THE SUBLITHOSPHERIC MANTLE AS THE SOURCE OF CONTINENTAL FLOOD BASALTS; THE CASE AGAINST THE CONTINENTAL LITHOSPHERE AND PLUME HEAD RESERVOIRS

Abstract

Continental flood basalt (CFB) provinces have been attributed to a variety of sources. Among these are: (1) ancient continental lithosphere (CL); (2) newly arrived plume heads from the core-mantle boundary; (3) steady-state plumes coincident with CL rifts; (4) fossil plume heads; and (5) contamination of deep upwellings (passive or active) by enriched sublithospheric shallow mantle. The criticism that CL is too cold to provide extensive magmatism has been countered by the proposal that the CL is wet, thereby lowering the melting point. This also lowers the viscosity and increases the local Rayleigh number. The calculated seismic velocities and viscosities in this proposed CL source show that it has a low velocity and is weak and has asthenosphere-like physical properties. It is apparent that what has been called ‘continental lithosphere’ is actually asthenosphere or the lower part of the thermal boundary layer (TBL) and is unlikely to be a long-lived part of the plate. However, a low density and low viscosity region of the sublithospheric mantle (the perisphere) is a suitable reservoir for the enriched component of CFB. It helps to isolate the deeper depleted reservoir from contamination due to recycling at subduction zones. Lithospheric pull-apart at cratonic boundaries, rather than stretching of uniform lithosphere, is suggested as the trigger for extensive continental magmatism.