INTRACRUSTAL REE FRACTIONATION AND IMPLICATIONS FOR SM-ND MODEL AGE CALCULATIONS IN LATE-STAGE GRANITIC-ROCKS - AN EXAMPLE FROM CENTRAL BRAZIL

The light rare-earth elements (LREE) Sm and Nd are generally regarded as behaving coherently during intracrustal processes such as weathering, metamorphism, partial melting and fractional crystallization. Sm-Nd model ages calculated with respect to a depleted mantle reservoir (t(DM)) are often interpreted by assuming that Sm and Nd are not substantially fractionated within the crust. In consequence, Sm-Nd model ages are commonly considered as crust formation ages which see through crustal reworking episodes and indicate the age of extraction of continental material from the mantle. However, major-element and REE data on late Proterozoic K-rich granitic rocks from southwestern Goias (central Brazil) reveal major fractionation of Sm and Nd with differentiation of the granitic suites. Late-stage differentiates have high Sm/Nd ratios and yield Sm-Nd model ages (t(DM)) which do not reflect differences in source rock ages. Microprobe studies on accessory minerals from two granitic plutons (Israelandia and Ipora granites) show that a significant proportion of their total REE is carried by allanite, which is relatively depleted in Sm compared to the other four LREE analyzed (La, Ce, Pr and Nd). Crystallization of a strongly LREE-enriched phase such as allanite results in the progressive depletion of the residual magma in LREE and a consequent increase of Sm/Nd ratios and t(DM)-values with progressive differentiation. Progressive allanite segregation can produce a large range of Sm-Nd model ages, unrelated to the age of crust formation.