Abstract

The Shelley Lake granite of northwestern Ontario contains five magnetic phases: deuteric and post-crystallization hematites, which are relatively abundant but carry only 1–4% of the natural remanent magnetization (NRM); primary magnetite in coarse (50–500 μm) grains, both optically homogeneous and subdivided by hematite lamellae; micrometre-size secondary magnetite in chloritized biotites; and submicrometre-size magnetite, whose presence is inferred from low blocking temperatures in thermal decay curves of the NRM. The NRM is a composite of type 1 and type 2 remanences, which differ in direction by about 90° (see companion paleomagnetic paper). Both NRM components occur in normal (N) and reverse (R) polarities. Type 1 remanences (1N/1R) have the hallmarks of multidomain (MD) behaviour: high blocking temperatures but low coercivities, exponential alternating field (AF) decay curves, generally MD results of the Lowrie–Fuller test, and MD to transitional values (0.3–10) of the Koenigsberger Qn ratio. Furthermore, intensities of 0.6 Oe (0.06 mT) laboratory thermoremanent magnetizations (TRM's) match those of 1R and some 1N NRM's. We argue on this evidence that 1R and at least part of 1N NRM's are TRM's residing in coarse MD-size primary magnetite. This primary TRM dates from initial cooling of the Shelley Lake pluton around 2580 Ma. Thermal decay spectra of single-component type 2 NRM's (2N/2R) resemble those of 1R. However, the considerable overlap of 2N/2R and 1R blocking temperatures in multivectorial NRM's demonstrates that type 2 remanence must be a chemical or thermochemical rather than a thermal overprint.

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