PRECAMBRIAN TECTONICS AND THE PHYSICOCHEMICAL EVOLUTION OF THE CONTINENTAL-CRUST .2. LITHOSPHERE DELAMINATION AND ENSIALIC OROGENY

Precambrian ensialic orogeny driven by gravitational instability of the lower crust and underlying lithospheric mantle as a consequence of decaying geotherms has been proposed as an alternative cause for tectonism than modern plate tectonics for the Precambrian. Based on a depleted mantle lithosphere, calculations show that this is unlikely. As a consequence, Precambrian orogenic events, whether at ancient plate margins or in an ensialic regime, had to be caused by modern style plate tectonics. Although the process of Wilson-cycle plate tectonics operated throughout Earth's history, there is evidence to suggest that the products of this process-plate margin volcanism and metamorphism-have changed significantly through time due to a secular decrease in heat flow. There is good evidence that some Proterozoic terranes initially developed in an ensialic extensional setting (Etheridge et al.). However, it is proposed here that the cause of later compressive orogenies in these belts must have been transfer of stresses from distant active plate margins and subduction zones. This conclusion necessarily requires that some Precambrian mobile belts must represent sutures which were formerly ancient active plate margins. Although much of the outcrop and metamorphic evidence is ambiguous in determining the ultimate driving force for compressive orogenesis, there is abundant independent evidence to support subduction in the Precambrian when mantle xenoliths, refractory inclusions in diamonds and the trace element compositions of continental basalts are considered. Isotopic data indicate even Archaean ages for some of these peridotite, harzburgite and eclogite xenoliths and diamond inclusions. In addition, the occurrence of high field strength (HFS) element depletions in many continental mafic dykes and volcanics ranging in age back to the Archaean, can only be satisfactorily explained by this geochemical signature ultimately being caused by subduction. This is correct irrespective of whether the crustal influence is due to subduction of sediment into the mantle or in situ crustal assimilation by ascending basalts. Alternative models for HFS element depletions involving delamination of lower crustal eclogite back into the mantle and its subsequent partial melting are rejected. It is argued that subcontinental lithosphere has remained attached to Precambrian shields as an integral part of moving plates in the Earth's history. Later reactivation of this lithospheric mantle beneath continents may explain the occurrence of HFS element depleted continental basalts, yet their absence from modern oceanic regions.