Géochronologie U-Pb par ablation laser et ICP-MS (LA-ICP-MS ...

Int J Earth Sci (Geol Rundsch) (2003) 92:338–347
DOI 10.1007/s00531-003-0321-3
ORIGINAL PAPER
O. Bruguier · J. F. Becq-Giraudon · N. Clauer ·
H. Maluski
From late Visean to Stephanian: pinpointing a two-stage basinal
evolution in the Variscan belt. A case study from the Bosmoreau basin
(French Massif Central) and its geodynamic implications
Received: 3 April 2002 / Accepted: 9 February 2003 / Published online: 17 April 2003
Springer-Verlag 2003
Abstract Post-convergence evolution of the Variscan
belt is characterized by the development of intramontane
coal-bearing basins containing volcano-sedimentary successions.
In the French Massif Central, K–Ar ages on clay
particles from fine-grained sediments of the Bosmoreau
basin (Limousin area), help pinpoint the evolution of the
basin. In the lower part of the sedimentary pile, illite in a
siltstone underlying a volcanic layer previously dated at
332€4 Ma by the U–Pb method on zircon, yields a
consistent K–Ar age of ca. 340 Ma. Upward in the
sedimentary succession, illite yields Stephanian K–Ar
ages, which can be combined to provide a mean
deposition age of 296.5€3.5 Ma. The Bosmoreau basin,
albeit mainly filled with Stephanian deposits, was initiated
during the late Visean, i.e. ca. 30 Ma earlier than
inferred from biostratigraphical constraints. During the
Stephanian, the same structure was reactivated and late
Visean deposits were eroded and subsequently blanketed
by thick clastic sediments. These results emphasise a twostage
evolution for the Bosmoreau basin, which is closely
related to extensional tectonics identified on basement
country rocks, and they are used to propose a geodynamic
evolution of the studied area.
O. Bruguier () )
Service ICP-MS, cc 056, ISTEEM,
UniversitØ de Montpellier II, Place E. Bataillon,
34095 Montpellier, France
e-mail: bruguier@dstu.univ-montp2.fr
J. F. Becq-Giraudon
BRGM, 3 Avenue C. Guillemin, BP 6009,
45060 OrlØans, France
N. Clauer
CGS-EOST, UniversitØ Louis Pasteur-CNRS,
1 rue Blessig, 67084 Strasbourg, France
H. Maluski
Laboratoire de GØophysique, Tectonique et SØdimentologie,
CNRS-UMR 5573, ISTEEM, UniversitØ de Montpellier II,
Place E. Bataillon, 34095 Montpellier, France
Keywords Delamination · French Massif Central ·
Intramontane basins · Stephanian · Visean
Introduction
Extensional tectonics is preferentially located along
orogenic belts with a thickened crust and collapse of
mountain belts represents an important feature of postcollisional
orogenic stages (e.g. Ratschbacher et al. 1989).
Implicit to this is the creation of a pervasive series of
continental basins accompanying extension in the upper
crust. Whereas most studies addressing evolution of
orogenic belts usually deal with basement and deep
crustal processes (Faure and Pons 1991), an alternative
approach, whenever possible, is to focus on sedimentary
records and to use sedimentary basins as tectonic markers,
considering that their formation mirrors deeper processes
(Zoback et al. 1993). Basin opening and sedimentary
infilling are often tectonically controlled (e.g. Bruguier et
al. 1997). A key issue, therefore, is to determine precisely
both the age of basin formation and the main stages of
infilling, and the relationships between basin initiation
and tectonic structures such as major fault systems. These
parameters potentially carry important information that
have implications for understanding the tectonic control
on the sedimentary record, and to punctuate the different
stages of extensional tectonics. The implied requirements
for such an approach to be fully operative are tightly
related to the possibility of selecting suitable sedimentary
records and appropriate chronological methods.
The French Massif Central is one of the most
important exposures of the Internal Zone of the Variscan
Belt, which extends along ca. 3,000 km from the Iberian
Massif in the West to the Bohemian Massif in the East
(Fig. 1). In the whole belt, the Late Carboniferous–Early
Permian time interval is characterized by numerous coalbearing
intramontane basins corresponding to isolated
troughs closely associated with fault-zones and filled with
coarse, clastic, non-marine sediments deposited unconformably
on the metamorphic and igneous basement. The