A palaeomagnetic investigation of the Mashonaland dolerites, north-east Zimbabwe

Abstract

Earlier palaeomagnetic work on the Mashonaland dolerite sills and dykes (1830 ± 230 Ma) showed that they record a geomagnetic field reversal. 28 new sites have been sampled in sills (26) and dykes (2) in north-east Zimbabwe. Detailed alternating-field demagnetization (AFD) is more effective in removing secondary components of remanence than thermal demagnetization (TD) and enables an assumed primary remanence to be isolated at 16 sill sites. Combined with earlier results there are 11 sill sites with a positive inclination (P) (D = 287°, I = 53°, k = 40) and 18 with a negative inclination (N) (D = 114°, I = -47°, k = 35). The data lead to a positive class-B reversal test. Field tests suggest that the N remanence is the younger and that the period of Mashonaland igneous activity spanned a reversal of the Earth's magnetic field from P to N. The areal distribution of polarity is not random: sills with only P remanence are found in the more elevated central region of the Mashonaland Igneous Province, whilst predominantly N-bearing sills occupy the lower surrounding terrain.