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.
Highlights
Magnetic susceptibility is a symmetric second rank tensor that is conventionally represented by three orthogonal principal axes, maximum (k1), intermediate (k2), and minimum (k3) susceptibility, which are usually illustrated as an ellipsoid
When laminar magma flow occurs in a magmatic sheet intrusion, it exerts a shear force that tends to orient prismatic and tabular mineral grains parallel to the flow direction, with often only a small angular deviation between the long axis of the grain and the flow direction[2]
The main advantage of anisotropy of magnetic susceptibility (AMS) compared to traditional textural and microstructural techniques is that a large number of samples can rapidly be measured and provide an accurate determination of the overall orientation of the sum of all mineral grains in each sample, it is naturally biased towards minerals that have high magnetic susceptibility[20]
Summary
Magnetic susceptibility is a symmetric second rank tensor that is conventionally represented by three orthogonal principal axes, maximum (k1), intermediate (k2), and minimum (k3) susceptibility, which are usually illustrated as an ellipsoid. Studies of Tertiary dykes in east Greenland demonstrated drawbacks with the approach of using k1, because sub-vertical magma flow was indicated using k1, which deviated strongly from the sub-horizontal flow directions interpreted from outcrop observations[5]. Numerous studies subsequently employed AMS to reveal information on magma flow from the measured magnetic signal, e.g.1,2,4,9,18,22, but the use of different interpretation schemes has led to a muddled view of how best to interpret AMS results. A number of studies have indicated that AMS alone might be insufficient to infer flow directions in igneous rocks and should ideally be complemented by other analytical information (e.g. fabric analysis in the field or shape and crystallographically preferred mineral orientation determined by optical or scanning electron microscopy)[5,7,9,23,24,25]. In connection with flow-test calculations[2,5] and a compilation of geologically plausible syn- and post-emplacement scenarios[7,8,9,10,11,12,14,15,27,28], we use the AMS results to explain sheet emplacement in the Alnö ring complex
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