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

Abstract

Rocks containing breakdown products of majoritic garnet, derived from the deep upper mantle, occur in kimberlite xenoliths and in orogenic peridotites from Otrøy in Norway. The Otrøy peridotites are banded harzburgites and dunites with similar compositions to mantle xenoliths from Precambrian cratons and Phanerozoic supra-subduction-zone peridotites. Pressure–temperature (P–T) paths deduced for the Otrøy peridotites and kimberlite xenoliths from South Africa are consistent with emplacement of deep mantle peridotites into cratonic lithosphere by asthenosphere diapirism. Numerical thermo-convection models provide insight into the possible P–T histories of deep upper-mantle rocks. In the models, material from the base of the convecting system is transported to depths of 60–100 km by convection and small (50–100 km) diapirs. Diapir intrusion induces small-scale convection in the low-viscosity deeper part of the thermochemically defined lithosphere. Small-scale convection in the craton root can produce complex P–T paths, complex recurrent melting histories and complex compositional structure in the craton. P–T paths derived from the numerical models for asthenosphere diapirism in a hot upper mantle are consistent with the sequence of sub-solidus P–T conditions deduced for the cratonic peridotites.