Magnetic overprinting in the Thanet gabbro complex, Ontario

Abstract

Multi-component paleomagnetic remanences are reported for the Thanet gabbro complex of the Grenville Province in Ontario. Forty-five samples (10 sites) attain stable reversed (A) directions in AF's (alternating fields) of 150 Oe (1.2 × 104 A/m) or less. Blocking temperatures (TB's) are 300–350 °C (5 sites) and 525–560° (5 sites). The A1 paleopole of 158.4°E, 38.0°S (k = 593, α95 = 3.1°) for the low TB sites differs significantly at the 95% confidence level from the A2 pole of 172.0°E, 32.3°S (k = 151, α95 = 6.2°) for the high TB sites. Directions from 18 additional samples (at 8 sites) follow great circle paths towards the A direction upon AF demagnetization to 1000 Oe (8.0 × 104 A/m). TB's of the lower coercivity (B) component are 560–580 °C, above TB's of both A components. Upon thermal demagnetization, directions swing away from the A direction and endpoints reached above 560 °C define a normal B paleopole at 159.3°E, 20.3°N (k = 9.6, α95 = 11.7°).If the A and B components are both of thermal origin, B predates A, and was overprinted by A during a regional metamorphic event which reset components with TB's <560 °C but left those unaffected with TB's > 560°C. The difference in the A2 and A1 directions represents apparent polar wander during cooling from above 500 °C to less than 300 °C. If the B component, however, was generated as a low temperature (<300 °C) chemical remanence, the relative age of A and B are reversed—B is younger than A. The applicaton of Thanet paleopoles to the problems of Grenville and Sveconorwegian tectonics is described.