Rare-earth elements distribution in granulite-facies marbles: a witness offluid-rock interaction

The rare earth element (REE) distribution of marbles from Tranomaro (SE Mad agascar) shows that the marbles have interacted with syn-metamorphic fluids during the Panafrican granulite-facies metamorphism. The Tranomaro area is characterized by an extensive development of pyroxenites (skarns) variably mineralized in uranothorianite and enriched in REE, Zr. Across a meter-sca le marble-pyroxenite contact, the REE content increases From the most remot e marble sample to the contact with the pyroxenite (La from 19.8 to 129 ppm ). REE patterns in enriched marbles display a strong negative Eu anomaly si milar to those of pyroxenites. This peculiar type of REE distribution resul ts from fluid infiltration. On a regional scale, some marbles have similar REE characteristics (progressive development of Eu anomaly together with RE E enrichment) showing that some of them have interacted with fluids. Infilt ration was hardly recognizable using C and O isotopic signatures because la rge isotopic variations were introduced during the pre-granulitic history. Then, REE distribution in high-grade marble may be helpful in monitoring sy n-metamorphic fluid flow. In the present case, it is a more reliable tracer of fluid infiltration than stable isotopic compositions. (C) 2000 Elsevier Science B.V. All rights reserved.