PALEOMAGNETISM AND GEOCHRONOLOGY OF WIDESPREAD DOLERITE SILLS IN BOTSWANA RELATED TO THE 1.1 GA UMKONDO LARGE IGNEOUS PROVINCE

The 1.1 Ga Umkondo igneous province comprises widespread within-plate igneous activity in southern Africa and may record the impact of a mantle plume on the Kalahari craton during assembly of the Rodinia supercontinent. The province includes dolerites in eastern Zimbabwe and bimodal magmatic rocks in western Botswana and Namibia that have U-Pb zircon and baddeleyite ages of 1107 ± 2 to 1099 ± 9 Ma. Extensive dolerite sills in SE Botswana and South Africa also are included in the province because they have similar paleomagnetic poles to the type Umkondo dolerites in eastern Zimbabwe (Jones and McElhinny, 1966). This correlation is supported to some extent by available Rb-Sr and K-Ar dates from the sills. In order to extend the paleomagnetic data base for these rocks, 26 sites were sampled from 13 large dolerite sills in SE Botswana, over an area of 29000 square km. All samples were subjected to step-wise AF demagnetization. Most samples lose > 90% of their remanence after 60 mT demagnetization, and a common direction of magnetization is typically revealed in the 60 to 100 mT range. 68 samples from 11 sites so far have been fully demagnetized, and their directions have been calculated by principal component analyses. The mean of 10 site means has a declination of 185º and an inclination of 0º and includes one site which is antipodal to this group. The scatter of the site means is well within the range of secular variations. The mean pole position of these 10 sites lies at 39ºE/66ºN with an A95 of 5.2º. One site yields a very different direction. Our new pole position is in excellent agreement with poles from other parts of the Umkondo province. U-Pb zircon and baddeleyite geochronology on the dolerite sills in SE Botswana is in progress in order to better constrain the timing of the mafic magmatism as well as the age of magnetization. Preliminary results from four separate sills have yielded U-Pb baddeleyite ages of 1112 ± 3, 1111 ± 2, 1108 ± 2 and 1107 ± 7 Ma. A single zircon grain from a fifth sill has yielded a U-Pb age of 1080 ± 4 Ma. The results support emplacement of large parts of the Umkondo province in a limited time frame at ~1.1 Ga, consistent with plume origin.