Trace-element abundances of the Holterkollen pluton complex, Oslo area, Norway

Abstract

The behaviour of Rb, Cs, Ba, Sr, U, Th, Zr, Hf, Ta, Sc and Y in the Holterkollen plutonic complex as a function of increasing differentiation index in the granite—quartz porphyry series and in the akerite—syenite—ekerite series is found to be of great geochemical importance. The present data indicate that the Holterkollen granites probably belong to two different types: differentiated and highly differentiated granites. The ekerites and the highly differentiated granites are characterised by high values of Rb, Th, U, Ta and Zr and low values of Ba, Sr, Sc and Y. However, the ekerites are much depleted in Sr and Ba. Variation in Ba/Rb ratios for the analysed samples gives a reasonable correlation, while there is a wide range of variation in Ba/K ratios and this is due to Ba enrichment, especially in the akerites and syenites. The highly differentiated granite with low K/Rb ratio represents the result of extreme differentiation of a granitic magma, whereas the akerite and syenite with a high K/Rb ratio are deemed to have undergone little differentiation of probably syenodioritic magma, and accordingly are of an early age of formation. The late differentiate of the latter magma will undoubtedly produce the ekerite.