Magma Flow Pattern Inferred from Magnetic Fabrics in a 100 km-long Dolerite Dike, Karoo LIP, South Africa
Abstract
Magma flow in dikes is not yet fully understood in part due to lack of direct observations. Recent literature has provided numerous examples of dikes in which fabrics did not match expected flow models. Causes of complication arise from the possible existence of imbrication fabrics, back-flow in a conduit, thermal contraction-related stresses, or post-solidification alteration. Magnetic fabrics, in particular the Anisotropy of Magnetic Susceptibility (AMS) has been extensively used as a proxy for magma flow. However AMS is particularly sensitive to all above listed complications. The Karoo Large Igneous Province (LIP) of South Africa offers prominent and unaltered dolerite dikes that can be used to constrain magma flow in a tabular vertical conduit. The studied dike is more than 100 km long and 30 m wide between Cradock and Middleburg. Samples were collected both along and across the dike in order to evaluate the dominant flow direction, as well as whether magma flow was consistent throughout the dike and if magma was fed through the whole dike or through a single conduit. A series of magnetic measurements including AMS, Anisotropy of Anhysteretic Remanent Magnetization (AARM), hysteresis and First Order Reversal Curve (FORC) were performed. In addition, the mineral fabric is investigated by image analysis on several phases to determine the magma flow fabric. The bulk magnetic susceptibility, Km, measured in low field, ranges from 1.5 to 6.1x10-3 [SI] across the dike. The magnitude of Km and petrographic observations indicate that the dominant carrier is ferromagnetic titanomagnetite. The variations of Km at the scale a hand specimen are very large (10 x), suggesting that titanomagnetite grains are not homogeneously distributed regardless of sample position across the dike. The corrected degree of anisotropy, P’, is generally rather small and also shows large variations across the dike and within individual sample. P’ tends to increase significantly along the dike margin. The shape factor T shows a broad pattern of oblate fabrics along the dike margins and prolate fabrics in the centre. This preliminary data will also allow to evaluate the role played by single domain (SD) magnetite, a possible cause of inverse fabric, in generating complex fabrics, especially along dike margins. The orientation of the AMS principal axes does not support an imbricat
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMGP43A0837R
- Keywords:
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- 1518 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic fabrics and anisotropy;
- 1519 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic mineralogy and petrology;
- 8434 VOLCANOLOGY / Magma migration and fragmentation;
- 8439 VOLCANOLOGY / Physics and chemistry of magma bodies