No abstract

This apparatus was built to determine rapidly the magnetic susceptibility and/or its anisotropy in rock specimens in a low a. c. field ("'" 0. 5 oersteds). It accepts cubic or right cylindrical specimens up to 22 cc in volume. It consists of two accurately balanced a . c. transformers coupled to a commercial low-noise high - gain amplifier and digital voltmeter which allows volume susceptibilities as low as 5 x io - 8 emu(cgs) to be measured . To measure susceptibility only, the specimen is placed in the centre of one of the two identical sets of coaxial transformer coils with the amount of unbalance created being linearly related to the susceptibility. Alternatively, the susceptibility may be measured by placing the specimen in the air gap of the ferrite core in one of the two identical incomplete -toroid transformers; the susceptibility anisotropy is measured by moving a ferrite slug into the other air gap to balance the specimen's axial susceptibility and then rotating the specimen to measure the susceptibility difference for its other axes. It takes about 15 seconds per specimen to measure the susceptibility and a further 2 minutes to measure its anisotropy. T h e construction and calibration of the apparatus are discussed.

A paleomagnetic study was done on 28 diabase and 6 lamprophyre dikes from the vicinity of Kaminak Lake in the District of Keewatin. These dikes are Proterozoic and cut across an area of Archean rocks. Their metamorphic grade within this region varies from nil to amphibolite facies. Two stable magnetization directions were obtained from the diabase on AF demagnetization; one from 3 unmetamorphosed diabase dikes of D = 205", I = 3", %, =24", with a corresponding pole position at 23.8'S, 122.3" W, a,, = 16.5"; the other from the metamorphosed diabase of D = 176", I = 65", %, = 4", with pole position at 20.4' N, 92. l o W, %, = 6.1". The first is believed to be thermoremanent and approximately 2300 Ma old, while the second is a metamorphic direction of approximately 1800 Ma. The six lamprophyres give a stable remanence of D = 174", I = 78", a,, = loo, with a pole position at 40.9"N, 92.8"W, %, = 17", essentially the same as the metamorphosed diabase.A number of samples were also thermally demagnetized. Most of the results obtained were the same as those obtained from the A F treatment. However, one diabase site gave adirection similar to that obtained from the 3 unaltered dikes, and one of the lamprophyres showed a reversal.The pole position of the altered Kaminak diabase relative to that for the unconformably overlying Dubawnt Group suggests that the polar wandering curve for this part of the Aphebian may be more complex than previously thought. It is quite probable that the earlier Aphebian pole positions (Matachewan, Nipissing), as usually plotted, are actually antipodal points in terms of the polar wandering curve.Une etude paleohagnetique fut poursuivie sur des dykes de diabase (28) et de lamprophyre (6) dans la region du Lac Kaminak dans le District de Keewatin. Ces dykes proterozoiques traversent des roches archeennes. Leur degre de metamorphisme dans cette region varie de nu1 afacies d'amphibolites. Deux directions stables de magnetisation ont Cte obtenues des diabases par la methode de demagnetisation S.F.; une des directions provient de 3 dykes non metamorphises de D = 205", I = 3", a,, = 24"avec la position du p81e correspondant a 23.8"S, 122.3" W, = 16.5" I'autre direction provient des diabases metamorphises de D = 176", I = 65", a95 = 4" avec la position du p8le a 20.4" N, 92. l o W, orgS = 6. lo. On croit que la premiere magnetisation est de type thermoremante et date de 2.300 Ma, la seconde est une direction magnttique datant d'environ 1.800 Ma. Les six lamprophyres donnent une remanence stable de D = 174", I = 78", = 10" avec la position du p6le a 40.9" N, 92.8" W, = 17' essentiellement la mbme que celle des diabases metamorphises.Un certain nombre d'echantillons avaient subi une demagnetisation thermique. La plupart des resultats obtenus etaient les mbmes que ceux de la mkthode AF. Cependant, I'un des diabases donnait une direction semblable a celle obtenue pour les trois diabases non alteres et un des lamprophyres montrait un magnetisme inverse.Les donnkes paleomagnetiques favorisent I'hypothese de ...

Northern Baffin Island is intersected by spectacular swarms of predominantly northwesterly trending diabase dykes. Cross-cutting relationships, K–Ar ages, and paleomagnetic contact tests indicate that these dykes represent at least two major episodes of Hadrynian igneous activity. The Borden dykes were emplaced during the earlier of these episodes. They are about 950 Ma old and have a paleomagnetic pole position of 153.3°E, 26.7°S, δm = 8.6e, δp = 6.1°. The later episode saw the emplacement of the previously defined Franklin dykes, which are about 750 Ma old. The newly defined Borden pole lies at the southern extremity of the Grenville Loop and has an age consistent with poles of similar position that were derived from rocks of the Grenville Province.All of the dykes probably reflect tension acting in an east-northeast–west-southwest direction. The directions of thick northwest trending dykes, which form a considerable angle to that expected from east-northeast tension, are thought to have been controlled by earlier (circa 1200 Ma) fractures of the Borden fault zones. The two main surges of magma may coincide with two -periods of Hadrynian uplift in the Boothia Peninsula area to the west.

Two stable, ancient, N.R.M. components were isolated by a.f. demagnetization of samples from 29 sites in the Helikian (Middle Proterozoic) anorthosite and adamellite suites of the Mealy Mountains. The directions of the two components are chiefly to the northwest, 20° upward (northwest component) and to the east, 65 °downward (east component). The northwest direction is thought to be the primary igneous T.R.M. and no reversely magnetized sites with this direction were encountered in the Mealy suite. The 11 sites from which the northwesterly component was isolated yielded a mean pole at 179 °W, 8 °N, α95 = 12°. This is some distance away from poles derived from rocks of comparable age from elsewhere in the Canadian Shield and may reflect a 50° clockwise rotation of this section of the Grenville Orogen relative to the Superior after the initial magnetization of the Mealy rocks. Evidence of a second stable component was found in 14 sites (east component); 11 of these are reversely magnetized and three are normally magnetized. The 14 sites with easterly (and reversed easterly) magnetization yielded a combined pole for this component at 182 °W, 38 °S, α95 = 9°. This pole position suggests that the east component was acquired after the rotation of this part of the Grenville Orogen. The cores from 3 sites exhibited two clear end-points during a.f. demagnetization and thus contain both the northwest and the east component.The Helikian Shabogamo Gabbro at the south end of the Labrador Trough was sampled at 13 sites on a line extending 80 km north from the Grenville Front. After a.f. cleaning these sites yielded a pole at 171 °W, 10 °N, α95 = 12°. This pole lies about 20° northwest of poles for rocks of similar age in the Superior Orogen so it is suggested that the rocks of the sample area, though lying a bit north of the line generally accepted as the 'Grenville Front', have been rotated clockwise (about 20°) since intrusion of the gabbro.