Heat generation versus depth of crystallization for Norwegian monzonitic rocks

Thorium, U and K analyses by γ-ray spectrometry of monzonitic intrusives from Lofoten-Vesterålen (ca. 1800 m.y.), Bjerkreim-Sogndal (ca. 1000 m.y.) and the Oslo region (ca. 275 m.y.) are used to calculate values of radioactive heat generation. A strong negative correlation between heat generation and depth of crystallization is demonstrated, indicating that the chemical zoning of the continental crust with respect to Th and U is of a rather regional and regular character, and was probably firmly established at least 1800 m.y. ago. The K contents, in contrast to Th and U, show an increase with depths of crystallization for these rocks. This may be explained by considerations of the pressure dependence of the partition coefficients for these elements between minerals and magma, and the importance of a fluid phase as a transport medium for the incompatible elements Th and U. A positive correlation between K content and age of intrusion is discussed on a tentative basis.