Magnetism at Depth: A view from an ancient continental collision zone

Abstract:

The contribution to lithospheric magnetization from deep crustal rocks remains enigmatic. Based on models from satellite studies, the magnetic signature of the crust is best fitted using a susceptibility value of ~0.04 SI. Current models use crustal thicknesses from seismic and thermal studies, and then attempt to fit satellite magnetic data using a magnetic response in the direction of the present day field. This susceptibility is significantly higher than is found in many continental rocks. It is an open question whether there are more magnetic rocks at depth, if parts of the mantle could contribute, or if assumptions in the model are incomplete.

The magnetic response of deep crustal rocks is strongly controlled by the amount and oxidation state of magnetic minerals, thermal structure of the crust, whether fluids were available during metamorphism, and at what Ts and Ps reactions occurred. Subduction zones have been proposed as areas of enhanced magnetic signal due to production of magnetite, however high-grade metamorphism to eclogite facies may destroy magnetic minerals as Fe is absorbed into garnet. Contributions from mantle rocks would depend on the compositions of olivine and pyroxene, fluid availability, and PT regimes capable of producing magnetite.

1450 samples from mafic and ultramafic bodies in the Western Gneiss Region (WGR), Norway, >300 from the Leka Ophiolite, and >350 of ultramafic rocks from the Seiland Igneous Province were studied in the Scandian collision zone. They show strong variations in petrophysical properties, oxide mineralogy and assemblages. Four groups of WGR samples, represent different metamorphic conditions and crustal depths: 1) Garnet-corona gabbros considered to have reached 600-750^oC and 1-1.5 Gpa, equivalent to depths up to 60 km; 2) Normal eclogite-facies rocks that reached 700-750^oC and 1.5 -2.5 Gpa, equivalent to depths of 60-90 km; 3) Ultrahigh-pressure eclogite-facies rocks that reached 750-850^oC and 2.5-4 Gpa, equivalent to depths 90-120 km; 4) Garnet peridotites that may have reached 850-900^oC and up to 6.5 GPa, equivalent to depths 200 km. In addition, samples from the partially serpentinized Leka Ophiolite, and Seiland ultramafic rocks (crystallized ~1500^oC) are compared for magnetic properties, mineralogy and contributions to crustal magnetism.