The Effects of Crystal Fractionation and Magma Mixing on Remanent and Induced Magnetic Anomalies over a Layered Intrusion

The Bjerkreim-Sokndal layered intrusion lies in the Rogaland Igneous Complex (~930 Ma) within the Baltic Shield in southern Norway. This 7 km-thick intrusion is divided into six Megacyclic units topped by mangerite and quartz-mangerite units. The course of crystal fractionation punctuated by the influx and mixing of more primitive magmas produces sequences of early plagioclase norites, intermediate hemo-ilmenite norites, and late magnetite-rich norites with subordinate ilmenite. Oriented samples were collected from 46 sites through the stratigraphy of the intrusion and subjected to petrophysical, paleomagnetic and rock magnetic measurements. Magnetic properties show a large range of values, with susceptibilities ranging from 1.47x10-4 to 2.15x10-1 SI, NRM intensities ranging from 0.104 to 58.8 A/m and corresponding Q values of 0.1 to 85. When induced and remanent magnetizations are averaged for each subdivided mega-unit a pattern of remanence-dominance at the base to induced-dominance at the top of each cycle is clear. Hysteresis properties indicate PSD to MD size magnetites with a continuous trend between them, indicative of the magnetite-rich rocks. Hysteresis properties falling outside the magnetite PSD-MD ranges are interpreted as hemo-ilmenite samples, in good agreement with the observed oxide mineralogy. Distinctive differences in the magnetic mineralogy also shows up in demagnetization behavior. Thermal plots show either a loss of magnetization at 580C, or above 600C. AF demagnetization plots show two separate populations - one with high coercivity (hemo-ilmenite) and one with low coercivity (magnetite). Magnetic anomalies over the body correspond directly to the magnetic properties, with positive (induced) anomalies over the magnetite-rich layers and magnetic lows (due to reversed magnetic signal) over layers with hemo-ilmenite present.