Cyclicity as Revealed by Stratigraphic Mineralogy and Geophysics in the Bushveld Complex, South Africa
The 65,000 km2 Bushveld Complex of South Africa has been well characterized, but most work has focused on relatively thin PGE-, Cr- and V-bearing horizons such as the Merensky Reef, UG-2 and Upper Zone magnetite layers. We have been acquiring a near-complete set of mineralogical, geochemical and geophysical data in a stratigraphic context for almost the entire 5,500 thickness of cumulate rocks in the Northern (Potgietersrus) Lobe, from 2 drill cores at Bellevue (roof, Upper Zone, Main Zone, ~3000 m) and at Moordkopjie (Main Zone, Platreef, basement, ~1500 m). Our dataset includes mineral compositions and WR major & trace elements (avg. every 10 m), magnetic susceptibility (avg. every 2 cm) and density (avg. every 2 m). The results document, for example, the broad-scale magmatic fractionation trends (An80 à An21, Mg# = 80 à 06), but there are prominent discontinuities and reversals at scales ranging from 100s to 10s of m, which probably represent injections of new magmas to actively forming cumulates. Some, but not all of these departures from continuous frationation trends are associated with similar patterns in physical properties, especially density. Of particular interest in the Main Zone, which is perceived by many as a largely homogeneous body of gabbronoritic rocks, is a prominent cyclicity on a scale of ~50-200 m, with progressive upward increases in density reflecting gradual upward increases in modal color index. Some of these cycles are associated with broad reversals in chemical fractionation trends (e.g. upward increases in Mg# of pyroxenes), arguing against simple fractionation processes. We suggest that such cycles may represent blending zones in which dense liquids and/or crystals drain downwards into the existing cumulate piles. If so, magmatic cumulates in large layered intrusions like Bushveld, and possibly smaller examples, were constructed by 10s to 100s of successive magmatic influxes. We intend to test this isotopically using mineral separates.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1036 Magma chamber processes (3618);
- 8145 Physics of magma and magma bodies;
- 8400 VOLCANOLOGY