Paleomagnetism of the Chibougamau greenstone belt,Quebec, and the effects of Grenvillian post-orogenic uplift

A paleomagnetic survey (34 sites, 200 core samples) of an Archean greenstone belt just north of the Grenville Front is described. Sixty per cent of the samples have blocking temperatures less than 400° C and remanent coercive forces predominantly below 100 Oe, and they provide no information about the Precambrian geomagnetic field. The other samples contain many magnetizations of which two (CS and CH) can be clearly recognized. The CS magnetization occurs in basic sills, and has high blocking temperature (550 to 650°C) and remanent coercive force (often exceeding 1000 Oe) and a direction (186°,?38°) that is not significantly different from that in Matachewan diabase dikes (2690 m.y.). CS is thought to have been caused by uplift following the Kenoran orogeny and its age is probably about 2600 m.y. No older magnetization has been found. The CH magnetization occurs in several different bodies and has a direction (119°, +51°) similar to that observed in rocks from other places close to the Grenville Front. CH has more variable blocking temperatures (200 to 650°C) and remanent coercive forces (200 to > 1000 Oe) and is considered to have been acquired by heating during uplift of the highly metamorphose Grenville Province to the south. Uplift occurred about 1000 m.y. ago, and the CH magnetization is considered to be of this age. Evidence is presented to support the suggestion that there was an extensive magnetization episode both within and adjacent to the Grenville Province at that time.