A Model for Flood Basalt Vulcanism

Abstract

The question of whether basaltic rocks in continental flood basalt provinces are primary magmas or whether they are descended in general from picritic parent magmas is reviewed. It is suggested that the latter is more likely to be correct on the evidence of phase relations and the relative rareness of mantle materials with appropriate Fe/Mg ratios.Major element variations in the residual liquids of fractional and equilibrium crystallization of basaltic magmas are modelled for a variety of crystallizing assemblages. It is concluded that crystallization of olivine, clinopyroxene, and plagioclase has a marked effect on buffering chemical change in many important elements. It is this effect which accounts for the apparent uniformity of large volumes of flood basalts, not, as has sometimes been supposed, a series of implausible coincidences in the amount of material fractionated from each magma batch. It is further argued that much of the variation seen in basalts may be imposed by polybaric fractionation operating throughout crustal depths, that is at pressures up to at least 12 kb. Parental picritic magmas rising from the mantle reach the surface in exceptional areas of crustal thinning. More usually, however, it is suggested that they intrude the base of the crust as a series of sills which differentiate into upper gabbroic and lower ultramafic portions. Much of the 'low pressure' fractionation of basaltic magmas may take place in this deep crustal sill complex and evolved liquids are transmitted to the surface as their density becomes sufficiently low. This implies that in areas of flood vulcanism a potentially large new contribution to the crust is made by underplating, the volumes of concealed cumulates being at least as large as the amount of erupted surface lava.

Were the Deccan Flood Basalts Derived in Part from Ancient Oceanic Crust Within the Indian Continental Lithosphere?

“…Huge piles of cumulates should then be expected in the subsurface under the Deccan. Cox (1980) indeed suggested extensive crustal or subcrustal sill complexes and cumulate layers, and strongly argued for the derivation of Deccan basalts from picritic parent liquids (Cox, 1999). He considered only a peridotite mantle source probable.…”

Sedimentation record in the Konkan–Kerala Basin: implications for the evolution of the Western Ghats and the Western Indian passive margin

“…There is a general lack of stratigraphic markers to constrain the escarpment's evolution and so palaeosurfaces have been used to reconstruct the denudational chronology of the margin (Radhakrishna, 1967;Vaidyanadhan, 1977;Radhakrishna, 1993;Gunnell, 1997;Widdowson, 1997).. Post-rift surface uplift is certainly evident along the Western Indian margin (Radhakrishna, 1967;Widdowson & Cox, 1996;Gunnell, 2001a) but its causes remain unclear. Plume-related regional surface uplift and secondary mantle convection have been proposed (Cox, 1980;Thakur & Nagarajan, 7 1992) but these mechanisms are transient and so cannot explain persistent surface uplift and denudation long after the initial rifting event. Lithospheric necking, lithospheric delamination and magmatic underplating have also been invoked but such mechanisms also operate at shorter timescales (McKenzie, 1978;Cox, 1980).…”

“…Plume-related regional surface uplift and secondary mantle convection have been proposed (Cox, 1980;Thakur & Nagarajan, 7 1992) but these mechanisms are transient and so cannot explain persistent surface uplift and denudation long after the initial rifting event. Lithospheric necking, lithospheric delamination and magmatic underplating have also been invoked but such mechanisms also operate at shorter timescales (McKenzie, 1978;Cox, 1980). Other mechanisms that result in surface uplift persisting beyond the transient thermal effects associated with rifting include the impact of the Himalayan collision since the late Miocene (Gowd et al, 1992), and flexural isostatic effects driven by denudational rebound onshore and sediment loading offshore (Widdowson & Cox, 1996;Widdowson, 1997;Gunnell & Fleitout, 1998.…”

Petrochemical study of the Jingpohu Holocene alkali basaltic rocks, northeastern China.

“…The low Cr and Ni contents of TP indicate a relatively evolved magma, consistent with their lower MgO and Mg # . Low Mg # usually means that magmas have undergone fractional crystallization en route to the surface (Cox, 1980;Prestvik and Goles, 1985). Such fractionation will tend to increase the incompatible element concentrations in the basaltic magmas relative to those of the more MgOrich primary magmas.…”