniferous volcano-sedimentary
basin in the southern part of the Vosges massif in eastern France [5-S]. The basin sediments were in- truded by a series of granitoid plutons and were
tectonically juxtaposed against a high-grade gneiss and migmatite terrain in the north (Fig. 1). The aim
of this study was to determine the precise age of
conformable volcanic and crosscutting plutonic rocks situated within and at the margins of the basin. respectively, and to compare this age to published
ages in nearby basement terrains, in order to demon- strate the existence of short-lived extension leading
to coeval voluminous magmatism, subsidence and exhumation in a post-thickening, but syn-convergent
extensional environment.
2. Geological setting
The Variscan erogenic belt, extending from Iberia
to Bohemia, was formed during the collision of Laurentia-Baltica with Gondwana in the Devonian/Carboniferous (between 420 and 280 Ma).
The Vosges mountains in eastern France (Fig. 1) are
one of the windows of Variscan crust emerging from the Mesozoic and Cenozoic sedimentary cover. The
Variscan orogeny featured some striking peculiari-
ties: it was of unusually long duration, lasting for 140 myr, suggesting that the evolution comprised several collisional events between continental plates; the number, position and vergence of oceanic sutures are still virtually unknown; and the erogenic evolu-
tion was dominated by a late-stage high-temperature
overprint, erasing most of the information from the prograde history and causing the formation of vast granite-migmatite terrains.
The Vosges massif is divided in two parts by a large shear zone, the Lalaye-Lubine Shear zone
(LLSZ: Fig. 1): The Northern Vosges, north of the
shear zone, belong to the Saxo-Thuringian domain and consist of non and weakly metamorphosed De- vonian to Lower Carboniferous active continental
margin sequences, intruded by a talc-alkaline mag-
matic series ranging from diorites to granites with
ages of around 335 Ma (Late Visean) [9,10]. The central and southern parts of the Vosges, south of the
LLSZ, belong to the Moldanubian domain of the Variscan Belt. The Saxo-Thuringian domain will
not be further considered in this paper.
2.1. The basement of the Central and Southern Vos-
ye.7
The Central Vosges consist of high grade meta-
morphic sequences that were intruded by numerous granitoid plutons (granodiorites, anatectic granites
and late-stage leucogranites). These plutons yielded Rb-Sr whole-rock ages scattered between 325 and
270 Ma, which are probably artefacts caused by
post-emplacement disturbance of the Rb-Sr isotopic system. K-Ar and Ar-Ar data have constrained
intrusion ages to between ca. 335 and 340 Ma
(Visean) for all granites dated so far [9]. The meta- morphic rock series intruded by these granitoids are
divided into an upper high-grade unit, containing
granulites with garnet peridotites and kinzigitic metapelites [I 11, and an underlying lower grade
metasedimentary unit, containing mainly migmatitic
gneisses and anatectic granites. Both underwent
high-T/low-P metamorphism and ductile deforma- tion during exhumation from the middle crust in the
Visean during detachment faulting [I]. Information about the composition and evolution
of the basement portion buried below the Southern
Vosges basin is scarce. From xenoliths in the gran-
ites and from components in Lower Carboniferous conglomerates we can conclude that it is composed
of continental and oceanic crust with additional man- tle rocks 181.
2.2. The Llolcanic and sedimentary formations of the Southern Vosges
The Southern Vosges host a well preserved Lower Carboniferous sedimentary basin (Fig. 2). This is bordered by the intrusions of the Ballons and C&es
406 U. Schaltegger et al. / Earth and Planeta? Science Letters 144 (1996) 403~-119
granites [ 121, the post-Carboniferous sedimentary cover and Tertiary Rhine Graben faults. The basin is
subdivided into a southern (more proximal) and a
northern, more distal part, the latter called the Mark-
stein Formation (Figs. 2 and 3). Both the southern and northern parts of the basin are slightly deformed
(slight folding and local thrusting), due to either
regional deformation prior to the intrusion of the
magmatic rocks and/or transtensional tectonics dur- ing emplacement.
Lithostratigraphic subdivisions of the basin sedi-
ments have been established by detailed mapping
and paleontological studies [ 13- 181, distinguishing
older (pre-Late Visean) and younger (Late Visean)
sequences. More recent investigations, however, have suggested that most of the fossil faunas represent
reworked mixtures of Toumaisian up to Late Visean age [ 19-211. Schneider [8] proposed a stratigraphic
division based on the depositional facies and tempo- ral evolution of the volcanism in the basin. The
different consecutive volcanic successions of the Southern Vosges are represented by a variety of
intrusive, effusive and pyroclastic facies [6,8,13,14].
2.2. I. Southern part of the basin Three consecutive sedimentary units may be dis-
tinguished within the southern part of the basin: (1) A Lower Unit (biostratigraphic age: Late
Devonian-base of Late Visean): this consists of turbidites that are associated with bimodal volcanic
rocks (basalts and low-K rhyolites), the basalts have
been diversely interpreted as oceanic arc tholeiites
[6] or continental tholeiites [5]. Volcanic activity and the degree of resedimentation of elastic material
from both older volcano-sedimentary units and the
high-grade basement increased markedly at the top of the Lower Unit. These trends strongly suggest that
a tectonic phase occurred at the end of the Early Visean (“intra-Visean extension”, [8,22]).
(2) The Middle Unit (base of the Late Visean) is
characterized by marine and mainly turbiditic sedi-
ments and by andesitic volcanism, hence a distinct change in the nature of volcanism in the basin. The
oldest andesites, however, were contemporaneous
with the youngest products of the bimodal volcanism of the Lower Unit, suggesting different sources for
coeval magmatic activity.
Ballon d'l\lSaC
GUEBWILLER
0 Post-Visean deposits
a Markstein Formation
m Diorftic rocks
Ligne des Klippes
Fig. 2. Geological sketch map of the volcano-sedimentary formations of the Southern Vosges with sample localities.
U. Schaltegger et al./Earth and Planeta? Science Letters 144 (1996) 403-419 401
Southern domain Northern domain (Markstein formation)
340*2Ma I
UPPER UNIT
w-93-12
-ca. 342Ma
f - su-43-11
34552 Ma I
w-94- I 366k2 Ma -
“KLIPPES” UNIT
.-_ 13/-5 Aa
Ezl Rhyoliiic lgnimbrites
Fig. 3. Schematic lithostratigraphic logs from both the northern
and southern domains of the Southern Vosges basin. Sample
locality and ages of volcanic rocks are indicated in italics. Thick-
nesses are not to scale. Geological timescale ages according to
E31.
(3) The Upper Unit (upper Late Visean) displays
a volcanic association, which evolved from trachyan- desites towards more felsic compositions (latitic rhy- olites, rhyodacites and rhyolites) that have a pro- nounced potassic chemistry. The sedimentation rapidly evolved from marine to terrestrial and in- volved mainly reworking of volcanic rocks.
Both the Middle and Upper Units were thus de-
posited during the Late Visean (ca. 340-325 Ma, [23]) according to biostratigraphic evidence. The
whole sequence was subsequently intruded by tra-
chytes.
2.2.2. Northern part of the basirl (Murkstein Forma-
tion) This up to 4000 m thick marine formation form-
ing the northern part of the basin (Fig. 2) contains very rare volcanic rocks and has been deposited from Famennian (Late Devonian, ca. 365 Ma) to Late
Visean (ca. 340-335 Ma) times. Detailed investiga-
tions by [8] pointed out that these strata represent the northern distal equivalent of formations located in
the southern part of the basin and that the volcanism
of the south is recorded by tuffs and reworked
volcanic components in the elastic sediments. A
layer containing lenses of high-grade gneisses, ser-
pentinites and mafic rocks with oceanic affinities
[24.25] of unknown age and origin and separating the
two basin domains, represents the base of the Mark- stein formation (‘Ligne des Klippes’; K, Fig. 2) and
has been interpreted as an olistostrome [26]. The olistostrome deposition occurred before the Late De-
vonian, because it is overlain by Famennian pelites
(green and red ‘Treh shales’, 1261) and grades up- wards into turbiditic sequences of alternating con-
glomerates, sandstones and pelites. This structure has
also been considered as a succession of tectonic
lenses (the ‘klippes’) marking a south-vergent thrust
fault. There is no agreement, whether the ‘Ligne des Klippes’ represents a tectonic or a stratigraphic con-
tact between the two basin domains. The rock material, which was sampled for this
study. includes a tuffaceous layer from the Markstein
Formation, several samples of rhyolitic flows and
ignimbrites from the southern part of the basin, in order to constrain the timing and age of basin sedi-
mentation. Various cross-cutting granitoid rocks as-
cribed to the Ballons and C&es plutons have also been sampled.
3. Methods
Short descriptions of the samples investigated are given in Appendix A. The samples consisted of
Tabl
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U-P
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of
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Num
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3.39
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330
340.
7
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34”
8
34”
I
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44
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33’)
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2
377
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6
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9
337
6
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21.5
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a 0.
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00
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5
312
2
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7
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b
312.
7 65
L,
euh
. tr
anbp
410 U. Schaltegger et al./ Earth and PlanetaT Science Letters 144 11996) 403-419
5-10 kg of fresh rock. Zircon was separated from a fraction smaller than 300 pm using standard meth-
ods. The bulk zircon fraction or a non-magnetic
zircon fraction from a Frantz magnetic separator was
examined under a binocular microscope and the suit-
able grains were selected for analysis, mostly accord-
ing to morphological criteria and colour. Analyzed multigrain fractions were homogeneous in terms of
morphology, length/width ratio, colour and trans-
parency. All analyzed grains were air-abraded to remove zones of marginal lead loss. Prior to analy-
sis, zircons were washed in warm 4 N nitric acid and
rinsed several times with distilled water and acetone in an ultrasonic bath. Dissolution and chemical ex-
traction of U and Pb were performed following [27],
using bombs and anion exchange columns that are
scaled down to 1 /IO of their original size. Total
procedure blanks were usually between 0.8 and 4 pg
Pb and 0.1-0.5 pg U, for sample sizes up to 0.05 mg. Some samples yielded higher common lead
concentrations that cannot be related to higher blank
contamination. Mass spectrometry and data reduction was carried out in the same way as described in [28].
The analytical data are compiled in Table 1. Cathodoluminescence imaging of zircons was car-
ried out in order to gain additional information on
growth conditions of the zircons.
4. Results
4.1. Treh shales, Markstein Formation, Cabane du
Treh (SU-94-l; Fig. 4)
The oldest ages of this study were found on two single zircons of a light-coloured tuffaceous layer
within the Treh shales, whose Late Devonian
(Famennian) age has been documented by conodonts [26]. A short prismatic grain (number 1, Table 1)
yielded a concordant age of 386 & 1 Ma; another grain (2, Table 1) had a greater uncertainty because of high common lead correction, but also provided a
206Pb I *?I
0.066
t
Treh shales Markstein formation
410
J o’mm 0.062
A 390 400 1
2 380
Age of detrital zircons: 386*2Ma
0.44 0.46 0.48 0.50
Fig. 4. U-Pb concordia diagram of sample SU-94-I. Late Devb-
nian (Famennian) tuffaceous layer from the base of the Markstein
Formation (Treh shales).
meaningful ‘O’Pb/ *” U age of 386 + 2 Ma. The
zircons are detrital and some 20 myr older than the deposition of the Famennian sediment (367-362.5
Ma [23]). The age of 386 Ma is similar to values given by [9] and [ 1 I], and reflects an earlier meta-
morphic or magmatic event in the basement rocks
around and below the Southern Vosges basin.
4.2. Low-K rhyolite from bimodal association, east
of Tremontkopj Doller Valley (W-93-11; Fig. 5a)
Six zircon fractions, each consisting of 8 to ca. 30
small grains, were analyzed from this rock. Three
concordant results (3, 6 and 8, Table 1) of short prismatic grains define a mean U-Pb age of 345 ? 2
Ma, interpreted as the extrusion of this flow. This sample represents one of the stratigraphically lowest rhyolite flows at the top of the Lower unit; the age of
345 Ma is of great importance because it represents the minimum age for the onset of basin opening. Three other fractions of long prismatic grains and
a abr. = abraded; anh. = anhedral; euh. = euhedral. cl. = colourless, incl. = inclusions, ndl = needles, pr. = prisms: tit. = titanite, transp. =
transparent. frags. = fragments; lpr. = long prismatic, spr. = short prismatic, z = zircon: P-type zircons according to [29].
h Calculated on the basis of radiogenic *‘*Pb/ “‘Pb ratios, assuming concordancy.
’ Corrected for fractionation and spike.
d Corrected for fractionation, spike, blank and common lead (according to [44]). Errors include uncertainties of common lead correction.
U. Schaltegger et al. /Earth und Planeta? Science Letters I44 (I 9961403-419 411
tips thereof (4, 5 and 7, Table 1) show minor amounts
of inherited lead, coupled with some secondary lead loss.
4.3. Rhyolite. Luuw Quarry, Doller Valley (SU-93- 12; Fig. 56)
Seven analyses were done on fractions of 9- 18
zircons of this sample. The zircons were generally of
poor quality, showing multiple inclusions and cracks. Most analyses (9, 10, 13-15, Table I) are slightly
Rennbaechelfels “Ligne des Klippes”
0.042
Zircon Upper intercept age:
339.5 f 2.5 Ma
to59f17Ma a 0.32 0.34 0.36 0.36 0.40 0.42
I Ballons granite 0.055 Lac d’Alfeld 345
0.054
0.052
0.051
Upper intercept age: 340 +4/-2 Ma
C
discordant and not colinear, indicating combined ef-
fects of inheritance and lead loss. Two analyses of euhedral tips ( 11 and 12) overlap the concordia
curve at 341 and 344 Ma, but the emplacement age
cannot be precisely determined.
4.4. Ignimbritic rhyolite to latite, ‘Molkenrain rhyo- lite’. red type. Co1 du Hundsruck (SiJ-93-7; Fig. 5cl
Eight fractions were analyzed for U and Pb iso-
topic composition. but only analysis 22 turned out to
0.052
I. b
0.36 0.39 0.40 0.41 0.42 0.43
Mean 207Pb1206Pb age:
Fig 5. U-Pb concordia diagrams of volcanic rocks the Southern Vosges volcano-sedimentary basin. (a) Low-K rhyolite from the bimodal
association at the top of the Lower Unit (SU-93-I I). (b) Rhyolite, Lauw (SU-93-12). (c) Molkenrain rhyolite, red facies, Cal du Hundsruck
(W-93-7). (d) Molkenrain rhyolite. grey and brecciated facies (Vieil Armand: SU-94-5). The ages bracket a period of volcanism to between
345 and 340 Ma; more details are given in the text.
412 CJ. Schaltegger et al. /Earth and Planetq Science Letters 144 C 1996) 403-419
be concordant (337.5 _+ 1 Ma). Five others (18-21
and 23, Table 1) are inversely discordant and, to-
gether with fraction 22, yield a discordia with inter-
cept ages of 655 f. 70 and 336 + 3/ - 5 Ma. We analyzed representative fractions of the whole range
of different morphological zircon types, in search for
an inheritance-free morphology. We found that
long-prismatic grains have the highest uranium con-
tent and the lowest degree of inheritance. but at least
one of these fractions (16) shows evidence of addi-
tional secondary lead loss. The lower intercept age of 336 Ma, very likely biased (lowered) by lead loss, is
regarded as a minimum age of emplacement and will not be considered for the discussion of emplacement
ages. Fraction 17 does not lie on the discordia and has a ‘O’Pb/ ‘06 Pb age of 670 Ma.
4.5. Ignimbritic rhyolite to latite, ‘Molkenrain rhyo- lite’, grey brecciated type, Vieil Armand (SU-94-5; Fig. 5d)
Six microfractions were analyzed from a typical
grey and breccia-like facies sampled near the type locality of this ignimbrite. Most zircon grains from
this sample show a characteristic rounding of edges
and tips, which has not been found in other volcanic
rocks of the Vosges, interpreted as the result of mechanical transport during ignimbrite deposition in
a high-temperature ash cloud. Subhedral crystals suggest the presence of morphological types
525/S24. S2O/S19 and S14 (according to the clas- sification of [29]), reminiscent of both Ballons and
C&es zircon populations. The CL imaging showed
very low degrees of luminescence and no inherited
cores. Analyses 25, 28 and 30 (Table 1) yielded analyti-
cally concordant ages with a mean value of 340 * 2 Ma; together with 24 and 29 they define a best-fit line through the origin and an upper intercept age of
340 k 2 Ma. Analysis 28 suffered high lead contami- nation during analysis. Analyses 24, 26 and 29 show secondary lead loss; analysis 26 an additional inher- ited component. Analysis 27 points to an inherited component of Proterozoic age. An emplacement age of 340 f 2 is, therefore, well established for this rhyolite. This age is identical within error limits with the lower intercept of 336 + 3/ - 5 Ma of sample X-93-7 from the same rock type, but is given a
higher confidence, because the data are less influ- enced by inheritance.
4.6. Microgranitic dyke, Rennbaechelfels. southeast of Grand Ballon (W-93-3; Fig. 6a)
Eight zircon and two titanite fractions were ana-
lyzed from this rock. The zircons are generally very
high in U, the least enriched being the most concor- dant ones. Zircon fractions 3 l-35 and 39 yield a
discordia line with an upper intercept age of 339.5 * 2.5 Ma (MSWD = 0.18) and a lower intercept at
59 + 17 Ma. Analysis 37 indicates the presence of minor amounts of inherited lead; fraction 38 con-
sisted of 13 large fragments of short prismatic grains that were considered as having incorporated the same
inherited component. The point is not colinear with the others and therefore was excluded from discordia
calculation. This exclusion is somewhat arbitrary; the
same line calculated with 38 yields a slightly higher age of 342 f 3.5 Ma, which is identical within error
limits. The lower intercept age of 59 Ma might
indicate lead loss during hydrothermal alteration re- lated to Rhine graben rifting.
Two concordant titanite fractions (36 and 40)
have a ‘06Pb/‘38U age of 336 + 3 Ma, which is
slightly, but not significantly, younger than the zir-
con upper intercept age. The 339.5 Ma age is taken
as the intrusion age of this granite, whereas titanite may have formed later in this rock at 336 Ma.
4.7. C&es granite, quarry at Metzeral, Munster Valley (SU-9.5-5; Fig. 6bj
Eight zircon fractions and one of titanite were analyzed from this sample. The zircons exhibit a
morphological range from S24/S25 to S19 and S14. Prismatic grains with a length/width ratio of at least 2-3 show oscillatory zoning and seem to be free of inherited components (cores). Equant grains, how- ever, commonly host inherited cores. The cores re- flect several generations of zircon growth, visible in cathodoluminescence, although the isotopic data in- dicate a single-stage inheritance. The proportion of core-bearing grains can be estimated at a minimum of 50%.
The zircons are considerably lower in U (around 400 ppm) than the ones from the dyke SU-94-3,
0. Schaltegger et al. / Earth and Planetary Science Letters 144 (1996) 403-419 413
except for analysis 42. Analyses 42 and 44-46 re-
vealed an inherited Pb component of Proterozoic age; the calculated best-fit line points to a well defined upper intercept age of 2.12 + 0.02 Ga (MSWD = 0.34). There is no indication for a younger
inheritance of Cadomian/Pan-African age, as in the
Molkenrain rhyolite (Fig. 5~). The well constrained lower intercept age of 340 f 1 Ma is interpreted as
the intrusion age. Analyses 41 and 47 have
‘07Pb/ ‘06 Pb ages of 342 and 344 Ma, respectively,
Age: 345 i 2 Ma
a 0.400 0.405 0.410 0.415
0.052
Molkenrain rhyolithe Col du Hundsruck Upper Visean
Lower interced
v 330 4 2.2 age: 336+31-5 ha
C I -
0.38 0.39 0.40 0.41 0.42 0.43
and are probably also influenced by minor amounts
of inherited lead. Analysis 43, with a ‘07Pb/ 206Pb age of 340 Ma, seems to be the only one without
inheritance. An age much higher than 340 Ma does not seem reasonable, because the C&es granite
cross-cuts the 345-340 Ma old volcano-sedimentary
sequences.
One third of the lead of the titanite analysis (48)
is common lead. responsible for the large analytical uncertainty of the ‘07Pb/ 23sU and Pb/Pb ages. The
0.057
0.056
0.055
0.054
0.052
Rhyolite Lauw quarry Upper Wean
Approximate age: 342 Ma
0.38 0.39 0.40 0.41 0.42
0.056
0.054
0.052
0.050
Molkenrain rhyolite Vieil Armand Upper Visean
Upper intercept age and mean U-Pb age:
340i2Ma
Fig. 6. U-Pb concordia diagrams of magmatic rocks from the southern Vosges. (a) Microgranite dyke from the Rennbaechelfels (SU-94-3).
(b) C&es granite, from the quarry at Metzeral (Munster Valley: SU-95-5). (c) Ballons granite (Doller Valley; SU-95-6). (d) Monzodiorite of
Langenfeld (Doller Valley; SU-95-7) and diorite near Chlteau-Lambert (south of Le Thillot; SU-95-15). The results cluster between 339.5
and 342 Ma. defining a 3 * 2 myr period of short-lived magma&m; more details dare given in the text.
414 U. Schaltegger et al/Earth and Planetan Science Letters 144 (19961403-419
‘06Pb/‘38U age of 338.5 + 2 Ma, however, is a
significant minimum age that overlaps the intrusion age derived from the discordia intercept.
4.8. Ballons granite, Luc d’Alfeld, Doller Valley (SU-95-6; Fig. 6~)
The investigated zircons of this granite exhibit
S24/S23, Sl9/S18, Sl2-S14 and S8 morphologies.
A statistically significant analysis of the morphologi-
cal types (> 100 zircons) by [29] yielded a maxi-
mum in the S I8 and S 13 fields. CL imaging revealed no inherited cores. The grains display fine low-CL
oscillatory zoning, abundant inclusions and had high
U concentrations. even in the least magnetic frac-
tions used for isotopic analysis. It can be concluded
from the zircon morphology that this granite is
slightly more evolved than the Crgtes granite.
Short prismatic zircons (analyses 50-54) turned out to be free of inherited lead components, whereas a fraction of six prismatic zircons (49) yielded a “‘Pb/ ‘06Pb age of 375 Ma. The Ballons granite
hosts titanite with a relatively high U concentration and is thus well suited for U-Pb dating. A precise
age of 339 + 2 Ma can be calculated from two
concordant titanite analyses (55 and 56). The intru-
sion age of this granite is best approximated by an
upper intercept age of 340 + 4/ - 2 Ma of a lead
loss line. calculated through both zircon and titanite
data points (50-56).
4.9. Langenfeld monzodiorite. west of L.uc d’Alfeld, Doller Valley (SU-95-7); and diorite, Ch&eau-Lam- bert, south qf Lx Thillot (W-95-15; Fig. 6d)
The small Langenfeld monzodiorite intrusion (SU-95-7) occurs at the southeastern tip of the Bal- lons granite body; a close genetic relationship was
therefore anticipated. The zircons exhibit completely different morphological characteristics to those of the granite zircons. The grains are rounded to sub-
round, partially facetted and never show completely euhedral morphology. The I21 1) and { 1011 faces can be recognized and S23/S18 type crystals could be determined (with some uncertainty). The zoning re- vealed by CL imaging is oscillatory and magmatic, but is characterized by much broader zones than in the granite zircons, and many grains are dominated
by sector zoning. Curved boundaries cutting older zones are common and are interpreted as magmatic features, because the isotopic data exclude the pres-
ence of more than one generation of zircon growth.
The zircon size ranges up to 500 pm and the U
concentrations up to 900 ppm. Five zircon fractions consisting of l-6 grains were analyzed and three of
them (57, 58 and 60) yield subconcordant results. Analyses 59 and 6 1 are slightly discordant, reflecting
some minor lead loss.
From the Chiteau-Lambert diorite (SU-96- 15). four zircon fractions consisting of 8-12 grains each
were analyzed. The analyses show that the zircons
suffered some lead loss, despite their low U contents around 300 ppm. No systematic difference between
anhedral (62 and 64) and euhedral grains (63 and 65)
could be detected, nor any trace of inherited lead. The emplacement of the two rocks is dated by a
mean “‘Pb/ ‘06Pb age of 342 * 1 Ma for all nine fractions analyzed, identical or slightly older than the
Molkenrain rhyolite (SU-94-5). the Ballons (SU-95- 6) and the C&es granite (SU-95-5). The zircon
U-Pb age of the ChGteau-Lambert diorite is thus some 20 myr younger than the published Ar-Ar
hornblende age of 360 + 6 Ma 191. pointing to the
presence of excess Ar.
5. Discussion
5.1. Southern Vosges basin erlolution
The basin evolution shows two important phases of tectonic activity: (1) A first, strong uplift phase
created the olistostrome deposits of (pre-?)Famen- nian (ca. 365 Ma) age at the base of the Markstein Formation. The chaotic deposits of the ‘Ligne des Klippes’ olistostrome are the oldest rocks of the
Southern Vosges basin, possibly contemporaneous with the initiation of basin opening and strong ero- sion into a small basin at sea level. (2) The sedi- ments at the base of the Middle Unit indicate re- working processes, syn-sedimentary deformation and block tilting, which has been assigned to a Visean extensional event [22]. The sedimentation rate was significantly higher and the volume of volcanic de- posits greater in the subsequent Middle and Upper Units, while the setting of sedimentation and volcan-
U. Schaltegger et al. /Earth and Planetan/ Science Letters 144 f I9961 403-419 415
ism quickly changed from marine to voluminous continental elastic sedimentation and sub-aerial vol- canism, both indicating uplift and emersion of the
basin. Slight deformation of the sedimentary pile, including shearing and folding, may be attributed to
a third tectonic phase slightly before or during the
emplacement of the magmatic rocks.
Subsidence of the Southern Vosges basin there- fore started in the Late Devonian (Famennian), ca.
365 Ma ago, characterized by slow marine or tur- biditic sedimentation. The volcanism within the basin
occurred within a relatively short time span of ca. 5 myr, between the lowest rhyolites of the bimodal
association and the uppermost Molkenrain ignimbrite (Fig. 3). The stratigraphic position of the latter has
been a matter of debate for a long time: For 1301, these pyroclastic sequences belonged to the last vol-
canic episode of the Southern Vosges basin, although Schneider [8] and Coulon et al. [IS] argued that the
ignimbrites were interbedded with the volcaniclastic formations at the base of the Upper Unit. The new
age determinations now reveal that these rocks do indeed correspond to the last volcanic products at the
top of the Upper Unit. Our age data confirm that
most of the sedimentary and volcanic deposits were emplaced within a short period of time in the Visean. starting before 345 Ma and ending at 339-342 Ma.
No Namurian deposits should be present, which is in
accordance with previous paleontological studies [14-161.
5.2. Emplacement of the magmatic rocks
The volcano-sedimentary deposits of the basin
were intruded by granitoid rocks, which are associ- ated with the CRtes (SU-94-3 and SU-95-5) and the
Ballons granites @U-95-6, SU-95-7 and SU-95-15).
The C&es granite cross-cuts the Markstein Forma- tion, forming its northern limit, and a dyke derived
from the C&es granite (SU-94-3) cuts through sedi- ments of both the northern and southern part of the
basin at the ‘Ligne des Klippes’. The Ballons granite intruded into the volcano-sedimentary sequences in
the centre of the basin and is situated in a weakly developed antiform. The intrusives have been found to have imposed weak contact metamorphism and syn-intrusive deformation onto the adjacent sedimen- tary rocks, together with peripheral mineral orienta-
tion parallel to the contacts (work in progress; Burg
and Maurin, pers. commun.). Several small dioritic to monzonitic intrusions
occurring along the northern and southern contacts
contain fine to coarse grained gabbroic cumulates
[ 121. The northern sequence has been reported as
having a tholeiitic and the southern a talc-alkaline
signature [5]; both were interpreted as being unre-
lated to the Ballons granite by [ 121. Gabbroic to
dioritic rocks from both northern and southern suites dated by U-Pb yield identical ages of 342 + 1 Ma
(Fig. 6d), showing that these marginal intrusions were coeval and probably co-genetic with the gran-
ite. The U-Pb zircon age of 342 $- I Ma is inconsis-
tent with an Ar-Ar plateau age of 360 i 6 Ma determined by Boutin et al. [9], indicating excess
‘“Ar in the hornblende dated. This strongly questions
the existence of older, pre-Visean mafic magmatism around the Ballons granite.
The talc-alkaline (but highly potassic) character
of the volcanic rocks has often been taken as an argument for an active continental margin setting
[3 11, which is difficult to fit within current models of
Variscan erogenic evolution that invoke termination
of oceanic subduction processes in the Devonian [32]. Volcanism with talc-alkaline and shoshonitic
affinities, unrelated to subduction but related to the motion of large crustal-scale strike-slip faults, has
been recognized within many erogenic belts [33]. The high-K volcanic rocks of the Upper Unit are
genetically related to the Ballons and C&es granites
[34]. Some of the rocks exhibit major crustal compo- nents. reflected by abundant inheritance of 640 Ma
(Molkenrain rhyoiite) and 2.12 Ga old lead (Cretes
granite) in the zircon fractions analyzed. The Ballons
granite zircons. on the other hand. seem to be mostly free of inheritance (Fig. 6~). This explains the differ-
ence in the initial Nd isotopic ratios (-6 for the C&es and - 3.4 for the Ballons granite [35]).
The isotopic ages for the Southern Vosges basin and its associated magmatism cluster around an age
of 339.5 (-t 2.5) to 342 (t- I> Ma (except for the 345 f 2 Ma old rhyolite of the bimodal association
from the Lower Unit), suggesting that all the Late Visean volcanism and plutonism occurred within a short time span of 3 f 2 myr, immediately following a period of rapid subsidence and sediment deposition within the basin. The granulite-facies gneiss and
416 U. Schaltegger et al. /Earth and Planetan Science Letters 144 (I 996) 403-419
migmatite terrain to the north underwent rapid ex-
humation associated with ductile deformation. at the
same time as graben subsidence with brittle faulting
in the south [ 1,361. Exhumation resulted in crustal anatexis, formation of migmatites and regional high-
temperature/low-pressure metamorphism, all dated at ca. 335 Ma (Ar-Ar biotite and hornblende ages [9]). The coeval relationship between basin subsi-
dence, magmatism and exhumation is therefore
demonstrated by geochronological methods.
5.3. Implications for the tectonic ellolution of the Variscan Belt between the Devonian and Upper Carboniferous
The oldest marine sediments in the southern Vos- ges basin were reported to be of Famennian age (ca.
365 Ma), indicating that the thickened crust of the
Variscan orogeny had already been thinned earlier. Any thickening or subduction must therefore be of
pre-Late Devonian age. High-grade gneiss compo-
nents from basin sediments as well as detrital zircons
yielded ages around 385 Ma ([9]; Fig. 4), possibly reflecting a major metamorphic or magmatic event
during Variscan crustal thickening in the basement rocks around the Southern Vosges basin.
Recent studies have emphasized the major contri-
bution of extension during the late Lower Carbonif- erous evolution of the Variscan orogen [1,37,38]. Our investigations support a model of extensional
formation of the southern Vosges basin in a tectonic
context, which was convergent at the scale of the
whole orogen [2]. The last convergence at ca. 310
Ma has been reported from the most external zones of the orogen (i.e., the Rheno-Hercynian domain in the north and the Montagne Noire in the south [3.4]).
The southern Vosges basin represents a local exten- sional domain situated between large dextral strike-
slip faults [39,40], which accommodated the dextral
slip component of oblique compression. The exten- sion was restricted in time and took place within a short period of 5 myr. The Southern Vosges basin is part of a larger zone, probably consisting of a num- ber of such basins, running from the northernmost Massif Central (Morvan-Beaujolais), through the Southern Vosges to the Black Forest (Badenweiler- Lenzkirch zone). suggesting formation by an orogen-wide tectonic process.
The Tibetan Plateau has been repeatedly proposed as a tectonic analogue, where syn-convergence and
orogen-parallel extension is caused by lateral escape
of overthickened crust [37,41]. The Lower and Mid-
dle Units of the Southern Vosges basin, however, contain sediments of marine character, indicating
that the deposition of the Famennian to pre-Late Visean (ca. 362-345 Ma) sediments occurred below sea level. This suggests the existence of a thinned
continental crust and incipient break-up, with basaltic volcanism or even formation of minor amounts of
oceanic crust [42]. A more appropriate example seems
to us to be the Aegean Sea, where the thickened continental crust has already been thinned by syn-
convergent extension and the exhumation of meta-
morphic core complexes, high-temperature metamor- phism and magmatism are known to be coeval with a sedimentary basin of Miocene age 1431.
6. Summary and conclusions
The subsidence in the area of the Southern Vos-
ges volcano-sedimentary basin started in the Late Devonian (Famennian; ca. 365 Ma) with marine sedimentation and proximal turbidites containing
olistoliths (‘Ligne des Klippes’). Sedimentation and
subsidence were accelerated at the beginning of the Late Visean; voluminous volcanism and elastic sedi-
mentation evolved over a relatively short time period of ca. 5 myr: Rhyolitic volcanism can be bracketed
by ages of 345 _t 2 Ma, for the oldest low-K calc-al- kaline rhyolites of the bimodal association at the top
of the Lower Unit (base of Late Visean), and 340 k 2 Ma, for the youngest ignimbritic high-K rhyolites in the Upper Unit (Molkenrain rhyolite; Fig. 3). Sedi-
mentary and volcanic rocks were intruded by grani- toid dykes, monzogabbroic to dioritic stocks and large granite plutons that yielded ages of 339.5 & 2.5,
342 i_ 1 Ma, 340 + 1 and 340 + 4/ - 2 Ma, all in- distinguishable from the age of the latest ignimbrites. Adjacent high-grade gneiss and migmatite terrains were exhumed at the same time, as indicated by Ar-Ar biotite and hornblende ages around 335 Ma [9]. Rapid subsidence, voluminous magmatism and exhumation are therefore concurrent processes. which within a few million years fundamentally changed the structure of the orogen. The coeval exhumation
U. Schaltegger et al. /Earth and Planetary Science Letters 144 (1996) 403-419 417
of adjacent basement areas post-dates the compres- sional emplacement of high-grade nappes. An age of
ca. 385 Ma for detrital zircons may reflect a high- grade metamorphic or magmatic event in the base- ment rocks around the Southern Vosges basin.
The high-K granite-rhyolite association is charac-
terized by crustal components of Late to Early Pro-
terozoic age: 0.6 Ga for the Molkenrain rhyolite and
2.12 Ga for the Cr&es granite; the Ballons granite, in
contrast, is nearly free of recognizable zircon inheri- tance. This mimics the difference in initial Nd iso-
topic compositions between the C&es ( eNd = - 6)
and the Ballons granite (Ed,, = -3.4) found by Langer et al. [35].
The Southern Vosges represent an example where basin subsidence and magmatism were contempora-
neous with exhumation of adjacent deep crustal units, situated in an extensional tectonic environment in the
centre of the orogen, while orogen-scale, convergent tectonics were still prevailing. This main phase of
basin evolution has been constrained to a time span of ca. 5 myr. This is the first precise time correlation
for rapid syn-convergent extension in an ancient
erogenic belt. This extensional episode of Visean age, related to high thermal gradients, magmatism
and exhumation, can be found elsewhere in the
Variscan belt and was responsible for the widespread
high-temperature/low-pressure retrogressive over- print of collisional structures and metamorphic as- semblages. The often cited modem tectonic analogue
of the Tibetan Plateau can be shown to be inappro- priate for the case of Visean syn-convergent exten-
sion. The Variscan Belt was at sea level by the Late
Devonian (Famennian, ca. 365 Ma) and probably never reached a high topographic elevation during its subsequent evolution. A comparison with the present
tectonic situation in the Aegean Sea seems to be more appropriate, where thickened continental crust
has been thinned during syn-convergence extension within a few million years.
Acknowledgements
The first author would like to thank B. Pod- stawskyj, R. Tlhiste and Y.Y. Kwok at the ROM in Toronto for support and the whole staff for friend- ship and hospitality (again!). The use of the facilities
at the Mineralogy departments of Bern and Base1
and at the Laboratory for Isotope Geology of Bern for sample preparation is kindly acknowledged. The help of V. KGppel, A. von Quad& G. Vavra and W. Wittwer, all at ETH Zurich, is highly appreciated.
Critical comments by J.P. Burg, J. Ridley and three
constructive anonymous reviews improved the
manuscript considerably.
The investigations were partly carried out during
short-time fellowships of U.S. at the ROM, sup-
ported by grants from the ‘Paul-Niggli-Stiftung’, Zurich, the ‘Stiftung Vulkaninstitut Immanuel
Friedlander’. Zurich, and ETH Zurich. These and
additional financial support by the French Geological
Survey (B.R.G.M. Orleans, C. Delor and Ph. Rossi) are all kindly acknowledged. [PTI
Appendix A. Sample descriptions
Tuffaceous layer, base of Markstein Formation, Cabane du Treh (SU-94-l): Very fine grained grey
layers (2-8 cm> in the Treh shales, overlying olis- tolithic sediments of the ‘Ligne des Klippes’. No
phenocrystic minerals. Late Devonian (Famennian)
age is indicated by conodonts [26].
Rhyolite, east of Tremontkopi Doller Valley (SU- 93-1 I): Stratigraphically lowest rhyolite, part of the bimodal volcanic suite (top of Lower Unit), interlay-
ered with arc tholeiites and shales. Massive rhyolite flow, grey in colour, with reddish spots of altered
glass components. No preferred orientation. Fine crystalline matrix with secondary calcite. Phe-
nocrysts of plagioclase and little K-feldspar; no quartz, biotite or hornblende.
Rhyolite, LLZUW quarry, Dolfer Valley (SU-93-12): Massive grey rhyolite assigned to the first group of
Upper Unit. probably of ignimbritic nature. Glassy matrix with exsolutions of Fe-Ti-oxides, flow tex-
tures. Phenocrysts of quartz. plagioclase. K-feldspar, biotite (all strongly altered) and a light green horn-
blende (actinolite).
Rhyolite to latite (‘Molkenrain rhyolite’), Co1 du Hundsruck fSU-93-7): Highest volcanic rock of the Southern Vosges basin (Upper Unit). Red ignimbritic rhyolite without preferred orientation. Sampled next to a major fault of Tertiary age, with secondary oxidation. Glassy matrix with dispersed Fe-Ti-oxides
418 U. Schaltegger et al. /Earth and Planetary Science Letters 144 11996) 403-419
and secondary calcite. Phenocrysts of completely altered biotite (chlorite + oxides), K-feldspar and
plagioclase; no hornblende. Rhyolite to latite f ‘Molkenrain rhyolite ‘). Vieil
Armand, Cantine Zeller (SU-94-5): Upper Unit; dark
grey welded ignimbrite with heterogeneous breccia-
like glassy matrix and clearly visible pumice clasts (‘fiamme’); microscopic flow or load textures. Phe-
nocrysts of quartz, biotite (altered), green horn- blende, relics of pyroxene, strongly altered plagio- clase and K-feldspar; secondary Fe-Ti-oxides.
Microgranitic dyke, Rennbaechelfels, south of Grand Ballon (SU-94-31: l-2 m thick, fine grained. grey-brownish microgranite dyke intruding into
pelites of the ‘Ligne des Klippes’; chilled margins.
Conspicuous phenocrysts of elongated biotite defin-
ing a flow texture. Attributed to the C&es granite, according to biotite morphology.
C&es granite, quarry at Metzeral, Munster Val- ley (SU-95-51: Sampled some hundred meters from
the intrusive contact, towards the sediments of the
Markstein formation. Medium grained, relatively
dark granite with thin lamellar K-feldspar phe-
nocrysts, up to 2 cm long. Dark patches consist of biotite and hornblende. Perthitic K-feldspar partially
transformed into microcline; plagioclase sericitized, quartz shows brittle deformation. Accessory miner- als: large euhedral titanite, apatite and zircon.
Ballons granite, Lac d’AJfeld, Doller Valley (SU- 9.5-6): Sampling site 200 m from the contact, to-
wards the volcano-sedimentary rocks of the Lower Unit. Coarse grained, porphyritic granite with 2-5
cm long, red K-feldspar phenocrysts. Biotite and hornblende strongly altered to chlorite, hematite and
Ti-oxides. Perthitic K-feldspar, twinned plagioclase
and quartz undeformed. Accessory minerals: Large epidote crystals, zircon, apatite and abundant titanite.
Monzodiorite, Langenfeld, west of Lac d’AIfeld, Doller Valley (SU-95-7): Relatively dark, greenish
rock, marginal intrusion of the Ballons granite. Bi- otite in large amoeboidal grains, poikilitic horn-
blende and some euhedral pyroxenes. partially re- placed by chlorite and actinolite fibres; sericitized euhedral plagioclase and xenocrystic K-feldspar, very little quartz. Accessory minerals: apatite (intergrown with opaque phases), epidote and zircon.
Diorite, east sf Chiiteau-Lambert, south of Le Thillot (W-95-15): Medium grained, greenish rock
containing large plagioclase phenocrysts, strongly
altered to sericite. Very little quartz and K-feldspar, euhedral biotite and a subhedral to euhedral green
hornblende (altered to chlorite and actinolite), host-
ing rare relicts of clinopyroxene. Accessory miner- als: apatite, abundant epidote, zircon, little titanite
and opaque phases.
References
[I] P. Rey. J.P. Burg and J.M. Caron, Middle and Late Carbonif-
erous extension in the Variscan belt: structural and petrologi-
cal evidence from the Vosges massif (eastern France).
Geodin. Acta 5, 17-36. 1992.
[2] R. Van Der Voo. Pre-Mesozoic paleomagnetism and plate
tectonics, Annu. Rev. Earth Planet. Sci. 10. 191-220, 1982.
[3] H. Ahrendt, N. Clauer, J.C. Hunziker and K. Weber, Migra-
tion of folding and metamorphism in the Rheinisches
Schiefergebirge deduced from K-Ar and Rb-Sr determina-
tions, in: Intracontinental Fold Belts, H. Martin and W. Eder,
eds.. Springer. Berlin, 1983.
[4] H. Maluski. S. Costa and H. Echtler, Late Variscan tectonic
evolution by thinning of earlier thickened crust. An 19Ar/‘“Ar
study of the Montagne Noire, southern Massif Central,
France, Lithos 26, 287-304. 1991.
[5] J. BCbien and C. Gagny, Le plutonisme viseen des Vosges
mtridionales: un nouvel exemple de combinaison magma-
tique entre roches tholCiitiques et calco-alcalines. C.R. Acad.
Sci. Paris 286/R, 1045-1048. 1978.
[6] C. Lefevre, M. Lakhrissi and J.L. Schneider, Les affinites
magmatiques du volcanisme dinantien des Vosges mtrid-
ionales (France): approche gtochimique et interpretation,
C.R. Acad. Sci.. Paris 319/II. 79-86. 1994.
[71
Bl
[91
[]Ol
[I 11
1121
M. Page1 and J. Leterrier. The subalkaline potassic magma-
tism of the Ballons massif (southern Vosges, France):
shoshonitic affinity. Lithos I?. l-10. 1980.
3.L. Schneider. Enregistrement de la dynamique varisque
dam les bassins volcano-stdimentaires devono-dinantiens:
exemple des Vosges du Sud (zone moldanubienne), Ph.D.
Thesis. Univ. Louis Pasteur, Strasbourg, 222 pp., 1990.
R. Boutin, R. Montigny and R. Thuizat. Chronologie K-Ar
et ‘9Ar/‘oAr du metamorphisme et du magmatisme des
Vosges. Comparaison avec les massifs varisques avoisinants
et determination de l’age de la limite VisCen inferieur-Vi&en
superieur. Gdol. Fr. 1. 3-25. 1995.
J.C. Hess. H.J. Lippolt and B. Kober, The age of the
Kagenfels granite (northern Vosges) and its bearing on the
intrusion scheme of late Variscan granitoids. Geol. Rundsch.
84, 568-577. 1995.
M. Bonhomme and P. Fluck, Nouvelles donnees isotopiques
Rb-Sr obtenues sur les granulites des Vosges. Age pro-
terozdique terminal de la serie volcanique calco-alcaline et
age acadien du mttamorphisme regional, C.R. Acad. Sci.
Paris 293,X 771-774, 1981.
F. Andre and J. Bibien. DCchirures continentales et pluton-
U. Schaltegger et al. / Earth and Planetag Science Letters 144 (I 996) 403-419 419
isme: etude petrologique et structurale comparee de l’associa-
tion ignee de GuevguCli (Macedoine grecque) et des forma-
tions plutoniques basiques sit&es en bordure nord du massif
des Ballons (Vosges. France). Bull. Sot. Geol. France 7.
XXV-3. 29 I-299, 1983.
[13] C. Fourquin. Contribution a la connaissance du tectorogene
varisque dans les Vosges mtridionales. I - Le Culm de la
region de Giromagny, Sci. GCol. Bull. 26/l, 3-42. 1973.
[ 141 M. Coulon. C. Fourquin. J.C. Paicheler and R. Point. Contri-
bution ‘a la connaissance du tectorogene varisque dans les
Vosges meridionales. II - Le Culm de la region comprise
entre Giromagny et Bourbach-le-Bas. Sci. Geol. Bull. 28/2.
IO9- 139. 1975.
[ 1.51 M. Coulon. C. Fourquin and J.C. Paicheler. Contribution h la
connaissance du tectorogene varisque dans les Vosges mtrid-
ionales. III - Le Culm entre Bourbach-ie-Haut et le
Molkenrain. Sci. Geol. Bull. 32/3, 117-129, 1979.
[16] P. Corsin. M. Coulon. C. Fourquin. J.C. Paicheler and R.
Point, Etude de la flore de la serie de Giromagny (Viseen
suptrieur des Vosges meridionales). Comparaison avec les
autres Bores du Culm des Vosges, Sci. GPol. Bull. 26/l.
43-68, 1973.
[17] M. Coulon, C. Fourquin. J.C. Paicheler and C. Heddebaut.
Mise au point stir 1’9ge des faunes de Bourbach-le-Haut et
sur la chronologie des differentes series du Culm des Vosges
du Sud. Sci. Gtol. Bull. 28/2. 141-148, 1975.
[ 181 M. Mattauer. Dtcouverte d’une faune viseenne p&s de Rim-
bath (Vosges meridionales). C.R. Acad. Sci. Paris 248/B,
433-435. 1959.
[19] C. Vogt, Benthonische Klein-Foraminiferen aus dem Unter-
Karbon der Siidvogesen, Neues Jahrb. Geol. Pallont.
Monatsh. 6. 363-384. 1981.
[20] G. Hahn, R. Hahn and R. Maass, Trilobiten aus dem Unter-
Karbon der Siidvogesen. Oberr. Geol. Abh. 30. I-26, 1981.
[21] R. Madss, Die Siidvogesen in variszischer Zeit. Neues Jahrb.
Geol. Pal:dont. Monatsh. 10. 61 I-638, 1988.
[22] J.L. Schneider. R. Maass, J.C. Gall and P. Duringer,
L’evenement intrdvisten dans la zone moldanubienne de la
chaine varisque d‘Europe: les don&es des formations vol-
cane-ddimentaires devono-dinantiennes du Massif Central
Francais, des Vosges du Sud (France) et de la For@t Noire
(R.F.A.), Geol. Rundsch. 78, 555-570, 1989.
[23] G.S. Odin, Geological time scale (1994). C.R. Acad. Sci.
Paris 318/B, 59-71, 1994.
[24] C. Pin and F. Carme. Ecailles de materiaux d’origine
octanique dam le charriage hercynien de la “Ligne des
Klippes”. Vosges mtridionales (France). C.R. Acad. Sci.
Paris 306/B. 217-222, 1988.
[25] J.L. Schneider. B. Hassenforder and J.C. Paicheler, Une ou
plusieurs “Lignes des Klippes” dans les Vosges du Sud
(France)? Nouvelles donntes sur la nature des “klippes” et
leur signification dans la dynamique varisque. CR. Acad.
Sci. Paris 31 l/II, 1221-1226, 1990.
[26] R. Maass and D. Stoppel. Nachweis von Oberdevon bei
Markstein (Bi. Munster, Siidvogesen), 2. Dtsch. Geol. Ges.
133, 403-408. 1982.
[27] T.E. Krogh. A low contamination method for the hydrother-
mal decomposition of zircon and extraction of U-Pb for
isotopic age determinations, Geochim. Cosmochim. Acta 37,
637-649. 1973. [28] U. Schaltegger and F. Corfu. Late Variscan “basin and
range” magmatism and tectonics in the Central Alps: Evi-
dence from U-Ph geochronology, Geodin. Acta 8. 82-98.
1995.
[29] J.P. Pupin. Zircon and granite petrology. Contrib. Mineral.
Petrol. 73. 207-220. 1980.
[30] F. Andre and C. Gagny. Proposition d’un age namurien pour
le granite porphyrdide des Ballons. temoin vosgien du plu-
tonisme g potentialhe molybdenifere au Carbonifere
auptrieur. Proc. 106th Congr. Nat. Sot. Sav., Perpignan, Vol.
111, pp. 287-296. 1981.
[31] G.H. Eisbacher. E. Liischen and F. Wickert. Crustal-scale
thrusting and extension in the Hercynian Schwarzwald and
Vosges, Central Europe. Tectonics 8, I-21. 1989.
[32] Ph. Matte. Accretionary history and crustal evolution of the
Variscan belt in Western Europe, Tectonophysics 196, 309-
337, 1991.
[33] N.B.W. Harris. J.A. Pearce and A.G. Tindle, Geochemical
characteristics of collision-zone magmatism, in: Collision
Tectonics, M.P. Coward and A.C. Ries. eds., Geol. Sot.
London Spec. Pub]. 19. 67-81. 1986.
[34] A. Bennouna. M. Pichavant and J.M. Stussi, Carboniferous
rhyolitic volcanism, Southern Vosges: Petrology, geochem-
istry and relation with plutonism. Terra Cognita Abstr. 7/2-
3. 360-361. 1987.
[35] C. Langer. R. Altherr. E. Hegner. M. Satir and F. Henjes-
Kunst. Moldanubian granitoids of the Vosges: evidence for
diverse crustal and mantle sources. Terra Abstr. 7/l. 299,
1995.
[36] P. Fluck. Mttamorphisme et magmatisme dam les Vosges
moyennes d’Alsace, Sci. Gtol. Mtm. 62, 248 pp., 1980.
[37] J.P. Burg. J. Van Den Driessche and J.P. Brun, Syn- to
post-thickening extension in the Variscan Belt of Western
Europe: Mode and structural consequences. Geol. Fr. 3,
33-51, 1994.
[38] H.P. Echtler and A. Chauvet. Carboniferous convergence and
subsequent crustal extension in the southern Schwarzwald
(SW Germany). Geodin. Acta 5. 39-49. 1992.
[39] J.B. Edel and K. Weber, Cadomian terranes, wrench faulting
and thrusting in the central Europe Variscides: geophysical
and geological evidence, Geol. Rundsch. 84, 412-432, 1995.
]40] P.A. Ziegler. Geodynamic model for the Paleozoic crustal
consolidation of Western and Central Europe, Tectono-
physics 126. 303-328. 1986.
[41] G. MCnard and P. Molnar, Collapse of a Hercynian Tibetan
Plateau into a late Paleozoic European Basin and Range
Province, Nature 334, 235-237. 1988.
]43] C. Pin. Variscan oceans: Ages, origins and geodynamic
implications inferred from geochemical and radiometric data,
Tectonphysics 177. 2 15-227. 1990.
[43] L. Jolivet, J.M. Daniel. C. Truffert and B. Gaffe, Exhuma-
tion of deep crustal metamorphic rocks and crustal extension
in arc and back-arc regions, Lithos 33, 3-30, 1994.
[44] J.S. Stacey and J.D. Kramers, Approximation of terrestrial
lead isotope evolution by a two-stage model, Earth Planet, Sci. Lett. 26, 207-221, 1975.
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