Proterozoic paleomagnetism in the Nipigon Embayment of northern Ontario: Pillar Lake Lava,Waweig Troctolite and Gunflint Formation tuffs

By using unusual combinations of demagnetization techniques, Proterozoic paleomagnetic vectors and paleopoles are provided for two recently discovered post-tectonic Proterozoic units near Armstrong, northern Ontario, and also for well-dated Gunflint Formation, which by previous techniques yielded problematical paleomagnetic data. The first paleomagnetic data are provided also for the Seagull Pluton and Inspiration Sills. Characteristic remanent magnetizations (ChRM) for the Pillar Lake Lavas indicate a Keweenawan age, more specifically ∼1000–1040 Ma by comparison with the well-established APWP for the Late Proterozoic Superior craton. Four combinations of demagnetization techniques yield declinations in the range 108–133° and inclinations in the range −65 to −70° (n = 100), which define paleopoles near 200 W/48 N corresponding to a location on the Keweenawan APWP near ∼1040 Ma. In the underlying basement a recently discovered Proterozoic igneous complex, the Waweig Troctolitic Complex, yields new paleomagnetic data with declination and inclination 42/−54 (n = 14) defining a paleopole at 238 W/09 N. Its ages may be 1400–1600 or ∼2000 Ma by comparison with the presently available, ambiguous and sparsely populated APWP. The first paleomagnetic results for the Seagull Pluton (U–Pb age 1113 Ma) yield a mean declination of 87.4/−75.7 (n = 32) corresponding to a Keweenawan paleopole near 233/42 N, consistent with other paleopoles near ∼1200 Ma. Tuffs of the oft studied but problematical Gunflint Formation (U–Pb age1878 Ma) yielded stable and presumably primary vectors using several different demagnetization techniques on the same specimens. Their mean primary declination and inclination ∼303/+48.8 (n = 17) yields a paleopole now located near 178 W/42 N, comparable with the published locations of paleopoles of ∼2000 Ma. Of broader interest, we recognized that low temperature demagnetization preceding conventional demagnetization techniques enhanced the isolation of characteristic vectors. Combining the conventional techniques (thermal and AF demagnetization) also improved the resolution of characteristic vectors not achieved by other means. Low grade metamorphism affected the non-tectonized Proterozoic cover to the Canadian shield, due to burial or hydrothermal effects, obfuscating or erasing primary vectors in some lithologies and especially at certain sites.