Magnetic fabrics in the Bjerkreim Sokndal Layered Intrusion, Rogaland, southern Norway: Mineral sources and geological significance

Andrea R. Biedermann, Florian Heidelbach, Mike Jackson, Dario Bilardello, Suzanne A. McEnroe

Research output: Contribution to journalArticlepeer-review

8 Scopus citations

Abstract

Magnetic anisotropy can provide important information about mineral fabrics, and thus magmatic processes, particularly when it is known how multiple mineral species contribute to the anisotropy. It may also affect the direction of induced or remanent magnetization, with important consequences for paleomagnetic studies or the interpretation of magnetic anomalies. Here, we aim at describing the magnetic fabrics in the Bjerkreim Sokndal Layered Intrusion and identifying their carriers. Anisotropies of magnetic susceptibility and remanence were measured on samples covering different geographic locations and stratigraphic units within the Bjerkreim Sokndal Layered Intrusion. The intrusion is characterized by magmatic layering and has a synform structure, with strong foliation on the limbs. Detailed comparison between magnetic and mineral fabric shows that they are not necessarily coaxial, but the minimum susceptibility, and minimum anhysteretic remanence are generally normal to the foliation or the magmatic layering. The minimum susceptibility and anhysteretic remanence are associated with pyroxene (100) axes, and the maximum susceptibility and anhysteretic remanence are sub-parallel to the pyroxene [001] axes in layers MCU IVc and MCU IVe for which electron backscatter data are available. However, the paramagnetic anisotropy of pyroxene is too weak to explain the observed anisotropy. We propose that the magnetic anisotropy of magnetite-free specimens is carried by hemo-ilmenite exsolutions within pyroxene, in addition to pyroxene itself. When present, multi-domain magnetite dominates both the anisotropy of magnetic susceptibility and anhysteretic remanence, due to shape-preferred orientation and distribution anisotropy. The orientation of the magnetic fabric appears independent of carrier, due to their common deformation history, but the degree of anisotropy is stronger for magnetite-bearing specimens. The results of this study will facilitate future structural interpretations and may be used to correct for magnetization deflection.

Original languageEnglish (US)
Pages (from-to)101-118
Number of pages18
JournalTectonophysics
Volume688
DOIs
StatePublished - Oct 12 2016

Bibliographical note

Funding Information:
Alexander Michels, Norwegian University of Science and Technology (NTNU), is thanked for assistance during fieldwork. We are grateful to Bjarne S.G. Almqvist, Uppsala University, who provided access to their Kappabridge for initial AMS measurements. Peat Solheid is thanked for technical assistance during sample preparation and measurements at the Institute for Rock Magnetism (IRM), University of Minnesota. This study was financed by the Swiss National Science Foundation , project P2EZP2-155517 to A. Biedermann, Norwegian University of Science and Technology and NFR grant 222666 to S. McEnroe. The first author had a visiting fellowship at the IRM, which is supported by the Instruments and Facilities program of the US National Science Foundation, Division of Earth Sciences. FH was supported through DFG grant He3258/2-1 . Eric C. Ferré and Philippe Robion are thanked for their careful and thorough reviews, and Philippe Agard for the handling of the manuscript and his additional comments, all of which helped to improve the manuscript.

Publisher Copyright:
© 2016 Elsevier B.V.

Keywords

  • AMS
  • Anisotropy of remanence
  • Bjerkreim Sokndal
  • CPO
  • Layered intrusion
  • Magnetic fabric

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