signature=5a4e238e0ad11da3ef224cf938d8331a,The use of hand-held magnetic susceptibility meters in th...

摘要：

Diabase dykes and sills, unmetamorphosed to very weakly metamorphosed, are typically magnetic, enough to cause minor compass deflections and to attract a small hand-held magnet in the field. On aeromagnetic maps, such intrusions reveal themselves by more or less well-defined magnetic anomalies that contrast with more variable and typically less magnetic bedrock signatures. With some experience, one may be able to tell different dyke swarms apart simply by gauging the deflection of a hand-held magnet in the field, particularly in cases where weakly metamorphosed dykes - with a greatly reduced magnetic susceptibility due to alteration and metamorphic reactions of Fe-Ti oxides to secondary minerals such as chlorite - are intermixed with younger, less altered dykes. Nevertheless, this simple observation remains somewhat subjective and fails to take full advantage of the wide range of magnetic susceptibilities intrinsic to intrusive rocks of differing compositions. Mafic dyke rocks show magnetic susceptibilities (MS) that may vary over two full -3 orders of magnitude, from ~2 to ~200 x 10-3 SI units (factor not repeated hereafter). Most fresh diabase dykes have MS >20, whereas dykes affected by weak alteration or incipient metamorphic recrystallization typically have reduced MS values <10. Fully hydrated metadiabases have MS values <1, typically converging on 0.7 ± 0.2. Here I present preliminary results on a systematic survey of dyke susceptibilities in various parts of Canada, and a comparison of the utility of two different hand-held susceptibility meters. It is concluded that hand-held susceptibility meters are of great use in field studies of dykes, with applications ranging from distinguishing between different dyke sets, and defining heterogeneities within individual dykes, to resolving dyke intersections.

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