• journal_title：Bulletin de la Societe Geologique de France
• Contributor：Christian Le Carlier De Veslud ; Paul Alexandre ; Michel Cuney ; Gilles Ruffet ; Alain Cheilletz ; Denis Virlogeux
• Publisher：Societe Geologique de France
• Date：2004-
• Format：text/html
• Language：en
• Identifier：10.2113/175.2.147
• journal_abbrev：Bulletin de la Societe Geologique de France
• issn：0037-9409
• volume：175
• issue：2
• firstpage：147
• section：Articles

The eastern part of the “Seuil du Poitou” area had been selected by the French National radioactive waste management agency (ANDRA) as a potential site for building an underground laboratory in granitic rocks. 17 cored bore holes, completed by petrographical, geochemical [Cuney et al., 2001], geophysical [Virlogeux et al., 1999] and structural [Gros and Genter, 1999] studies, have provided a detailed knowledge of the Charroux-Civray complex, hidden under a Jurassic sedimentary cover. Three main types of magmas were distinguished : medium-K calk-alkaline, high-K calk-alkaline and peraluminous. The first two types are largely predominant and belong to the “Limousin Tonalitic line” (LTL) [Peiffer, 1985 ; 1986]. They were emplaced between 356 ± 5 Ma to 349 ± 5 Ma from U-Pb dating of zircon [Bertrand et al., 2001], at a depth of 14 ± 2 km [Freiberger et al., 2001].

This work aims to reconstruct the thermochronological evolution of the Charroux-Civray complex from ⁴⁰Ar/³⁹Ar dating of biotite and amphibole, combined with available U/Pb ages [Bertrand et al., 2001] and thermo-barometric data derived from mineral paragenesis and fluid inclusions [Freiberger et al., 2001]. This reconstruction may provide interesting constraints on a stage not well understood in the evolution of the French Massif Central : the emplacement of the LTL granitoids. The datings were performed on alteration-free, single grain of biotite and amphibole from the main petrologic types, according to the procedure described by Ruffet et al. [1991] and [1995]. The closure temperatures of the isotopic systems have been assumed to be 300 ± 30 ^oC for biotites, 500 ± 50 ^oC for amphiboles, and 850 ± 50 ^oC for zircons [Villa, 1998 ; McDougall and Harrison, 1999]. Six samples were dated : two tonalites (samples 112 and 212t), a monzogranite (sample 106), a monzogabbro-diorite (sample 115), a monzodiorite (sample 104), and a granodiorite (sample 105). Some of the analyses have been performed twice to test the reproducibility of the ⁴⁰Ar/³⁹Ar measurements.

The 14 age spectra obtained may be divided into four groups :

These results show that : (1) the closure of the isotopic system of the amphiboles occurred at approximately the same time over the entire complex (about 348 Ma) ; (2) the closure of the isotopic system of the biotites occurred slightly after the closure of the amphiboles, but spread over a larger time interval (350–343 Ma), (3) all the samples display high temperature gradients between 500 and 300 ^oC (> 40 ^oC.my-1). These results are in good agreement with mineralogical and fluid inclusion thermo-barometric data [Freiberger et al., 2001]. Two scénarios may be invoked to explain such high temperature gradients :

These data confirm the existence of a major calk-alkaline magmatic event on the Seuil du Poitou, at about 355–350 Ma, which would be synchronous with the emplacement of the large peraluminous Guéret-type granodiorites in the northern Limousin. The ⁴⁰Ar/³⁹Ar biotite ages indicate that a regional temperature of 250–300 ^oC was reached at ca. 340 Ma at a depth of about 9 km.