The Bushveld Complex and Lunar Mare Basalts
Lewis D. Ashwal
School of Geosciences, Wits University, Wits, South Africa
Both the Bushveld Complex and lunar mare basalts represent some of the largest mantle-derived magmatic products on their planetary bodies, yet their origin in terms of heat sources and tectonic settings are not yet understood. In both cases the magmas have been suggested to have formed by meteorite impact, but vehement arguments against this have been voiced; this idea should probably be discarded. An interesting comparison between Bushveld and lunar mare basalt formation relates to the rates of magma production. We now know from precise geochronology and thermal modeling that Bushveld magmatic products formed quickly, probably in a few m.y. or less at 2054 Ma. Using a minimum volume for Bushveld of 1 x 106 km3, this yields estimated magma emplacement rates of 0.2-2 km3/yr, which are comparable to the high output rates of terrestrial flood basalts. In contrast, lunar mare basalt output rates have been estimated at 2.4 x 10-6 km3/yr to 1.5 x 10-6 km3/yr, which would be far less than individual terrestrial volcanoes such as Vesuvius or Kilauea. These estimates are unreasonably low because they assume a constant eruption rate throughout selenologic Periods whose ages are poorly constrained: Imbrian (3.2-3.8 Ga), Erastothenian (1.1-3.2 Ga), Copernican (0-1.1 Ga). Better estimates will require new lunar field work and more isotopic age determinations—my prediction is that lunar basalt eruption rates were comparable to those of terrestrial LIPs. This again raises the question of the causative heat source(s). For Bushveld, deep mantle plume theory is attractive, but fails to account for emplacement in an actively subsiding intra-continental sedimentary basin. For lunar basalts, endogenic heating by earth-Moon tidal friction is a possible solution, but seems far-fetched. We still have a lot to learn about the most prominent magmatic products on the Earth and Moon.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery