Magma transport in sheet intrusions of the Alnö carbonatite complex, central Sweden

Abstract

Magma transport through the Earth’s crust occurs dominantly via sheet intrusions, such as dykes and cone-sheets, and is fundamental to crustal evolution, volcanic eruptions and geochemical element cycling. However, reliable methods to reconstruct flow direction in solidified sheet intrusions have proved elusive. Anisotropy of magnetic susceptibility (AMS) in magmatic sheets is often interpreted as primary magma flow, but magnetic fabrics can be modified by post-emplacement processes, making interpretation of AMS data ambiguous. Here we present AMS data from cone-sheets in the Alnö carbonatite complex, central Sweden. We discuss six scenarios of syn- and post-emplacement processes that can modify AMS fabrics and offer a conceptual framework for systematic interpretation of magma movements in sheet intrusions. The AMS fabrics in the Alnö cone-sheets are dominantly oblate with magnetic foliations parallel to sheet orientations. These fabrics may result from primary lateral flow or from sheet closure at the terminal stage of magma transport. As the cone-sheets are discontinuous along their strike direction, sheet closure is the most probable process to explain the observed AMS fabrics. We argue that these fabrics may be common to cone-sheets and an integrated geology, petrology and AMS approach can be used to distinguish them from primary flow fabrics.

Unravelling the internal architecture of the Alnö alkaline and carbonatite complex (central Sweden) using 3D models of gravity and magnetic data

“…Prior to the preparation of sub-samples (21 mm long) suitable for AMS measurements (Andersson et al, 2016), the core A total of 40 measurements are plotted.…”

“…In total 610 gravity data points were used, covering the main complex and the surroundings. We estimate about 0.1 mGal data error in the preparation of the data until complete Bouguer response; hence we used an RMS misfit of 0.2 mGal during 10 the inversion (Malehmir et al, 2016).…”

“…As part of an interdisciplinary geophysical study involving reflection seismics , anisotropy of magnetic susceptibility (Andersson et al, 2016) and potential field methods (this study), new surface gravity and magnetic data were 20 collected to provide information about the deeper interior of the complex and with the aim to test if a shallow magma-chamber can be verified independently by 3D modelling of potential field data over the complex. We propose that by estimating the thickness of the intrusion it is possible to indirectly infer the location of the magma chamber.…”

“…A recently conducted study focussing on the anisotropy of magnetic susceptibility (AMS) of carbonatite dykes revealed a pattern of the magnetic fabric that follows the orientation of the dykes (Andersson et al, 2016). AMS results are commonly presented as susceptibility ellipsoids and it was found that most of the AMS measurements showed oblate ellipsoid shape.…”

“…AMS results are commonly presented as susceptibility ellipsoids and it was found that most of the AMS measurements showed oblate ellipsoid shape. The dominance of oblate shape was interpreted to represent re-orientation or deformation of magnetite grains due to dyke closure at a late stage of magma transport, probably due to pressure drop in the dyke (Andersson et al, 2016).…”