Anisotropy of the Magnetic Susceptibility of the Alnö alkaline and carbonatite igneous complex

The Alnö igneous complex in central Sweden is one of the largest (radius ~2.5 km) of the few well-known alkaline and carbonatite ring-intrusions in the world. The lithologies span from alkaline silicate rocks (nepheline syenite, ijolite, and pyroxenite) to a range of carbonatite dykes (e.g. sövite) with variable composition. The depth extent, dip, and dip direction of the alkaline and carbonatite rocks have been inferred from surface geological mapping, and a dome-shaped magma chamber with the roof at ~2 km below the palaeosurface was inferred to have supplied steeply dipping radial dykes and (shallowly dipping) cone sheets. Recent high-resolution reflection seismic profiles and gravity and ground magnetic measurements suggest, in turn, a saucer-shaped magma chamber at ~3 km depth below present day land surface. To provide further insight into the internal flow mechanics of these dykes and into their emplacement mechanisms, we have measured the anisotropy of magnetic susceptibility (AMS). About 250 samples from 119 oriented cores were collected with a handheld drilling machine from 26 localities within the Alnö complex. Prior to preparation of discrete samples for AMS, the cores were measured for their density and for ultrasonic P- and S-wave velocities. Most of the sampling locations lie on a transect through the intrusion. Three locations have been sampled in detail, to determine the variation of AMS within individual carbonatite dykes. The AMS of samples were measured in low-field, using a KLY-2 Kappabridge. Bulk magnetic susceptibility ranges from 3.01e-5 to 2.50e-1 SI, and correlates with lithology. The sövites have the widest range of susceptibility (average 4.32e-2, with a range from 3.01e-5 to 2.50e-1 SI), whereas fenites have the lowest average susceptibility (average 2.06e-3, with a range from 9.86e-5 to 1.47e-2 SI); nepheline-syenite, ijolite and pyroxenite have susceptibilities between these two end member lithologies. Sövite consists mainly of calcite; but holds varying concentrations of magnetite, pyrochlore, and biotite. These minerals explain the wide range of susceptibilities found in the sövites. Within sövite dykes, the magnetic fabric is consistent however, and has strong association with the orientation of the dyke. Maximum susceptibility (k1) tends to be sub-parallel to the strike of the dyke, displaying mostly sub-horizontal orientations. The minimum susceptibility axes (k3) are generally oriented perpendicular to the strike of the dyke. The magnetic fabric in the transect displays greater variation and complexity in the orientation of the principal susceptibility axes. The majority of samples have an oblate shape susceptibility ellipsoid, but many have triaxial and a few have prolate shapes. The close affinity of the AMS with the orientation of the dyke, together with geological mapping of outcrops, helps to infer the flow direction of the magma. The sub-horizontal direction of the maximum susceptibility can be interpreted such that the flow direction of magma during intrusion might have been horizontal rather than vertical in the investigated sövite dykes. This assumes that the anisotropy is from primary flow, but it is also possible that it was created due to post-emplacement flattening. The full nature of fabrics will help to further unravel the history of the Alnö complex and test hypotheses of flow patterns.