Aspects of Precambrian palaeomagnetism, with new data from the Limpopo mobile belt and Kaapvaal craton in Southern Africa

Abstract

Palaeomagnetic results from Precambrian rocks suggest that the geomagnetic field in the Precambrian was similar in major respects to the Recent field, although there is a possibility that reversals were less frequent. Limitations of precision in Proterozoic palaeomagnetic and geochronological data, and inadequate geographic spread of these data, prevent the making of complete Proterozoic continental reconstructions. Nevertheless, where the data are most abundant, Precambrian apparent polar wander (APW) paths can be used in determining the relative movement of cratons and groups of cratons. Relative movement between Africa, Laurentia and Australia during at least part of the Proterozoic seems to be indicated, but the lack of significant differences between the APW paths of several pairs of presently adjacent Proterozoic cratons suggests that some Proterozoic orogenic belts were not formed by the collision of previously widely separated blocks. Palaeomagnetic studies on metamorphic and igneous rocks from within Proterozoic orogenic belts, notably the Grenville belt in Canada, show that the effects of slow post-orogenic cooling are commonly detectable palaeomagnetically. The problems and potential of Precambrian palaeomagnetism are illustrated by studies on rocks from the Kaapvaal craton and adjacent Limpopo mobile belt in southern Africa, and new data are presented from (i) a stratigraphically important horizon in the Waterberg Supergroup, (ii) the Palabora igneous complex and (iii) the metamorphic rocks of the Limpopo belt. The results from the Palabora complex indicate two distinct bursts of igneous activity at ca. 2000 and ca. 1900 My. The rocks of the Limpopo belt have two stable magnetizations, these sometimes residing together in the same specimens and resolvable only by vector analysis of detailed demagnetization data. Component A is thought to have been acquired during cooling of the belt at about 2000 My, and this result, together with one previously published, suggests diachronous cooling across the belt with a polarity change occurring during the cooling interval. Component B is thought to be a later CRM but its exact age is unknown. The new data assist in refining the African APW path for the interval ca. 2000 to ca. 1850 My. This segment of the African APW path is shown to have a distinctive polarity pattern, and a similar pattern is also recognised in the Laurentian APW path for this interval. These polarity patterns help to resolve the ambiguity of geographical polarity to be assigned to these APW paths and, following from this, evidence is presented which suggests that Africa and Laurentia were in relative motion at about 1950 My.