PALEOMAGNETISM AND GEOCHRONOLOGY OF NEOPROTEROZOIC INTRAPLATE IGNEOUS ROCKS IN THE SW KALAHARI CRATON, NAMIBIA AND SOUTH AFRICA

Neoproterozoic anorogenic magmatic assemblages in the SW Kalahari craton include subvolcanic granite-syenite intrusions of the Richtersveld Igneous Complex (RIC) and Older Bremen Complex (OBC), and the younger, regionally extensive Gannakouriep dolerite dike swarm (GDS). Their emplacement preceded and accompanied rifting leading to opening of the Adamastor Ocean to the west. Parts of the RIC within the study area have yielded published U-Pb zircon ages of 830-800 Ma. Our new CA-TIMS U-Pb zircon dating indicates that older parts of the RIC (Sjambok River pluton) were emplaced at ~890 Ma, and syenite within the OBC has also yielded a zircon date of ~890 Ma. Two sites in the 1.5-km-wide Gannakouriep dike (the largest dike of the GDS, for which the swarm is named) have yielded baddeleyite dates of ~790 and ~785 Ma, indicating the likelihood that different parts of the dike were emplaced over a significant time span. Dating of additional units in the RIC, OBC, and GDS is in progress.

Progressive AF demagnetization was performed on samples from 6 RIC sites, 5 OBC sites, and 21 GDS sites, distributed over ~10⁴ km². Seven sites carried two antipodal directions. This observation suggests remagnetization because it is highly improbable that these sites acquired their magnetizations during a polarity transition. The time span between the acquisition of these two components must be sufficiently short so that their directions were not affected by plate motion. Remagnetization is also suggested by the fact that, in spite of the different crystallization ages of the rock units, the 25 poles of normal and 14 poles of reversed polarity are similar and can be combined to yield a mean pole position at 74° N, 238° E, A95=6.6°. One possible interpretation is that the RIC was remagnetized by the Gannakouriep dikes. Comparison with the African APWP raises the alternate possibility that all rocks were remagnetized during the early stages of the opening of the South Atlantic Ocean. Preliminary U-Pb data on apatite grains seem to support the first alternative.