Intracrustal REE fractionation and implications for SmNd model age calculations in late-stage granitic rocks: An example from central Brazil

Abstract

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 ( tDM) 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 Goiás (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 ( tDM) which do not reflect differences in source rock ages. Microprobe studies on accessory minerals from two granitic plutons (Israelândia and Iporá 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 tDM- values with progressive differentiation. Progressive allanite segregation can produce a large range of Sm-Nd model ages, unrelated to the age of crust formation.