Palaeomagnetism of Precambrian Birrimian and Tarkwaian Rocks of West Africa

Abstract

Summary This paper reports the first palaeomagnetic results from Precambrian rocks of the West African region. Five statistically-significant sites collected from a greenstone body (2200 My) in the Ashanti gold mines at Obuasi, Ghana, give a total NRM grouping of D = 320, I = 26(01~~ = 19). This collection does not respond to a.f. cleaning, and IRM studies suggest that the NRM may be carried by multidomain magnetite. A dolerite dyke cutting the greenstone body gives a direction of magnetization D = 327 and I = -3(cr,, = 11). The grouping of five sites from basic intrusions in Tarkwaian rocks at Tarkwa, Ghana, is improved when they are corrected for the dip of the intruded sediments when they combine to give a mean direction of D = 156, I = 40(01,~ = 14). The well-grouped sites from this collection show high stability to a.f. demagnetization. Only one of five surface sites in post-orogenic dolerites in eastern Ivory Coast is statistically significant after a.f. cleaning and it gives a direction of D = 281,Z = 14 (ag5 = 39). The palaeomagnetic poles (either north or south) derived from this study all lie in the south-central Indian ocean and are in the vicinity of poles of similar age from Guyana and Suriname after allowance for Mesozoic and later continental drift. The new West African poles, which refer approximately to the time interval 2200-2000 My, lie on the smooth continuation of the polar wander curve derived from southern Africa for the interval 2700-2300 My partially bridging the gap between that curve and the next southern African data (c. 1950 My). This suggests that later intercratonic movement between western and southern Africa has been slight, though more data are needed to confirm this.