Intracrustal REE fractionation and implications for SmNd model age calculations in late-stage granitic rocks: An example from central Brazil
1991; Elsevier BV; Volume: 86; Issue: 2 Linguagem: Inglês
10.1016/0168-9622(91)90058-5
ISSN1878-5999
AutoresMárcio Martins Pimentel, Norman Charnley,
Tópico(s)High-pressure geophysics and materials
ResumoAbstract 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. SmNd 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, SmNd 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 (Israelândia 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.
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