Abstract

The Nipissing diabase (ND), sampled 2 km northwest of the Grenville Front near Temagami, Ontario, has four distinct components of natural remanent magnetization (NRM). The lowest unblocking temperature (TUB) component is attributed to the present Earth's field, whereas the second lowest TUB component is due to overprinting during the late stages of the Grenvillian Orogeny (~900 Ma). Similar Grenvillian overprint directions of NRM are found in Archean (~2600 Ma) country rock. The second highest TUB component (B) has an unknown origin. Low-temperature demagnetization (LTD) reveals that this component is significantly contaminated by overlap with the Grenvillian overprint and with the highest TUB component (A). If the A NRM is primary, its direction should agree with previously published ND poles after structural correction for subvertical tilting of sites. However, the structurally corrected A direction does not agree with known ND primary directions without additional rotation about a vertical axis. This may indicate sequential block rotation and tilting or could be caused by a single tectonic rotation about an inclined axis. The ND in the Temagami area shows a variety of characteristic magnetizations, which may reflect a complicated structural history and (or) hydrothermal activity before Grenvillian uplift, whereas west of Lake Temagami and in the Wanapitei Lake area ND results agree with the previously reported N1 paleopole. This contrast suggests quite different tectonic histories east and west of Lake Temagami.

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