Correction for deviations due to magnetic anisotropy of palaeomagnetic directions from the Mulcahy Gabbro, north-west Ontario

Abstract

SUMMARY The Mulcahy Gabbro is an Archean, layered intrusion in the Wabigoon subprovince of north-western Ontario. Alternating field and thermal demagnetization of specimens from it have provided consistent within-site characteristic remanence (ChRM) directions in eight of 19 sites. Antipodal polarities of remanence are recovered and the overall in situ formation mean is D = 240, I = -51, k = 21. This direction is unlike any reported Archean direction from the Superior province of North America. The degree of anisotropy of low-field magnetic susceptibility is high with K,/K3 values up to 1.3. Cooling selected specimens in weak fields indicate that TRM is deflected towards the magnetic fabric plane. Accordingly the in situ ChRM was corrected for the effect of magnetic anisotropy. Two methods of correction give similar results. However, the resultant formation mean remains essentially unchanged as a consequence of sufficient variability in the attitudes of K,-K, planes. Although uncertainty in the initial dip renders the tilt-correction problematical, partial tilt-correction (36 per cent) results in a formational mean with better precision than the in situ formation mean. Furthermore partial untilting of remanence directions results in a positive reversal test with a,better classification than that obtained using in situ directions. This partially tilt-corrected result (D = 202, I = -49, k = 35) yields a pole (latitude = 62N, longitude = 42E) which is Archean in aspect.