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U, Th, and K distribution in a differentiated charnockite-granite intrusion and associated rocks from SW Sweden

Published online by Cambridge University Press:  05 July 2018

J. L. Constable  and
F. H. Hubbard
J. L. Constable
Affiliation:
Department of Geology, The University, Dundee, DD1 4HN, Scotland
F. H. Hubbard
Affiliation:
Department of Geology, The University, Dundee, DD1 4HN, Scotland
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Abstract

The concentrations of U, Th, and K in a charnockite-alkali granite intrusive complex from SW Sweden and associated, partly charnockitized country rocks are presented. The charnockitic components have constant low U and Th levels throughout the compositional range with a mean Th/U ratio of 1.2 to 1.5. The transition from charnockite to granite is marked by a sharp increase in Th and U concentrations with mean Th/U ratios increasing to 3.4 4.4. However, the K concentration increases relatively smoothly with increasing acidity through the compositional range studied. Late shearing of the alkali granite led to significant Th depletion with little change in the U and K concentrations. Charnockitization of the country rock granite gneisses is marked by depletion in Th and K while the U content is almost unchanged.

It is suggested that the behaviour of U and Th reflects the tendency for Th to associate with magma-dissolved water whilst U shows a greater affinity for coexisting free volatiles. In the absence of a significant free volatile phase during crystallization of this charnockite-granite suite, U was incorporated into primary mineral lattice sites whereas Th entered sites from which it could be liberated more easily.

Type
Research Article
Information
Mineralogical Magazine , Volume 44 , Issue 336 , December 1981 , pp. 409 - 415
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1981

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Footnotes

*

Present address: Department of Geology, University of Keele, Keele, Staffordshire.

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