THE EARTH’S CRUST IN PRECAMBRIAN TIMES

Abstract

SUMMARY Primitive continental crustal masses rich in granitic material, land-areas subject to erosion, and primitive oceans in which sedimentation took place were already in existence when the oldest known rocks were formed about 3800 Ma ago. The initial development of a concentrically zoned earth (which overlapped with the main phases of activity on the moon) had taken place before the beginning of the geological record as we know it. The extent, distribution and thickness of the early continental masses have still to be established. The continental crust appears to have received large intermittent accessions of granitic material from the mantle over a very long time but it is not clear whether the crust was continually enlarged or whether some process of recycling kept its volume roughly constant. Certain crust-forming magmatic processes originating in the mantle appear to have been peculiar to the period prior to 2500 Ma. Surface processes associated with organic activities began at least as early as 3000 Ma and changed markedly with time. Considered in broad terms, the structural patterns characteristic of Archaean domains formed before about 2500 Ma differ from those of younger domains which are known to have been produced in response to the motions of large and effectively rigid lithospheric plates. Large-scale variations in structural style and in metamorphic grade suggest regional variations either in the composition of the crust and upper mantle or in geothermal gradients. The forms and distribution of layered gabbro-anorthositc complexes have a bearing on this question. The large volumes of material of granitic composition which characterise Archaean provinces show distinctive relationships with other rock units which may record the effects of substantial enlargement of the continental crust. Profound changes in the style of sedimentation, tectonic activity and magmatism beginning at about 2700 Ma ushered in the Proterozoic stage of crustal development.