A HISTORICAL APPROACH TO CONTINENTAL FLOOD BASALT VOLCANISM: INSIGHTS INTO PRE-VOLCANIC RIFTING, SEDIMENTATION, AND EARLY ALKALINE MAGMATISM

Abstract

Continental flood basalts are widely thought to be produced from mantle plume heads. However, plume theories do not observe any role for lithospheric rifting before flood basalt events, and consider irrelevant the fact that most, if not all, continental flood basalts have erupted through deep rifts containing thick sedimentary sequences. At best, plume theories invoke selective capture of such deep rifts or lithospheric thinspots by rising mantle plume heads. However, the fact that CFBs of the world erupted through deep, ancient rift zones, and alternative dynamical considerations of flood basalt genesis, directly lead towards a new, historical approach to flood basalt emplacement. This approach takes cognizance of the basic unity of geological history and processes, satisfactorily explains pre-volcanic rifting, sedimentation, mantle metasomatism, and early, pre-tholeiite, enriched alkaline magmatism for tens of millions of years. Both incubating and impacting plume heads ought to lead to pre-volcanic lithospheric doming which is usually not observed in flood basalts. Continental or oceanic flood basalt events instead seem to be derived by convective partial melting during sudden lithospheric pull-apart (splitting) along pre-existing lithospheric discontinuities such as deep rifts or fracture zones.