From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Bureau Seminar, October 22, 2004

Paleomagnetic Data from the Precambrian of Africa:
a Cautionary Note to Plate Pushers

Wulf Gose
Research Scientist
Department of Geological Sciences
The University of Texas at Austin
512-471-3596

Abstract:

The Kalahari craton of southern Africa is a key block in the Rodinia supercontinent. It has been suggested by Dalziel, Mosher, and Gahagan (2000) that it was the indentor that collided with Texas about 1,100 Ma. That speculation was permissible with available paleomagnetic and radiometric data; however, both data sets were of marginal quality. New paleomagnetic and age data resulted in a different interpretation (Hanson et al., 2004).

The Umkondo sills are widespread within Proterozoic successions in the Kalahari craton, as well as in the Grunehogna Province of East Antarctica. Single-crystal U-Pb baddeleyite and zircon analyses yielded ages ranging from 1,106 to 1,112 Ma. Forty-seven paleomagnetic sites gave consistent directions of magnetization, but thirteen additional sites, which were mapped as Umkondo and even in thin section look like Umkondo dolerites, gave a distinctively different direction. Geochronological analyses dated these sites between 1,868 and 1,881 Ma.

The apparent polar wander path for the Kalahari craton has no data between 1,100 and 1,000 Ma, making Rodinia reconstructions highly ambiguous. In an attempt to fill the data gap, I collected rocks in the Namaqua-Natal belt in the KwaZulu-Natal Province that had zircon SHRIMP ages of 1,082±2 Ma and 1,025±8 Ma. Thermochronological studies showed that this metamorphic belt cooled below the 500°C isotherm only at about 1,005 Ma. The magnetite-residing magnetic remanence of the Port Edward charnockite was acquired at that time. But the older Equeefa dikes have magnetic blocking temperatures of about 350°C; they were remagnetized during burial and exhumation at 530 Ma.
This work points out the need of establishing the time of magnetization before using paleomagnetic data for plate reconstructions.

Dalziel, Mosher, Gahagan: J. Geology, 108, 499–513, 2000.
Hanson et al.: Science, 304, 1126–1129, 2004.