Magnetic Properties of Rocks of the Kapuskasing Uplift (Ontario, Canada) and Origin of Long-Wavelength Magnetic Anomalies

The sources of long wavelength magnetic anomalies (LWA) in the crust are poorly understood. We have measured remanent and induced magnetizations of 210 samples of anorthosite, tonalite and mafic gneiss from the Kapuskasing uplift, an exposed partial crustal cross-section in northwestern Ontario, Canada. Anorthosites have generally high Q ratios of remanent/ induced magnetization, in the range 0.3-60, and their natural remanent magnetization (NRM) is resistant to both thermal and alternating-field (AF) demagnetization. However, anorthosite NRMs (0.001 to 0.3 A/m) are too weak to explain LWA amplitudes. Mafic gneisses also have relatively high Q ratios, peaking in the range 1-10, and the NRM is resistant to thermal and AF demagnetization. NRM and induced magnetization (IM) intensities are in the ranges 0.01-2 and 0.01-0.6 A/m, respectively. Tonalites have a bimodal distribution of magnetization. The more strongly magnetic group has both NRM and IM intensities in the range 0.1-5 A/m and wide-ranging Q values, from 0.1-10 approximately. Some tonalites could be an LWA source, although the long-term stability of their NRMs at high temperature in the crust is questionable because unblocking temperatures are broadly distributed from 100 to 600°C. In general, Q values measured at surface temperatures overestimate remanence at depth. In the deep crust, IM remains more or less constant but remanence decreases both reversibly and irreversibly, leading to Q ratios of <0.2-0.3 for multidomain grains and ≈1-3 for single-domain grains near their blocking temperatures. Thermoviscous magnetization over the Brunhes chron could add substantially to the effective induced magnetization. Typically induced + thermoviscous magnetization in the direction of the present Earth’s field will outweigh remanence in the direction of an ancient field as a source of LWA originating in the deepest crust. Remanence may play a larger role for mid-crustal sources where single-domain grains are well below their blocking temperatures. A compilation of NRM and IM values for our samples, with Qn as a parameter. Samples with Qn > 1 are remanence-dominated at room temperature but not necessarily at their original crustal depths.