Magnetic overprinting in the Thanet gabbro complex, Ontario

Author:

Buchan Kenneth L.

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.

Publisher

Canadian Science Publishing

Subject

General Earth and Planetary Sciences

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