Paleomagnetism of the Caldwell lavas, Eastern Townships, Québec.

Forty two oriented samples (97 specimens) were obtained from 17 sites in metavolcanic rocks from the Caldwell Group of the Appalachians of Southern Québec (longitude: 71 u 00'-71"30' W, latitude: 46°00'-46"10'N). These metavolcanics of Lower Cambrian age are massive or pillowed lavas of andesitic and mainly basaltic composition metamorphosed to the sub-greenschist facies. Magnetite and occasionally hematite are the fre-quent magnetic memory carrier. In order to obtain some pertinent information relative to the stability of the remanent magnetization component, stepwise alternating field demagnetization was conducted on 35% of the specimens and the others were demagnetized at an optimum alternating field. After AF treatment, the paleopole position of the tilted formation from 16 localities is 148°E, 43°N (dp=11.3°, dm = 22.4°). After omission of 3 localities for which a95>30°, the new paleopole position obtained is 173°E, 26°N. This formation of Early Cambrian age is characterized by a reversed polarity.

In order to obtain some pertinent information relative to the stability of the remanent magnetization component, stepwise alternating field demagnetization was conducted on 35% of the specimens and the others were demagnetized at an optimum alternating field. After AF treatment, the paleopole position of the tilted formation from 16 localities is 148°E, 43°N (dp=11.3°, dm = 22.4°). After omission of 3 localities for which a95>30°, the new paleopole position obtained is 173°E, 26°N. This formation of Early Cambrian age is characterized by a reversed polarity.

GEOLOGY
The Caldwell Group is composed of sedimentary and volcanic rocks. The bottom part of this group is mainly constituted of green, purplish to red, grey and black shales, slates, siltstones and schists overlain by thick-bedded, green, reddish and grey, arkosic quartzites and sandstones, with some intermingled grey, green, and red fissile arenaceous slates and shales. The majority of the beds are thin and some are quartzitic, sandy and silty. Immediately beneath the Caldwell lavas, there is an appreciable thickness of highly fissile pure grey slate which passes to a green and more frequently to a red colour near the top; it is apparently a variety of tuff.
The Caldwell volcanics composed of massive and pillowed lavas, breccias, agglomerates and tuffs, occupy the top part of the Caldwell Group. Only the lavas and in particular the pillowed lavas were sampled in this sequence. The Caldwell lavas are predominantly basaltic, with a little associated andesite.
The Sutton-Bennett schists located to the south west of the Thetford Mines-Black Lake area grade into the Caldwell Group as the metamorphism decreases gradually in a northeasterly direction. The Bennett chloritoschists are the metamorphosed and schistose equivalent of lavas and tuffs of the Caldwell Group (Cooke, 1938;St-Julien and Hubert, 1975).
FIG. 1 -Graph of the stability index of 5 specimens of the Caldwell Group.

AGE
The Caldwell Group is located stratigraphically below an ophiolitic sequence which gave a radiogenic (K/Ar) age of 550 m.y. Consequently, the Caldwell Group which is definitely overlying the Grenville basement (late Precambrian) has an age between that of Grenville basement and 550 m.y.

SAMPLING PROCEDURE
42 oriented samples (97 specimens) were collected at 17 different sites; the orientation was done with a Brunton or a solar compass. The number of samples per site varied between 2 and 4 and the number of specimens per sample bettween 2 and 5. The samples were drilled in the field with a portable diamond drill. In this survey, efforts were made to select sites where there was an excellent exposure and where the structural geology was rather simple and well understood. Great care was also taken to select relatively fresh material. The sample locations were chosen by reference to detailed geological maps published by Cooke (1938) at a scale of 1 mile to the inch.

MEASUREMENTS
The direction and intensity of remanent magnetization were measured with a Princeton Applied Research Model SM-1 spinner magnetometer (sensitivity: 10~7 cgs emu). Alternating field demagnetization was carried out to remove unwanted secondary components using a demagnetizer built at the University of Laval (maximum peak Held intensity of 1800 0rsteds) the performance of which was improved by adding 3 large concentric mu-metal cylinders around the solenoid. The increase in a 95 and the decrease of K suggest that at least a fraction of the N.R.M. component is post-folding.

AF DEMAGNETIZATION
A minimum of 2 specimens (usually 3 or 4) from each site were demagnetized, i nsteps of 50 0rsteds, from 50 to 500 0rsteds and in steps of 100 0rsteds, from 500 to 800 0rsteds in the absence of an ambient field. A few treatments up to 1500 0rsteds were done. Guided by the stepwise changes in orientation and intensity of the pilot specimens from a site, 2 stregths of AF demagnatization were selected using an "specimens from a site were AF demagnetized and their residual remanence measured. Pilot tests show that the AF strength at with the primary thermoremanent component is most thoroughly isolated is located in the 150-250 0rsteds range or the 500-700 0rsteds range (figure 1). Normalized demagnatization intensity curves for the same test specimens from different sites are shown in figure 2.
The behaviour of the orientation and intensity of the residual remanence is quite complicated and difficult to interpret After AF demagnetization in the 500-700 0rsted range and tilting of the formation, the paleopole position of the Caldwell Group is 328°E, 43"S (north pole), i.e., 148°E, 43°N (d m = 22A°, d p = 11.3° , K -23.6, reverse polarity). After exclusion of the 3 sites for which a 95-30°, the orientation of the residual remanence is 142°, -30" (a 95-19.5°) and the corresponding paleopole position is 352", 26"S (AO, i.e. 172°E, 26°N (R). This last value is retained as the best estimate of the paleopole position for the Caldwell Group.

DISCUSSION OF THE RESULTS
This paleomagnetic study is the first to be conducted in the Caldwell Group which is one of the oldest lithological unit of the Appalachians of southern Quebec. In spite of difficulties encountered in the interpretation of the AF demagnetized results, it is more probable that the original thermoremanent component was successfully isolated and a fair amount of confidence may be attached to the paleopole position (172°E, 26°N) of the Caldwell Group which is of Early Cambrian age. Three reliable pole available from Cambrian rocks of North America are listed in McElhinny (1973). Their mean position is 141°E, 08"N (a 95-22°, if = 31) and in all cases, the polarities are mixed or reversed. The paleopole position obtained in this study is thus located further to the east. The paleopole position obtained after AF treatment in the 150-250 0rsteds range and without tilting of the formation could logically coincide with the Taconian orogeny (Late Ordovician-Early Silurian time). The pole position obtained is 147°E, 28°N (d m = 25.2°, d"= 12.9°, K = 34.7, reverse polarity); it falls midway between the Ordovician and Silurian pole position (McElhinny, 1973). On this basis, it appears that both the original thermoremanent and the superimposed orogenic events were imprinted on Caldwell rock formations and that these magnetic events can be isolated by progressive AF demagnetization.

ACKNOWLEDGMENTS
The writer thanks Mr. D. Lafond who made many of the measurements and Mrss. J.L. Roy and P. Lapointe of the Division of Geomagnetism, Earth Physics Branch, Ottawa, who contributed to this study throughout numerous discussions and advice. This research was supported by the National Research Council of Canada (Grant No: A7070-110 [1974-75]) and by the Energy, Mines and Resources, Canada, Research Agreement No.: 1135-D13 -3 -55/75-76.