A Discussion on the evolution of the Precambrian crust - Problems of the evolution of the continental crust

• Published in: Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 1973
• Subjects: Shackleton; Iceland
• Online Access: http://dx.doi.org/10.1098/rsta.1973.0003; https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.1973.0003

In introducing this symposium I shall draw attention to some of the main problems concerning the early stages in the evolution of the continental crust. First, we wish to know how early the continental crust began to form, how fast it accumulated, when and by what processes the granitic and granodioritic rocks which form so much of it separated from the mantle, and whether these granitic materials were added continuously to the crust or in sharply defined phases. A few years ago it was thought that by studying 87Sr/86Sr initial ratios, it would be possible to discriminate between granitic rocks derived directly from the mantle and those derived from pre-existing crust. This method has shown that increments of granitic material are still being added to the crust from the mantle, as in Iceland (Moorbath & Walker 1965). But recycling of crustal material through the mantle by way of subduction zones, depletion of the deeper parts of the crust in rubidium and the smaller difference between mantle and crustal strontium isotope ratios early in the Earth’s history make this method less decisive when it is applied to very old granites such as those which surround the greenstone belts of the Rhodesian craton and the Barberton mountain land. It has been proposed that the greenstones in these regions accumulated in an island arc environment on a thin crust (Anhaeusser, Mason, Viljoen & Viljoen 1969) and that most of the enveloping granites represent subsequent additions to the crust (White, Jakes & Christie 1971). Yet before 2550 Ma ago when the Great Dyke of Rhodesia was intruded, the granites which constitute much of the Rhodesian craton had already been emplaced and the crust there must have reached almost its present thickness. Structural and stratigraphic evidence suggest that much of the granitic material represents reactivated continental crust older than the rocks of the greenstone belts (Shackleton 1970). Isotopic studies of lead from the Tanzanian, Rhodesian and Canadian cratons indicate that a ...