THE HESKESTAD ANOMALY OF THE BJERKREIM-SOKNDAL LAYERED INTRUSION, NORWAY: TRACKING THE SOURCE OF EXTREME MAGNETIC REMANENCE

The 7000m-thick, 930 Ma, Bjerkreim-Sokndal Layered Intrusion, with a present area of 250 km², lies in a tight, doubly plunging syncline produced during high-temperature solid-state deformation. It contains six magmatic megacyclic units, each beginning with intrusion of a noritic magma, which, over the entire episode, gradually became more reduced and more mafic. Typically each megacycle began with intrusion of new magma that mixed with more evolved magma already in the chamber, temporarily producing plagioclase- and hemo-ilmenite-rich cumulates. Later the magma returned to normal fractional crystallization, leading to evolved compositions, and precipitation of Fe-richer silicates, Ti-richer ilmenite, and magnetite.

The dominant oxides in the layers result in negative remanent magnetic anomalies over hemo-ilmenite–rich cumulates and positive induced anomalies over cumulates dominated by magnetite and Ti-rich ilmenite. Magnetic contrasts are most striking over the top megacyclic Unit IV, where a negative remanent anomaly traces for >15 km in the northern Bjerkreim Lobe of the intrusion. Here magnetic intensity in the trough of the ground anomaly varies, depending on position in the south-plunging syncline. At the northern hinge, where layering and foliation dip south, the trough is at 47,600nT in an Earth field of 50,400nT at this latitude. On the east limb at Heskestad, 15 km away, where the distal edge of Unit IV of the layered series abuts against the gneissic basement below the intrusion, layering and foliation are vertical, parallel to the reversed early Neoproterozoic magnetizing field. Here the ground anomaly locally decreases to 19,850nT, canceling out >60% of the Earth field, though returning to average Earth values barely 150m higher in the cumulate section.

Measurements on typical Heskestad samples yielded mean induced magnetization 4.5 A/m, NRM 23 A/m, Q=5.1. The origin of the extreme anomaly remains enigmatic, potentially involving hemo-ilmenite, pyroxenes with abundant oriented exsolutions of hemo-ilmenite and rare magnetite, and abundant multi-domain magnetite. Structural position appears to be important. Strong lattice-preferred orientations measured by EBSD show hemo-ilmenite (001) planes and pyroxene c-axes lie quasi-parallel to the Proterozoic magnetizing field.