Paleomagnetism of the mesozoic lamprophyre dikes in north-central Newfoundland

Prasad, Jagat Nandan (1981) Paleomagnetism of the mesozoic lamprophyre dikes in north-central Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

A paleomagnetic study of 86 samples from 18 lamprophyre dikes collected from the Notre Dame Bay area in north-central Newfoundland was done to define their age of emplacement. Published K-Ar dates of 144 to 115 m.y. (Late Jurassic to Early Cretaceous) from the dikes place these among the youngest rocks in eastern Canada. Measurements of natural remanence (NRM) give directions closely aligned to the present field at the site. A detailed alternating-field (AF) demagnetization yielded stable directions with optimum vector grouping for 9 dikes at 10-35 mT peak field. Similarly, the thermal treatment showed stability of remanence with tightest grouping at 300°c for 8 dikes and at 500°c for 7 dikes. The overall mean directions corresponding to thermal and AF cleaning are indistinguishable at 95% confidence level, which gives confidence in the stability of remanence obtained from two independent tests. The AF and thermal treatment coupled with thermomagnetic analysis and an isothermal remanence (IRM) study indicate that the stable remanence resides in a single component of magnetite or titanomagnetite. Studies of Rayleigh loops and high-field hysteresis loops suggest that the natural remanence is carried by a mixture of fine-grained single-domain and pseudo-single domain magnetites of stoichiometric to cation-deficient composition. The AF and thermal mean directions correspond to poles 75°N, 168°W (dp, dm = 7.4°, 9.7°) and 72°N, 152°W (dp, dm = 5.2°, 6.6°) respectively, which are in good agreement with each other. Bath pales are of normal polarity, though the present study revealed for the first time the occurrence of a reversed polarity (with well-grouped directions in one dike only) among the Notre Dame Bay lamprophyres. These pole positions are intermediate between the poles reported previously from two other lamprophyre localities in the Notre Dame Bay area, and also are intermediate between the Lower Cretaceous poles and Jurassic poles reported from other parts of the Appalachians. This suggests a good correlation between the age of magnetization and the range of available radiometric dates.

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