Emplacement of Deep Upper-Mantle Rocks into Cratonic Lithosphere by Convection and Diapiric Upwelling

Rocks containing breakdown products of majoritic garnet, derivedfrom the deep upper mantle, occur in kimberlite xenoliths andin orogenic peridotites from Otrøy in Norway. The Otrøyperidotites are banded harzburgites and dunites with similarcompositions to mantle xenoliths from Precambrian cratons andPhanerozoic supra-subduction-zone peridotites. Pressure–temperature(P–T) paths deduced for the Otrøy peridotites andkimberlite xenoliths from South Africa are consistent with emplacementof deep mantle peridotites into cratonic lithosphere by asthenospherediapirism. Numerical thermo-convection models provide insightinto the possible P–T histories of deep upper-mantle rocks.In the models, material from the base of the convecting systemis transported to depths of 60–100 km by convection andsmall (50–100 km) diapirs. Diapir intrusion induces small-scaleconvection in the low-viscosity deeper part of the thermochemicallydefined lithosphere. Small-scale convection in the craton rootcan produce complex P–T paths, complex recurrent meltinghistories and complex compositional structure in the craton.P–T paths derived from the numerical models for asthenospherediapirism in a hot upper mantle are consistent with the sequenceof sub-solidus P–T conditions deduced for the cratonicperidotites. KEY WORDS: asthenosphere diapirs; cratonic lithosphere; deep upper mantle; majoritic garnet