MAGNETIC PROPERTIES RELATED TO A PRONOUNCED NEGATIVE AEROMAGNETIC ANOMALY, MORIN ANORTHOSITE COMPLEX, CANADA

The Morin Anorthosite Complex, north of Montreal in the Canadian Grenville Province, is delineated by a strongly negative aeromagnetic anomaly some 2000 nT below background. The 1.15 Ga anorthosite and related jotunite and mangerite plutons were emplaced into a package of metasedimentary and igneous rocks and subsequently metamorphosed to ca. 750°C. To investigate the nature of the negative anomaly, and the magnetic properties of the associated rocks, we studied samples of anorthosite, jotunite and mangerite from the Morin Complex. Measurements of density, magnetic susceptibility, natural remanent magnetization (NRM), and hysteresis properties were collected on a suite of samples. Magnetic susceptibility ranges over three orders of magnitude from 2 x 10-4 to 3 x 10-1 (SI units). Jotunites and mangerites were in the stronger group, but the anorthosites were widely distributed over the entire range. NRM values also showed considerable variability, from 0.03 A/m to 13 A/m, with anorthosite again providing both the lowest and highest values. Anorthosite properties are strongly controlled by location, with the ~1500 km2 western lobe (which preserves igneous textures) having high NRM values and high susceptibility values, while the ~1000 km2 eastern lobe (which is dominated by annealed mylonites) has low NRM and low susceptibility. Calculations of the Koenigsberger ratio (remanent magnetization to induced magnetization), Q, reveal all the anorthosites have ratios greater than 0.6, indicating that remanence dominates the anomaly. Jotunite and mangerite have most Q values less than 0.5, indicating induced magnetization prevails over these units. Hysteresis properties indicate multidomain magnetite is present, albeit in very small amounts, and this masks the hemo-ilmenite observable in thin sections. As Plio-Pleistocene basalt has average NRM values of about 4 A/m, the anorthosites from the western lobe possess surprisingly large magnetizations for rocks possessing ~1% opaque minerals. As shown by Irving et al. (1978) the paleomagnetic signature of Morin samples is steeply negative; this combined with the high Q values of the anorthosite, indicate the magnetic anomaly is a remanent-dominated anomaly related to strong magnetization of hemo-ilmenites in the Morin anorthosite.