PHYSICAL AND CHEMICAL EVIDENCE ON THE CAUSE AND SOURCE CHARACTERISTICS OF FLOOD-BASALT VOLCANISM

Flood basalt volcanism, defined by huge accumulations of magma erupted over relatively short time intervals, is a primary means of extracting melts from the mantle to form new crust. Even though the duration of volcanism is short within any given province, the volumetric sum of oceanic and continental flood basalt accumulations in the Phanerozoic rivals the amount of crust created in the worldwide system of convergent margins. The larger flood basalt provinces consist of millions of cubic kilometres of tholeiitic basalt erupted over time intervals as short as a million years or less. Magma production rates required to supply flood basalt volcanism are at least an order of magnitude greater than is typical of intraplate or convergent margin volcanism. The very high melting rates associated with flood basalt volcanism are difficult to explain by simple adiabatic decompression of mantle rising beneath an extensional continental terrain and may require a contribution from anomalously hot mantle. In most cases, flood basalt eruptions can be correlated in time and space with the initial stages of activity of volcanic hot spots leading to the conclusion that the anomalously hot mantle may be supplied by a plume from the deep mantle. In their chemical and isotopic characteristics, some flood basalts mimic the composition of the basalts associated with the companion intraoceanic hot spot thereby supporting the flood basalt-hot spot link. Others trend toward compositions similar to oceanic lavas derived from depleted mantle. Many flood basalts, however, have chemical characteristics typical of shallow (i.e. lithospheric) fractionation and isotopic signatures more typical of continental crust than of the convecting mantle. Some of these characteristics may be imposed upon magmas by crustal or lithospheric-mantle contamination, others may be signatures of magma sources in the subcontinental lithospheric mantle.