C. Le Carlier De Veslud, Paul Alexandre, Michel Cuney, Gilles Ruffet, Alain Cheilletz, Denis Virlogeux
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 :
i. plateau ages, which provide robust ages for the amphiboles of samples 104, 106, 112, and biotites from samples 106, 115 and 212t ;
ii. pseudo-plateaux ages : three biotites (samples 104, 105 and 112) display spectrum shapes that could be interpreted as resulting from 39Ar recoil, related to an incipient chloritisation [Ruffet et al., 1991 ; McDougall and Harrison, 1999]. The most reliable ages are therefore close to the apparent ages given by the high temperature steps ;
iii. 40Ar* excess, as suggested by the spectrum shape of the amphibole from sample 212t [McDougall and Harrison, 1999]. The preferred age is defined with 83 % of released gas, and has been confirmed by a duplicate analysis ;
iv. a meaningless spectrum has been obtained on the amphiboles from sample 115. A duplicate analysis provided an approximate age of 347 ± 1 Ma, calculated on a relatively flat segment of the age spectrum.
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 :
i. a fast exhumation episode (several mm/y) during the 350-340 Ma period. This model is not acceptable because it is incompatible with pre- and post-intrusion conditions constrained by thermo-barometric data ;
ii. a fast thermal equilibration of the complex with surrounding rocks at the end of a succession of nearly-synchronous emplacement of calk-alkaline intrusions. First-order numerical models were used to simulate the thermal equilibration of the intrusive bodies with surrounding rocks, assuming a purely conductive heat regime [Carslaw and Jaeger, 1959]. These models show that according to the size of intrusions, the thermal equilibrium with surrounding rocks is reached in less than 5 to 10 m.y. The calculated temperature gradients derived from these models are compatible with those deduced from 40Ar/39Ar ages.
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.
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