Proterozoic diabase sills of northern Ontario: Magnetic properties and history

Magnetic fabrics show that a series of massive diabase sills (“Logan sills”) possess flow fabrics away from the midcontinental Proterozoic rift system, near its triple junction on the north shore of Lake Superior. Their intense remanences are commonly antiparallel to the geomagnetic field, requiring some knowledge of natural remanent magnetization in order to interpret of ground magnetic surveys. Thermal demagnetization, low‐temperature demagnetization, and the combined techniques yield differently oriented characteristic remanence vectors in some of these sills. Although the commonly reported “Keweenawan” reversed paleofield direction is verified by most A components, the orientation of vectors within sites and through the thickness of sills is large and may be attributed to reversals, excursions, and secular variation. B components are mostly down, but their orientation distribution is poorly concentrated. Low‐temperature demagnetization, prior to thermal demagnetization, tends to improve the consistency of vectors defined in subsequent thermal demagnetization. Moreover, in some cases, three to five cycles of low‐temperature demagnetization alone produces the same result as the more time‐consuming thermal demagnetization. Some within‐site scatter of the A components is large, and some anomalous specimen directions and alternate polarities may be associated with platinum group mineralization. Whereas the grand mean direction for all sites lies on the available apparent polar wander path between 1100 and 1300 Ma, the mean paleopole locations differ at the 95% level for different demagnetization treatments of specimens from the same sites.