Magmatism and the geodynamics of basin formation

Abstract

The magmatic response to extension of the continental lithosphere, with the resultant formation of sedimentary basins, varies widely. Quantitative models (e.g. McKenzie and Bickle, 1988), based upon an assumption of anhydrous partial melting in both lithosphere and asthenosphere, suggest that the temperature of the sub-lithospheric upper mantle, upwelling beneath the zone of extension, exerts a fundamental control on both the chemistry and volume of the erupted magmas. High volcanicity rifts are, in general, only likely to occur when the asthenospheric potential temperature is ca. 100–300°C greater than normal, associated with the activity of mantle plumes. In addition, such models also predict a strong correlation between the volume of magma produced and the amount of lithospheric stretching (β). More complex models (e.g. Gallagher and Hawkesworth, 1992) include partial melting of volatile-rich mantle source regions with lower than normal solidus temperatures. These may reside within mantle plumes or within enriched domains within the continental lithospheric mantle. Quantitative modelling of the magma generation process in the latter case is limited by our rather poor understanding of volatile-present partial melting of the range of potential source components. Geochemical and isotopic (Sr—Nd—Pb—O) studies of the igneous rocks emplaced within a basin are of fundamental importance in evaluating the relative roles of lithospheric versus asthenospheric source components in the petrogenesis of the magmas and the depth and degree of partial melting. In addition, these data may provide important constraints on the amount of crustal contamination which has occurred, particularly by partial melting of the lower crust. Knowledge of the latter is important in the development of models to explain crustal thinning. In many basins the bulk of the mantle-derived magmas may never actually reach the surface, but are underplating and intruding the lower crust instead. Identification of such a mafic underplate is critical if the amount of crustal thinning is to be evaluated correctly. The location of eruptive sites within individual basins is frequently determined by pre-existing basement structures. Within the extensive Mesozoic rift system of West and Central Africa, Pan-African lithospheric shear zones and deep basement faults have acted as foci for rift basin development, with the faults acting as magma pathways to the surface. In western and central Europe, Neogene—Recent extension has generated an extremely varied magmatic response which can be correlated with the Hercynian structural grain of the lithosphere. The Rhine graben, trending at a high angle to the Hercynian terrane boundaries is largely amagmatic. In contrast the Ohre rift of Czechoslovakia, which trends parallel to a major (Saxothuringian—Moldanubian) terrane boundary, is a classic example of a pure shear rift with major volcanic complexes located along the axial zone.