PALEOMAGNETISM OF THE LATE ARCHEAN NYANZIAN SYSTEM, WESTERN KENYA

The Nyanzian System lavas of western Kenya are believed to be the olde st rocks of the Tanzanian Craton. Intrusive age relationships suggest an age greater than or equal to 2850 Ma although direct attempts at da ting the Nyanzian have produced disparate results. Our study involves a suite of samples collected from the Nyanzian basalts, pillow basalts , andesites and rhyolites from sixteen sites in western Kenya. These r ocks yield a tilt-corrected paleomagnetic pole at 14 degrees N, 150 de grees E (K=59, dp=5 degrees, dm=7 degrees). This pole is constrained t o be older than the first (D1) deformation (>2472+/-30 Ma) by positive fold, conglomerate and reversal tests. Analysis of the paleomagnetic data base for three African cratonic nuclei (Tanzanian, Kaapvaal/Zimba bwe and West Africa) for the time period from 2.0 Ga to 3.0 Ga demonst rates a paucity of well-dated poles, although there are several poles from the Kaapvaal/Zimbabwe and Tanzanian Cratons which allow ''spot-re adings'' of their relative positions. We demonstrate, based on these d ata, that the Kaapvaal/Zimbabwe and Tanzanian Cratons were drifting in dependently at similar to 2875 Ma, similar to 2700 Ma and similar to 2 450 Ma. This independent motion of the Tanzanian and Kaapvaal/Zimbabwe Cratons indicates that previously proposed models involving African c ratonic coherence can no longer be considered valid for the time perio d from 2850 to 2500 Ma.