BIASED ND-MODEL AGES IN HIGHLY EVOLVED GRANITES - AN EXAMPLE FROM THE BUDDUSO INTRUSION (NE SARDINIA, ITALY)

Nd-model ages calculated with respect to a depleted mantle source (T(DM)) have been widely applied to granitoids in order to date events of mantle-crust differentiation, often referred as "crustal residence ages". The basic assumption for a Nd-model age to represent a crustal residence age is that intracrustal processes such as interactions with fluids, partial melting and fractional crystallization cause little or no fractionation of the Sm - Nd pair. The present study reports on the geochemistry and Sm - Nd isotopes of leucogranites from the Budduso' intrusion, belonging to the Hercynian orogeny in Northern Sardinia (Italy). It is shown that fractionation of a LREE-rich mineral phase as allanite causes a significant variation of the Sm/Nd ratio in the residual melt, thus inducing biased Nd-model ages which have no direct meaning of crustal residence ages.