Aeromagnetic anomalies, magnetic petrology, and rock magnetism of hemo-ilmenite- and magnetite-rich cumulate rocks from the Sokndal Region, South Rogaland, Norway

Aeromagnetic maps of the Egersund Mid-Proterozoic igneous province show a s pectacular range of positive and negative magnetic anomalies with a contras t up to 15 600 nT. The, positive magnetic anomalies are over magnetite nori tes and overlying mangerites and quartz mangerites of the Bjerkreim-Sokndal layered intrusion. These rocks are dominated by multi-domain (MD) magnetit e. The negative magnetic anomalies are over ilmenite-rich norites of the Bj erkreim-Sokndal layered intrusion, the Tellnes ilmenite norite ore deposit, and massif anorthosites. These rocks are dominated by hemo-ilmenite and/or by silicates containing fine-grained oxide exsolution lamellae. Electron m icroprobe analyzes of coexisting Fe-Ti oxides in the layered intrusion conf irm earlier observations that oxides in early magmatic rocks are dominated by hemo-ilmenite with minor end-member magnetite, followed by more reduced oxides dominated by titanomagnetite with minor near end-member ilmenite. Wh at is not fully understood is the property of ilmenite with hematite exsolu tion lamellae, or, even more striking, hematite with ilmenite lamellae, to produce strong remanent magnetization of high coercivity and with a Neel te mperature equal to or above the Curie temperature of magnetite. This proper ty makes the rhombohedral oxides an important candidate to explain some hig h-amplitude deep-crustal anomalies on earth, or strong remanent magnetizati on on other planets. A remarkable feature in the Egersund province is that primitive magmas produced rocks rich in hemo-ilmenite causing negative magn etic anomalies related to magnetic remanence, and more evolved magmas produ ced rocks rich in magnetite related to positive induced magnetic anomalies, all in the course of crystallization-differentiation.