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

Abstract

S U M M A R YEarlier palaeomagnetic work on the Mashonaland dolerite sills and dykes (1830-t 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.

Mesoproterozoic intraplate magmatism in the Kalahari Craton: A review

“…7), and whole-rock samples of the dyke plot on the same Rb-Sr errorchron as samples of the Umkondo sills and lavas in eastern Zimbabwe (Allsopp et al, 1989b). Some 90 km to the east of this dyke, palaeomagnetic data suggest a possible Umkondo-age remagnetization at one site in a Mashonaland sheet that elsewhere has a stable, Mashonaland-type magnetization direction (Bates and Jones, 1996). Mushayandebvu et al (1994) found that the Umvimeela Dyke, a satellite of the Archaean Great Dyke, has a widespread secondary component of magnetization with a direction similar to the Umkondo pole, and they attributed the secondary component to chemical alteration associated with regional Umkondo magmatism.…”

“…Widespread dolerite sheets intrude Archaean basement over a large area in the northeastern part of the country, but palaeomagnetic data show them in general to belong to the older, Palaeoproterozoic Mashonaland suite (McElhinny and Opdyke, 1964;Bates and Jones, 1996). Mashonaland and Umkondo dolerites are difficult to distinguish in the field, however, and it is possible that some intrusions assigned to the Mashonaland suite belong instead to the Umkondo Province.…”

Baddeleyite U–Pb ages and gechemistry of the 1875–1835 Ma Black Hills Dyke Swarm across north-eastern South Africa: part of a trans-Kalahari Craton back-arc setting?

“…1(A)), dated by a poorly constrained Rb-Sr age of 1900 ± 600 Ma and palaeomagnetic results (Wilson et al 1987), are regarded with caution, because their depleted geochemical signatures do not match (Stubbs et al 1999;. Söderlund et al (2010) has shown how the Sebanga swarm, previously matched to Mashonaland sills on the basis of palaeomagnetic results (Bates & Jones 1996;Hanson et al 2004a), are c. 600-500 Ma older and also unrelated with the "Mashonaland-Soutpansberg" event. Nevertheless, the Mashonaland and post-Waterberg sills cover an even larger region, extending from the Waterberg Group on the Kaapvaal Craton to the northern part of the Zimbabwe Craton (indicated in green in Fig.…”

Paleoproterozoic intraplate magmatism and basin development on the Kaapvaal Craton: Age, paleomagnetism and geochemistry of 1.93 to 1.87 Ga post-Waterberg dolerites

“…Additional age constraints are provided by relations between the northern part of the Waterberg Group and Formation (Evans et al, 2002); LPA: Limpopo gneiss group A (Morgan, 1985); Mash: Mashonaland dolerites (Bates and Jones, 1996); PA1, PA2: Phalaborwa Complex (Morgan and Briden, 1981); Seb-1, Seb-2: Sebanga dykes in Zimbabwe (Bates and Jones, 1996); SR: Sand River dykes (Morgan, 1985). See text for age controls on poles.…”