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Sm-Nd isotopic study of garnets and their metamorphic host rocks

Published online by Cambridge University Press:  03 November 2011

O. van Breemen
Affiliation:
Isotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU, Scotland.
C. J. Hawkesworth
Affiliation:
Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, England.

Abstract

In this feasibility study, Sm/Nd and 143Nd/144Nd ratios have been determined on garnets and coexisting mineral and whole-rock phases from a selection of amphibolite facies rocks, granulite facies rocks and an eclogite. In two micaceous schists and an eclogite the distribution coefficient of the Sm/Nd ratio in garnet relative to other phases is in the range 2-3, i.e. comparable to values for phenocrysts in igneous rocks. In two granulite facies rocks the garnets have Sm/Nd distribution coefficients relative to other phases in the range 13-20 and have yielded precise ages.

Garnet from a granulite facies shear zone from the Limpopo belt in southern Africa, of Archaean age, has yielded concordant duplicate ages of 1974±14 Ma and 1988±14 Ma. These results have been interpreted as relating to the last tectonic event in the region, long after the main gneiss-forming event. Garnet from a granulite facies xenolith from a Carboniferous vent in the Midland Valley of Scotland has yielded an age of 356±10 Ma; this is related to the onset of basic magmatism in the region.

Type
Research Article
Copyright
Copyright © Royal Society of Edinburgh 1980

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