Further paleomagnetic results from the Seal Group igneous rocks, Quebec

Abstract

This paleomagnetic study involves further demagnetization (alternating field, thermal) of specimens from part of the previously published Seal Group (volcanics and sills). A previous study uncovered two stable magnetizations nominated M2 (herein B) and M3 (herein A). In the present work, the A magnetization has been better defined, found to contain reversals of direction, and has been found also in the sills (6 sites, specimen unit weight; 289°, +11°; α95 = 11°) as well as the volcanics (5 sites; 293°, +13°; α95 = 13°) for a combined direction with respect to bedding of 291°, +12 °(11 sites, R = 10.76, α95 = 7°). Of the total number of specimen directions (63; 1 or 2 per sample), 17 were obtained by vector subtraction (VS); all but four of the latter were the means of at least two determinations. It is suggested that the A magnetization residing in hematite was acquired first, and B, residing in magnetite, later, as a result of remagnetization during the Grenville 'Orogeny'. At several sites, where there is no evidence of hematite, a reverse B direction may be present. The A pole is at 17°N, 195°E (δp = 4°, δm = 7°), near the Seal Group red bed pole (05°N, 205°E), and the B pole (direction with respect to horizontal) at 17°S, 158°E (7°, 9°).It is apparent in this and other studies that specimens that contain both normal and reverse magnetizations display a very characteristic behaviour under demagnetization. This behaviour and its implications for deriving good approximations to the true direction by the use of VS—even when remanent coercive force or unblocking temperature spectra overlap substantially—are revealed by means of a model using artificial data.