Magnetisation of some New Zealand igneous rocks

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Abstract
Values are given for the magnetisation of thirteen groups of igneous rocks from New Zealand: 2 rhyolites, 1 ignimbrite, 1 trachyte, 2 andesites, 6 basalts and 1 peridotite; and ranging in age from Permian:Jurassic to Quaternary. Values of intensity of magnetisation (J), intensity of induced and remanent components of magnetisation (Ji, Jc), and of the Koenigsberger ratio (Qn) vary widely within each group; usually between two and four orders of magnitude. The frequency distributions of these values are skewed towards high values (positive skewness) and fit a log-normal distribution. In 73% of the specimens the remanent component of magnetisation is of greater intensity than the induced component and hence cannot be ignored when determining the intensity and direction of magnetisation for interpreting magnetic anomalies. The best estimate of J to use in interpreting magnetic anomalies over an igneous body in New Zealand is eµ+½σ2 , where µ and σ are the mean and standard deviation of In J of specimens from the body. Basic igneous rock bodies are not necessarily more strongly magnetised than those of intermediate and acidic composition: for Quaternary rocks from the Central Volcanic Region the best estimate of J for rhyolites is 2.67 A/m, for ignimbrites 2.36 A/m, for andesites 9.9 A/m and for basalts 5.32 A/m. In some groups the directions of magnetisation of individual specimens are widely scattered, but the vector sum directions of all except one group are close to the present field direction.