Reassessing modes of Plume-Rift Magmatism

Several types of magmatism are typically associated with continental stretching and rift formation. The South Atlantic Rifted Margin is a particularly well-studied system that exhibits: (1) Thousands-km-long massive dike swarms – likely linked to the Tristan Plume/hotspot; (2) the Parana-Etendeka continental flood basalt (CFB); and (3) the formation of extensive seaward dipping reflector sequences (SDRs) along the southern portion of this rifted margin. Here we review the distribution, timing, and volumes of these different modes of rift-related magmatism in relation to rift evolution.

Great dike swarms formed prior to, during, and soon after the Parana-Etendeka flood basalt event at 136.5-135.5 Ma. Although comparable in spatial extent and volume to the well-known Proterozoic Mackenzie dike swarm that similarly extended from a continental flood basalt, summed dike volumes appear to only be ~10% (0.15e6 km^3) of the Parana CFB magmatism (~1.5e6) and ~2% of total magmatism (~6e6) associated with South Atlantic Rifting including SDR provinces.

The defining characteristic of the CFB event is that it occurred very rapidly, which appears most consistent with a sudden lithospheric thinning event (e.g. lower lithospheric delamination) in the presence of hot plume material. A plume-head rising under thick continental lithosphere simply could not create this sudden burst of volcanic activity, thus an abrupt lithospheric thinning event appears needed to explain this melting anomaly. Note that there is seismic evidence consistent with such a delamination event both in the thinned lower lithosphere beneath Parana and the presence of a delaminated lithospheric fragment in the transition zone near the site of the modern Tristan Plume.

Finally, the largest volcanism associated with South Atlantic rifting is linked to the SDR province including associated underplated magmas offshore the southern margins of South American and Africa. This post-CFB magmatic activity can be quantitatively explained by more extensive melting of southward flowing Tristan Plume material after extensive rifting has thinned the extending lithosphere to <~80km. The later timing of this activity (~130-125 Ma) relative to the CFB (136.5-135.5 Ma) suggests that it, too, was not linked to the arrival of a plume head, but rather the persistent ‘tail’ of the Tristan Plume.  We will also briefly discuss potential implications for the epeirogeny linked to plume-rift evolution.