Continental crust formation seen through the SR and Nd isotope systematics of S-type granites in the Hercynian belt of western France

The isotopic composition of Sr, Nd and Pd in leucogranites which are intercorrelated (Bernard-Griffiths et al., 1985) may be explained by the mixing of ancient basement 1800 Ma) with juvenile crust (late Precambrian or early Palaeozoic). This hypothesis does not involve the existence of Mid-Proterozoic crust, as apparently indicated by the TDM model ages of the leucogranites (ranging between 1600 and 1100 Ma). The Nd isotopes reveal the crustal reworking while Sr isotopes mainly record juvenile crust formation. This paradox is explained by the geochemical heterogeneity of the sources involved. (orig.)