Gravity interpretation of the Egersund anorthosite complex, Norway: Its petrological and geothermal significance

The Egersund anorthosite complex consists of several bodies of anorthosite and a large syncline of layered norite and anorthosite with granitic rocks in the core. This igneous complex is emplaced in granulite-facies gneisses. These gneisses have a mean density of 2.70 g/cm3, indicative of granitic rocks. A Bouguer gravity anomaly map shows no distinctive gravity anomaly over anorthosite, but a sharp 25-mgal positive anomaly is present over the norite syncline. Gravity models indicate that the norite syncline is about 4 km thick and that relatively minor amounts of granitic gneiss are present in the core of the syncline. Anorthosite masses cannot be modeled directly but can be inferred to have a thickness of at least 4 km. Relative amounts of the rock types in the anorthosite complex are anorthosite, 70%; norite, 25%; and granitic rocks, 5%. No evidence is found for a dense mafic residuum that would be formed if the anorthosite differentiated from a basaltic magma in place. If the rocks of the complex are cogenetic, the parent magma would be noritic anorthosite, and the volume of granitic rocks is so small that it would not change the presumed composition of the parent magma appreciably. Heat flow through the anorthosite is so low (0.45 HFU) that mantle heat flow in this area can hardly be greater than 0.2 to 0.3 HFU. Granulitic gneisses or other rocks produced at low heat must compose the entire crust beneath the anorthosite complex.