LOW-LATITUDE GLACIATION IN THE PALAEOPROTEROZOIC ERA

One of the most fundamental enigmas of the Earth's palaeo-climate conc erns the temporal and spatial distributions of Precambrian glaciations . Through four billion years of Precambrian history, unequivocally gla cial deposits have been found only in the Palaeoproterozoic and Neopro terozoic record(1). Nonetheless, some of these deposits are closely as sociated with tropical- rather than just polar-palaeolatitudinal indic ators such as carbonate rocks, red beds, and evaporites(1,2). These ob servations are quantitatively supported by palaeomagnetic results indi cating a similar to 5 degrees latitude for Neoproterozoic glaciogenic rocks in Australia(3-5). Similarly reliable palaeolatitudes for the ol der, Palaeoproterozoic glaciogenic rocks have not yet been obtained, a s such deposits commonly suffer from poor preservation and secondary m agnetic overprinting. The Archaean-Palaeoproterozoic 'Transvaal Superg roup' on the Kaapvaal craton in South Africa is, however, exceptionall y well preserved, and is thus amenable to the palaeomagnetic determina tion of depositional palaeolatitudes. Within this supergroup the simil ar to 2.2 billion-year old Ongeluk lavas are a regionally extensive, l argely undeformed and unmetamorphosed, extrusive volcanic succession(6 ), which conformably overlies glaciogenic deposits (the Makganyene dia mictite). Here we report a palaeomagnetic estimate of 11 +/- 5 degrees depositional latitude for the lavas, and hence for the underlying con temporaneous glacial rocks. The palaeoclimate enigma is thus deepened; a largely ice-free Precambrian world was apparently punctuated by two long ice ages, both yielding glacial deposits well within tropical la titudes.