Normal and anomalous AMS fabrics in gabbroic sills: examples from the Karoo Large Igneous Province
Abstract
The magnetic fabric of plutonic rocks has often been used as a proxy for magma flow. Yet, a number of studies suggest that the relationship between the principal axes of the anisotropy of magnetic susceptibility (AMS) and the flow referencial axes are not simple. Several case studies on mafic dikes have shown that complications may arise from the contribution of single domain (SD) magnetite grains, imbricated symmetric fabrics along dike margins and post-solidification thermal stresses. Fewer investigations have been dedicated to subhorizontal tabular intrusions despite the fact that they could also yield valuable clues regarding the various processes that might cause anomalous AMS fabrics. The Karoo Large Igneous Province in South Africa hosts a remarkably impressive set of undeformed, stacked gabbroic sills that were intruded parallel to bedding in the Karoo Basin. The sills range in thickness from about 1 m up to 1000 m and have a relatively constant petrological composition of gabbros and gabbro-norites. Theses sills are distributed throughout the whole Karoo Basin and were emplaced at various stratigraphic heights / depths in the Karoo stratigraphic column. Oriented core samples were collected from 30 different sills and yielded 1598 specimens for AMS, AARM and paleomagnetic measurements. The low-field magnetic susceptibility Km ranges widely from about 100 to 20,000 x 10-6 [SI], while the degree of anisotropy P' ranges from 1.01 to 1.10. A broad correlation between Km and P' is observed. Thermomagnetic experiments reveal that the main magnetic carrier is titanomagnetite with variable ulvöspinel content. This is confirmed by measurement of hysteresis properties that also indicate that titanomagnetite in general has a pseudo-single domain grain size. The directional data is consistent with the nearly horizontal attitude of the sill in 23 out of 30 sills, with subvertical K3 axes. In 5 out of 30 sills, K3 axes are subhorizontal, characterized by scattered directional data and are considered anomalous AMS fabrics. K1 axes are systematically subhorizontal. Preliminary AARM data suggests that titanomagnetite grains in anomalous stations are slightly flattened in the vertical plane while being subhorizontally elongated. Such fabrics could be explained by near solidus thermal stresses similar to those that occur in columnar basalts. Alternatively, the internal structure of titanomagnetite grains, which is some cases exhibits complex ilmentite exsolution microstructures, could account for the anomalous AMS.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMGP41A1033L
- Keywords:
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- 1518 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic fabrics and anisotropy;
- 1519 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic mineralogy and petrology;
- 8414 VOLCANOLOGY / Eruption mechanisms and flow emplacement;
- 8439 VOLCANOLOGY / Physics and chemistry of magma bodies