CONTINENTAL MAGMATIC UNDERPLATING

Three arguments based on geological evidence are put forward to support the importance of magmatic underplating processes during continental flood basalt vulcanism. (1) Petrological evidence of gabbro fractionation in erupted basaltic sequences allows estimates to be made of the minimum total mass of concealed cumulate material, which is retained in deep crustal magma chambers. Possibly along the Moho, and is comparable in amount to the erupted material. (2) In the Karoo province (southern Africa) large volumes of rhvolite along the S.E. continental margin were generated from basaltic precursors, either as partial melts of already-emplaced solid basic material or as crystal fractionation products of large volumes of basic magma. In either case very substantial volumes of concealed basic rocks are at least locally implied. (3) Studies of geomorphology suggest that the area of the Karoo province experienced at least 1 km of permanent uplift associated with the vulcanism. This appears to be the consequence of the emplacement of an underplated gabbroic layer ca. 5 km thick.