The occurrence of inverse magnetic fabric in dikes is very common and may either reflect the presence of singledomain (SD) grains or it may result from different orientation mechanisms of magnetic minerals in magmas of different viscosities. Here we present the results of a multidisciplinary study in the Alftafjordur volcanic system (Eastern Iceland), including a: (1) structural field study to identify kinematic and thermal indicators of dikes; (2) low field anisotropy of magnetic susceptibility (AMS) analysis, to investigate the magnetic fabric and reconstruct the flow direction of 25 dikes; (3) field variation of low field AMS, first order reversal curve (FORC) diagrams and thermomagnetic properties on selected dikes to define the magnetic mineralogy; (4) petrofabric and image analyses at different microscopic scales to investigate the origin of the magnetic fabric and compare the AMS results with mineral texture. Half of dikes (14 out of 25) show well defined inverse fabric and high anisotropy degrees. 7 dikes have a normal fabric and other 6 dikes have an intermediate fabric. All the inverse fabric dikes are characterized by intensive subhorizontal cooling joints, orthogonally oriented with respect to the dike margins. Each fabric is carried by clear magnetic assemblages made of low Ticontent coarse magnetite for the normal one and high Ticontent elongated magnetite grains for the inverse fabric. No SD grains are observed for these inverse fabric. The orientation of the elongate high Ticontent magnetite grains is comparable with that of the maximum and minimum axes of the AMS ellipsoids. We interpret the inverse fabric due to the late crystallization of elongated magnetic minerals during formation of horizontal cooling joints system.
Porreca, M., Urbani, S., Trippanera, D., Acocella, V., Kissel, C., Sagnotti, L.:. (2015). Interpreting inverse magnetic fabric in dikes from Eastern Iceland. In 26th IUGG General Assembly.