Probing the southern African lithosphere with magnetotellurics, Part II,
linking electrical conductivity, composition and tectono-magmatic
evolution.
Abstract
The tectonic history of Southern Africa includes Archean formation of
cratons, multiple episodes of subduction and rifting and some of the
world’s most significant magmatic events. These processes left behind a
compositional trail that can be observed in xenoliths and measured by
geophysical methods. The abundance of kimberlites in southern Africa
makes it an ideal place to test and calibrate mantle geophysical
interpretations that can then be applied to less well-constrained
regions. Magnetotellurics (MT) is a particularly useful tool for
understanding tectonic history because electrical conductivity is
sensitive to temperature, bulk composition, accessory minerals and rock
fabric. We produced three-dimensional MT models of the southern African
mantle taken from the SAMTEX MT dataset, mapped the properties of
$\sim36000$ garnet xenocrysts from Group I kimberlites,
and compared the results. We found that depleted regions of the mantle
are uniformly associated with high electrical resistivities. The
conductivity of fertile regions is more complex and depends on the
specific tectonic and metasomatic history of the region, including the
compositions of metasomatic fluids or melts and the emplacement of
metasomatic minerals. The mantle beneath the $\sim
2.05$ Ga Bushveld Complex is highly conductive, probably caused by
magmas flowing along a lithospheric weakness zone and precipitating
interconnected, conductive accessory minerals such as graphite and
sulfides. Kimberlites tend to be emplaced near the edges of the cratons
where the mantle below 100 km depth is not highly resistive. Kimberlites
avoid strong mantle conductors, suggesting a systematic relationship
between their emplacement and mantle composition.