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Anorthosites as Sources of Magnetic Anomalies

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The Earth's Magnetic Interior

Part of the book series: IAGA Special Sopron Book Series ((IAGA,volume 1))

Abstract

Magnetic anomalies provide information about location, size and composition of earth structures, ore bodies and tectonic features even in bodies containing only a few percent magnetic minerals. Here we investigate the magnetic properties and oxide mineralogy of anorthosites, rocks rich in plagioclase (>90%), and compare their magnetic signatures to aeromagnetic anomaly maps of the regions. Two of the anorthosite complexes have large negative anomalies associated with them; both have low susceptibility and high remanence related to hemo-ilmenite mineralogy and remanent directions antiparallel to the present field. One complex has appreciable natural remanent magnetization quasi-parallel to the present field, and strong susceptibility, creating an enhanced positive anomaly. The fourth anorthosite has little or no magnetic anomaly over much of its area, in accordance with the weak remanence, low susceptibility and variable magnetic mineralogy observed. The anorthosite samples producing significant anomalies, and maintaining strong and stable natural remanent magnetization over geologic time all contain oxides of the hematite-ilmenite series. This study adds support to ‘lamellar magnetization’ whereby exsolved phases in the ilmenite-hematite system produce strong and stable magnetization with only minor amounts of oxide material.

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Acknowledgements

Parts of this research have been funded by NSF (USA), NFR (Norway) and NGU. Thanks to Peter Robinson for field assistance, Chris Koteas and Weining Zhu for GIS assistance, John Valley for support of past petrologic studies of the Morin Complex, and to Institute for Rock Magnetism, University of Minnesota, supported by a NSF instrument and facility grant, for use of their instruments.

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Authors and Affiliations

  1. Department of Geosciences, University of Massachusetts, Amherst, MA, 01003, USA

    Laurie L. Brown

  2. Norwegian Geological Survey, N-7491, Trondheim, Norway

    Suzanne A. McEnroe & Lars Petter Nilsson

  3. Department of Geology, Colgate University, Hamilton, NY, 13346, USA

    William H. Peck

Authors
  1. Laurie L. Brown
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  2. Suzanne A. McEnroe
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  3. William H. Peck
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  4. Lars Petter Nilsson
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Corresponding author

Correspondence to Laurie L. Brown .

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Editors and Affiliations

  1. Geophysical Institute, ASCR Praha, Bocni II / 1401, Praha 4, 141 31, Czech Republic

    Eduard Petrovský

  2. School of Mathematics & Statistics, Dept. Pure Mathematics, University of Sydney, Sydney, 2006, New South Wales, Australia

    David Ivers

  3. National Geophysical Research Institute, Uppal Road, Hyderabad, 500007, India

    T Harinarayana

  4. School of Ocean & Earth Science &, Technology (SOEST), University of Hawaii, East-West Road 1680, Honolulu, 96822, Hawaii, USA

    Emilio Herrero-Bervera

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Brown, L.L., McEnroe, S.A., Peck, W.H., Nilsson, L.P. (2011). Anorthosites as Sources of Magnetic Anomalies. In: Petrovský, E., Ivers, D., Harinarayana, T., Herrero-Bervera, E. (eds) The Earth's Magnetic Interior. IAGA Special Sopron Book Series, vol 1. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-0323-0_23

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