Source variability in a single granitic pluton : Example from the St Sylvestre Leucogranite complex

Abstract : The Upper carboniferous Variscan St Sylvestre Leucogranite Complex (SSLC), located in the western Massif Central (Limousin, France) is composed of four main granitic facies : a biotite-only facies (Brame), a two-mica facies (St Sylvestre) ; a coarse grained two-mica facies (St Goussaud) ; a fine grained two-mica facies (Châteauponsac) emplaced as sills within the Brame facies.We studied magmatic and inherited zircons using U/Pb datations and Lu/Hf isotopic signature. Even though some zircons present cases of lead loss and/or common lead, concordant magmatic ages provide overlapping ages for each different facies between 3283 and 3273 (St Sylvestre facies) and 3203 Ma (Châteauponsac). The Brâme facies confirms an existing age of 3243 Ma and zircons from the St Goussaud facies gives a Discordia with a upper intercept at 32713 Ma. Inherited zircons and Hf isotopes reveal differences of sources between the facies. Despite age uncertainties on single zircon, a eHf(t) vs. time diagram reveals different patterns for each granitic facies. The Model ages (TDM2) for the differentiation of crustal to the depleted mantle provide a range from 1.0 to 2.5 Ga for each phase. However, the ages of inherited zircon recorded by each phase argue in favour of different sources. The Brâme and Châteauponsac facies would be related to a source with rather homogeneous U/Pb ages only ranging between 500 and 540 Ma. On the contrary, the St Sylvestre facies provide a significantly wider range of inherited ages between 480 and 660 Ma. Unfortunately the St Goussaud facies does not provide sufficiently good data to be interpreted in a similar way. These results may suggest that the St-Sylvestre leucogranitic complex is produced from the melting of a heterogeneous Cadomian crust mostly composed of orthogneiss derived from a granitic protolith of Cambrian age and Paragneisses derived from sediment deposited in a Cadomian basin collecting crustal material formed between Brioverian and Cambrian.
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Submitted on : Tuesday, March 22, 2016 - 10:01:41 AM
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  • HAL Id : insu-01291778, version 1

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Arnaud Villaros, Oscar Laurent, Michel Pichavant, Armin Zeh, Michel Cuney, et al.. Source variability in a single granitic pluton : Example from the St Sylvestre Leucogranite complex. 24ème Réunion des Sciences de la Terre, Oct 2014, Pau, France. ⟨insu-01291778⟩

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