Mass estimates of cumulates and residues after anatexis in the Oslo graben

Abstract

The magmatic province of the Permian Oslo Rift, southeast Norway, is dominated by rocks of monzonitic to granitic composition. Most of the monzonitic rocks are believed to have formed by fractional crystallization from mantle‐derived magmas, whereas melts produced by anatexis in the lower crust are important to the formation of the syenitic and granitic rocks. These processes have left dense cumulates and dense residues after melting in the crust. The surplus masses of cumulates and residues at depth in the crust can be estimated by using petrological and geological information on the rocks exposed today. Quantitative estimates for the Oslo Graben are comparable to corresponding data derived from geophysical crustal models. The petrological/geological estimates suggest excess masses of 7–10×1010 kg per linear meter (assuming prismatic bodies) along the rift axis in the southern, and 5–7×1010 kg/m in the northern part of the Oslo Region. These estimates are dependent upon the volume proportion of trapped liquid in the cumulates, and the ratio of cumulates to residues after anatectic melting in the crust; the amount of trapped liquid appears to be low (less than 25%). The results agree well with estimates by Wessel and Husebye (1986) of excess mass in the crust based on gravimetric data, which gave 5–7×1010 kg/m. This agreement testifies to the validity of making mass estimates on the basis of petrological and geological data. The data bear on the composition and evolution of the upper lithosphere in the graben area; some cumulates may reside in the subgraben mantle.