Mantle plumes, flood basalts, and thermal models for melt generation beneath continents: Assessment of a conductive heating model and application to the Paraná

Abstract

Growing evidence suggests that there is more than one type of continental flood basalt (CFB). Many CFB, such as the Deccan, were probably derived by decompression melting of asthenospheric peridotite in a mantle plume resulting in high eruption rates (∼1 km3 yr−1) and geochemical signatures of uncontaminated basalt which are similar to ocean island basalts. However, several geochemical studies have concluded that other CFB were derived from the subcontinental lithospheric mantle. This requires that melting occurs via heating of lithospheric peridotite above a mantle plume instead of melting within the plume itself. The feasibility of this model is tested with a simple one‐dimensional, time‐dependent thermal model. The results show that 1–2 km thick CFB may be derived from the lithospheric mantie without melting occurring within the underlying mantle plume if (1) the mantle potential temperature of the plume is between 1380 and 1580°C; (2) the lithospheric mantle is composed of volatile‐enriched peridotite; (3) the overlying lithosphere is >100 km thick; (4) eruption occurs over 10–15 Myr; and (5) melting occurs over an area similar to the surface distribution of basalt. Small volumes of alkalic basalt may precede the eruption of tholeiites and if higher plume temperatures prevail, or extension/thermal erosion leads to lithospheric thicknesses ≤100 km, melts from the plume will rapidly dominate those from the lithosphere and eruption rates will increase (magmatic underplating may reduce the strength of the lithosphere sufficiently to initiate or focus rifting if the lithosphere is already under tension). These predictions can be used to discriminate between different CFB and are illustrated by application to the Paraná CFB using published data along with new geochemical data from a borehole through the thickest section of basalt.