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