U, Th, and K Distribution in a Differentiated Charnockite-Granite Intrusion and Associated Rocks from SW Sweden

J. L. Constable* and F. H. Hubbard
Department of Geology, The University, Dundee, DD1 4HN, Scotland
*Present address: Department of Geology, University of Keele, Keele, Staffordshire.

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.

Mineralogical Magazine; December 1981 v. 44; no. 336; p. 409-415; DOI: 10.1180/minmag.1981.044.336.06
© 1981, The Mineralogical Society
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