Rare earth, Th-Hf-Ta and large-ion lithophile element variations in metabasites from the Proterozoic amphibolite-granulite transition zone at Arendal, south Norway

Details Rare earth, Th-Hf-Ta and large-ion lithophile element variations in metabasites from the Proterozoic amphibolite-granulite transition zone at Arendal, south Norway Rare earth, Th-Hf-Ta and large-ion lithophile element variations in metabasites from the Proterozoic amphibolite-granulite transition zone at Arendal, south Norway

Jun 1983

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983e%26psl..63..446s&link_type=abstract

Earth and Planetary Science Letters, Volume 63, Issue 3, p. 446-458.

Computer Science

4

Scientific paper

The Proterozoic Arendal metabasites are minor intrusions which possess wholly metamorphic textures and mineral assemblages. Data are presented for eight selected samples.
Chondrite-normalised REE patterns are relatively flat, between 10 and 20 × chondrite abundances. They are characterised by 1 <= CeN/YbN < 4, but with CeN/NdN <= 1. Normalised to ``primordial mantle'' values, the incompatible elements reveal variable enrichment in the large-ion lithophile elements, except Th, and low Ta. They are similar to many destructive plate margin basalts.
On a Th-Hf-Ta diagram, the rocks plot in a sub-linear trend reflecting variation in Th/Hf. Samples with high Th/Hf ratios come from the zone of highest metamorphic grade, where the host gneisses are deficient in Th: consequently, the patterns in the metabasites could not have been produced by any secondary metamorphic fractionation, or by bulk contamination by the country rocks.
There is no evidence for any post-magmatic mobility of any of the elements except Rb, which is depleted in the highest-grade rocks relative to their lower-grade counterparts. The trace element distributions are largely reflecting those of the parental magmas, but consideration of bulk distribution coefficients shows how they may have been modified in some aspects (e.g. CeN/NdN) by the imposition of the metamorphic mineralogies. The data are consistent with a model involving: (a) derivation from a mantle source region which had been affected by subduction-related hydrous metasomatism or partial melting, and (b) crystallisation of the magmas directly under the high-grade conditions.

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