EVOLUTION OF THE PRECAMBRIAN LITHOSPHERE - SEISMOLOGICAL AND GEOCHEMICAL CONSTRAINTS

Several recent models of crustal evolution are based on the belief tha t the thickness of the continental crust is proportional to its age, w ith ancient crust being the thickest. A worldwide review of seismic st ructure contradicts this belief and falsifies these models, at least f or the Archean. Proterozoic crust has a thickness of 40-55 km and a su bstantial high-velocity (>7 km/s) layer at its base, while Archean cru st is only 2740 km thick (except at the site of younger rifts and coll isional boundaries) and lacks the basal high-velocity layer. Seismolog y also provides evidence that the lithosphere is thickest beneath Arch ean cratons, while diamond ages show that this lithospheric keel must have already existed in the Archean. Geochemical data also indicate si gnificant differences between Archean and Proterozoic lithosphere. Maj or and trace element studies of sediments show a change in upper crust al composition between the Archean and Proterozoic. Archean rocks are depleted in Si and K and enriched in Na, Ca, and Mg. There is also a m arked change in the Eu/Eu ratio. Mantle xenoliths and continental flo od basalts show that the mantle lithosphere beneath Archean crust is u ltradepleted in FeO compared to that beneath post-Archean crust. The s ecular change in the crust-forming process is attributed to a decline in mantle temperature, leading to a change in the composition of the l ithospheric mantle. The higher temperature of the Archean mantle led t o the eruption of komatiitic lavas, producing a refractory lithospheri c mantle which is ultradepleted in FeO and volatiles. The resultant li thospheric keel is intrinsically less dense than the surrounding mantl e and thus not susceptible to delamination. It was sufficiently thick and cool for diamonds to form during the Archean. In contrast, Protero zoic crust developed above fertile mantle. The eruption of continental flood basalts and underplating of basaltic sills is attributed to sub sequent heating and partial melting of the lithospheric mantle. Conseq uently, Proterozoic crust is thickened and has a high-velocity basal l ayer.