Rare-earth element geochemistry of regional metamorphic rocks

Abstract

A considerable body of data on the distribution and behaviour of rare-earth elements (REE) in igneous and sedimentary rocks has been accumulated over the past decade. Metamorphosed and metasomatized rocks have by comparison received scant attention. Some investigators have reported REE mobility, but most of the available data suggests the REE levels and patterns are essentially unaffected by metamorphism. However, the hypothesis of REE “immobility” during metamorphism has not been rigorously tested. We present new evidence from amphibolite and granulite facies terrains which bears upon the mobility of rare-earth elements during high-grade metamorphism. Metasomatic zones provide an excellent test of element mobility. Examples are described from an amphibolite facies terrain in which these zones have retained their original REE chemistry, as well as their Ti, P, Y, Zr, Hf, Nb and Ta contents. In the associated metavolcanic suite strong linear correlations exist between the REEs and the other “immobile” elements. Such correlations are frequently explained in cogenetic volcanic suites as being produced by fractional crystallization. It seems unlikely that element migration during metamorphism would preserve such coherent behaviour of elements with different chemical properties. We conclude that in these cases metamorphism and metasomatism has affected neither the REE abundances and patterns of this metavolcanic suite, nor its Ti, P, Y, Zr, Hf, Nb or Ta content. Only these elements provide a reliable guide to the parentage of the metasomatized volcanics. By contrast, our work on Archean granulite-facies gneisses from the Lewisian of N.W. Scotland shows that these deep crustal rocks are depleted in REEs (except Eu) by a factor of 2 or 3 relative to upper crustal values. A distinct positive europium anomaly supports the idea of preferential retention of Eu in the deep crust proposed by Jakeš and Taylor (1974) and Nance and Taylor (1976). Mineral REE patterns in the granulites and their distribution coefficients closely resemble those reported for phenocrysts in dacites, suggesting that the minerals equilibrated with a granodioritic liquid. The trace and major element chemistry of this terrain is compatible with the model of a granulite facies residuum, left after the generation of a granodioritic upper crust. Rare-earth elements provide a promising new tool in the determination of the parentage of metamorphosed and metasomatized rocks up to the upper amphibolite facies of regional metamorphism and may prove valuable in assessing the importance of anatexis in the evolution of the lower continental crust.