A large-scale geochemical zonation of the African LLSVP recorded by mantle plumes?

Abstract:

It is generally accepted that age-progressive volcanism is the surface expression of deep-rooted mantle plumes, which carry geochemical signals from the lower mantle. The association between mantles plumes and two continental-sized anomalous seismic structures at the base of the lower mantle, the African and Pacific Large Low-Shear-Velocity Provinces (LLSVPs), has been established, but the nature of their interaction remains enigmatic. The South Atlantic contains the Tristan-Gough, Discovery and Shona age-progressive seamount chains, and combined geochronological, geochemical, numerical and seismic studies provide evidence that these hotspots are derived from the lower mantle. Since all three South Atlantic mantle plumes share a common enriched mantle one (EMI)-type composition and are located above the margin of the African LLSVP, their source reservoir is most likely located either within the LLSVP or represent the ambient mantle flowing against its steep outer margin.

New geochemical and geochronological data from the Tristian-Gough and Shona hotspot lavas show evidence of younger (post-erosional) seamounts on the oldest portions of these submarine hotspot tracks, which preserve a distinct HIMU-type composition. These HIMU-type seamounts are part of two age-progressive lineaments that extend in the direction of plate motion, towards Southwest Africa. The combination of all these studies provides evidence for two compositionally-distinct, age-progressive paired hotspot tracks with a temporal offset of 30-40 Ma, which corresponds to a distance of 900-1200 km between the EMI- and HIMU-type hotspots sources. In contrast to the primary EMI-type hotspots, the secondary HIMU-type hotspots and St. Helena lavas are located above a more central portion of the African LLSVP, possibly reflecting a large-scale geochemical zonation subparallel to the LLSVP margin. We propose that upwelling of HIMU material from a more internal domain in the form of secondary plumes may be triggered by the extraction of large volumes of EMI-type mantle from the margin of the LLSVP by primary plume heads.