Journal Article

Dual polarity directions in basaltic-andesitic dykes—reversal record or self-reversed magnetization?

U. Liebke, E. Appel, U. Neumann and L. Ding

in Geophysical Journal International

Volume 190, issue 2, pages 887-899
Published in print August 2012 | ISSN: 0956-540X
Published online August 2012 | e-ISSN: 1365-246X | DOI: https://dx.doi.org/10.1111/j.1365-246X.2012.05543.x
Dual polarity directions in basaltic-andesitic dykes—reversal record or self-reversed magnetization?

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Summary

Rock magnetic analyses were performed on basaltic-andesitic dyke samples from the Lhasa Block, which were previously used for palaeomagnetic investigations of the India–Asia collision. The aim of the study is to prove whether antipodal directions separated in these samples represent different polarities of the Earth's magnetic field or a self-reversal magnetization. Reflected light microscopy, domain pattern observations and energy dispersive X-ray analyses revealed two generations of titanomagnetite (TM) grains, one consisting of large-sized (∼20 μm to more than 100 μm) and inhomogeneous Ti-poor TM, and another with small (smaller than ∼10 μm) and rather homogeneous Ti-rich TM grains. Partial thermoremant magnetization experiments and reflected light microscopy show that magnetostatic or superexchange interaction between these two phases is unlikely. Temperature dependence of saturation magnetization and low-temperature curves of isothermal remanent magnetization disprove a possible N-type behaviour of the samples. A self-reversal due to ionic reordering can be ruled out as it requires a high degree of oxidation of the Ti-rich TMs, which was not observed. In summary, a self-reversal magnetization is very unlikely in the studied dyke samples, and therefore the observed antipodal directions most probably represent a record of different polarity epochs of the Earth's magnetic field. This conclusion implies that the time of remanence acquisition in the studied dykes was sufficiently long to average out paleosecular variation, supporting the significance and reliability of the palaeomagnetic results.

Keywords: Paleomagnetism applied to tectonics; Reversals: process, timescale, magnetostratigraphy; Rock and mineral magnetism

Journal Article.  8355 words.  Illustrated.

Subjects: Geophysics

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