Precise U Pb chronometry of 345-340 Ma old magmatism related to syn-convergence extension in the...

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 intruded 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 metamorphic 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 deformation 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 mantle 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 during 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 temporal 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 rhyolites, 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

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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 rhyolite’. 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 rhyolite’, 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 contamination during analysis. Analyses 24, 26 and 29 show secondary lead loss; analysis 26 an additional inherited 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 influenced 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 concordant 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 related 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, however, commonly host inherited cores. The cores reflect several generations of zircon growth, visible in cathodoluminescence, although the isotopic data indicate 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 revealed 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 sediments at the base of the Middle Unit indicate reworking 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 deposits 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 volcanism, 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 therefore started in the Late Devonian (Famennian), ca.

365 Ma ago, characterized by slow marine or turbiditic 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 associated 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 sediments 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 sedimentary 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 components. 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 extensional domain situated between large dextral strike-

slip faults [39,40], which accommodated the dextral

slip component of oblique compression. The extension 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 number 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 metamorphism 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-alkaline 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 granitoid 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 basement 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 inheritance. 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 overprint 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 olistolithic 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 hornblende, relics of pyroxene, strongly altered plagioclase 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 minerals: 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 minerals: apatite, abundant epidote, zircon, little titanite

and opaque phases.

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Precise U Pb chronometry of 345-340 Ma old magmatism related to syn-convergence extension in the...

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