13 Posters II

Tuesday, 17 March 2009
8:00 AM - 6:00 PM

NEW PALEOMAGNETIC RESULTS FOR NEOPROTEROZOIC INTRAPLATE IGNEOUS ROCKS IN SOUTHERN NAMIBIA AND ADJACENT PARTS OF SOUTH AFRICA

Paper number: 13-21
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

There are no reliable, well-dated paleomagnetic poles in the literature for the Kalahari craton (southern Africa) in the critical time period 900-750 Ma, between breakup of the Rodinia supercontinent and assembly of Gondwana. We report paleomagnetic data for intraplate igneous rocks in the SW part of the Kalahari craton, which were emplaced prior to formation of the Neoproterozoic Adamastor Ocean to the west (in present coordinates). Our sample sites extend over an area of 10,000 km2 and include granitic-syenitic intrusions and felsic dikes of the Richtersveld Igneous Complex (RIC) and Older Bremen Complex (OBC), as well as the widespread Gannakouriep diabase dike swarm (GDS). One sampled intrusion within the RIC has a published U-Pb zircon date of ~830 Ma. We have obtained CA-TIMS U-Pb zircon dates of ~890 Ma from the OBC and the main batholith within the RIC, and baddeleyite dates of ~790-785 Ma for the GDS.

Alternating-field and thermal demagnetization yielded usable data from 36 sites in the RIC, OBC, and GDS. Rock-magnetic experiments indicate that magnetite is the main magnetic carrier. Seventeen sites have directions with normal polarity, and 9 have reversed directions; samples from another 10 sites show both polarities. The two directions are statistically antipodal, both at the site and between-site level. The mean poles for the RIC, OBC, and GDS are nearly identical and yield a combined pole of 74° N, 234° E (A95 = 4.5°). The fact that units with ages 100 m.y. apart yield the same pole position implies that at least some of the sampled units have been remagnetized. The mean pole is far from the 520 Ma pole for Africa, indicating that Pan-African orogenesis did not cause the remagnetization. The pole does fall on the ~70 Ma portion of the African APWP, but there is no known event at that time that could cause such widespread remagnetization. We infer that the mean pole records remagnetization during emplacement of the widespread GDS and thus has an age near 790 Ma. Comparison with sparse paleomagnetic data from broadly coeval units in the Congo craton suggests that a wide ocean may have separated Kalahari and Congo prior to Gondwana assembly, but additional data are needed to rigorously test this hypothesis.