Flood basalts and large igneous provinces from deep mantle plumes: fact, fiction, and fallacy

Abstract

Flood basalts, large igneous provinces and all intraplate magmatism are almost unanimously ascribed to features called mantle plumes in current geodynamics literature. In the starting plume model, flood basalts and any associated hotspot tracks have been ascribed to the melting of a large plume head and a narrow tail respectively, and the Deccan flood basalt province of India and the Laccadives–Maldives–Chagos–Réunion Island volcanic chain in the Indian Ocean are widely thought to be an excellent example supporting this model. However, the existing geodynamic, thermal, petrological, geochemical and geochronological constraints on Deccan volcanism do not require or support a mantle plume origin. A new interpretation made here rules out any role for the Réunion plume in Deccan volcanism, and even questions the notion that the Réunion `hotspot' is the manifestation of an underlying Réunion `plume'. The Laccadives–Réunion hotspot track is viewed here as caused by a southward-propagating fracture, based on the published radiometric age data, and the ultimate cause of Deccan volcanism is argued to be mega-scale lithospheric rifting. The appreciation of the great original areal extent of the Deccan, nearly as far as the southern tip of peninsular India, solves the hitherto unsolved problem of the cause of the uplift of the 1500-km-long Western Ghats escarpment, this cause being combined surface erosion and magmatic underplating. Hypothesized mantle plumes do not appear responsible for most large igneous provinces; instead, their very existence is questionable. No geological evidence of any kind — geochemical, petrological, thermal, topographic — requires mantle plumes. The self-contradictions and special pleading that proliferate in plume explanations are unjustified and unnecessary if one accepts, based on various grounds, that a globe-encircling enriched mantle layer (the `perisphere') may overlie a deeper depleted mantle, which is the source of MORB. The systematic geochemical variations along the Laccadive–Réunion chain can be explained by this model and are not consistent with a plume tail experiencing entrainment from surrounding MORB mantle, as in the current plume model. Hotspot tracks may not reflect plate motion above stationary deep mantle upwellings but the stress state of the lithosphere, as originally proposed decades ago. The locations of large igneous provinces, hotspots and hotspot tracks are strongly controlled by lithospheric architecture and history: they are not placed randomly on the globe, indifferent to the surface geology, neither are large igneous provinces sudden chance events within the vastness of geological time (which plume theories typically allege).