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C. R. Geoscience 335 (2003) 1081–1089 Geochemistry (Isotope Geochemistry) An Early-Cambrian U–Pb apatite cooling age for the high-temperature regional metamorphism in the Piancó area, Borborema Province (NE Brazil): initial conclusions Bruno Dhuime a , Delphine Bosch a , Olivier Bruguier b , Renaud Caby a,∗ , Carlos Archanjo c a Laboratoire de tectonophysique, université Montpellier-2, UMR 5568–CNRS/UMII, place Eugène-Bataillon, 34095 Montpellier cedex 05, France b Service ICP–MS, ISTEEM, cc 056, université de Montpellier-2, place Eugène-Bataillon, 34095 Montpellier cedex 05, France c Instituto de Geociências, University of São Paulo, Brazil Received 28 February 2003; accepted 24 September 2003 Presented by Georges Pédro Abstract The Borborema Province (BP) of northeastern Brazil is a complex crustal assemblage, which has undergone a polycyclic evolution during the Proterozoic. In the Piancó-Alto Brígida belt, a metamorphosed leucosome vein inserted in amphibolites has a trace element pattern suggesting a T-MORB protolith. Apatites yield a REE pattern indicating growth in equilibrium with garnet, thus pointing to its metamorphic origin. U–Pb analyses yield an age of 540±5 Ma interpreted as a cooling age following amphibolite facies regional metamorphism associated with granitic emplacement at ca. 580 Ma. The resulting slow cooling rates (ranging from ca. 2.5 to 5 ◦ CMa −1 ) are consistent with underplating of mafic magmas, or crustal thickening caused by nappe stacking, as possible processes governing the metamorphic evolution of the BP. To cite this article: B. Dhuime et al., C. R. Geoscience 335 (2003). © 2003 Académie des sciences. Published by Elsevier SAS. All rights reserved. Résumé Âge de refroidissement U–Pb sur apatite du Cambrien inférieur pour le métamorphisme de haute température de la région de Piancó, province de Borborema (Nord-Est du Brésil) : premières conclusions. La Province Borborema (BP), dans le Nord-Est du Brésil, est un assemblage complexe ayant subi une évolution polycyclique durant le Protérozoïque. Dans la ceinture de Piancó-Alto Brígida, un leucosome métamorphisé, associé à des amphibolites, présente des caractéristiques d’éléments en trace, suggérant un protolithe de type T-MORB. Les apatites présentent un spectre de terres rares indiquant une croissance à l’équilibre avec le grenat, ce qui traduit une origine métamorphique. Les analyses U–Pb fournissent un âge de 540 ± 5 Ma, interprété comme un âge de refroidissement suivant le pic du métamorphisme régional daté à 580 Ma. Les taux de refroidissement faibles, compris entre 2.5 et 5 ◦ CMa −1 , sont compatibles avec des processus de sous-placage magmatique * Corresponding author. E-mail addresses: dhuime@dstu.univ-montp2.fr (B. Dhuime), caby@dstu.univ-montp2.fr (R. Caby). 1631-0713/$ – see front matter © 2003 Académie des sciences. Published by Elsevier SAS. All rights reserved. doi:10.1016/j.crte.2003.09.012
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
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