Geodynamic Setting of the Bushveld Complex

American Museum Natural History, New York NY

Investigators

Abstract

This project is supported by the Petrology & Geochemistry and the Education & Human Resources programs in the Division of Earth Sciences and by the Africa, Near East and South Asia program of the Office of International Science and Engineering. The Bushveld Complex of South Africa is a fossil magma chamber that solidified in the shallow crust about 2 billion years ago. The body is well-known to geologists because it holds the world's largest reserves of platinum and related elements and is a major source of chromium and vanadium. The Bushveld is also notable for its size, which has been estimated to be up to 600,000 cubic kilometers, and for the fact that its structure reflects a long and complex solidification history. Despite its enormity, geochemical characteristics of the rocks indicate that some amount of the original magma had escaped from the chamber and is thus missing from the present body. This raises the questions of what was the nature of this additional magma and where could it have gone. These questions need to be answered if geologists are ever to understand how and under what the conditions the Bushveld Complex solidified. Research to date by the team has led them to hypothesize that the missing magma is now represented by a sequence of lavas known as the Rooiberg Group (plus their intrusive equivalents, the Rashoop granophyres), the ages of which are identical to the Bushveld age within uncertainty estimates. The present research is directed at testing this hypothesis. As part of the project, Mathez and colleagues will also organize a week-long workshop for 20-25 South African earth science teachers on the mineral resources of South Africa. The workshop will be taught by Mathez, VanTongeren, and two African-American New York City school teachers, who will thus gain knowledge of both mineral deposits and unique experience in teaching across cultural divides. More specifically, the research seeks to test the hypothesis that the Rooiberg lavas and associated rocks originated from the Bushveld Upper Zone. This will be accomplished by a detailed study of the trace element geochemistry of the Upper Zone to determine the compositions of the liquids in equilibrium with the cumulate minerals. The derived liquid compositions will then be compared with Rooiberg and Rashoop rock compositions. The top members of the Rooiberg Group are dominated by rhyolite, whereas the Bushveld Upper Zone, which is the most chemically evolved part of the intrusion, is dominated by cumulate rocks consisting of assemblages with most or all of the minerals plagioclase, Ca-poor pyroxene, Ca-rich pyroxene, magnetite, ilmenite, Fe-rich olivine, and apatite. Therefore, the possibility that a rhyolite could be in equilibrium with these phases bears on one of the most fundamental and debated questions in igneous petrology, namely whether mafic magmas differentiate toward Fe-rich or Si-rich compositions. In addition, if the Rooiberg and Rashoop rocks indeed originated from the Bushveld magma chamber, this implies that the Bushveld intruded into a much shallower level of the crust than has been generally appreciated, thus helping to define the geodynamic setting of the intrusion.