SETTING AND SIGNIFICANCE OF ZIRCON/BADDELEYITE IN BASAL UNIT OF ROOIBERG GROUP, STAVOREN INLIER, 2.06 GA BUSHVELD COMPLEX, SOUTH AFRICA

The Bushveld Igneous Complex (BIC, 67,000 km²) occupies four overlapping basins superimposed on the Transvaal Supergroup. Four siliceous formations of the ≤5 km Rooiberg Group (Schweitzer et al., 1995) form its roof. As the Rooiberg meltrocks (non-genetic term) accumulated, massive sills of the mafic Rustenburg Layered Suite (RLS, ≤8 km) and Lebowa Granite Suite (LGS, ≤3 km) sequentially intruded their base (Hatton and Schweitzer, 1995). LGS flooded BIC basins, leaving only islands (tens of km wide) of Transvaal rocks as paired inliers in the E and W basins (Hartzer, 2000). In each inlier, a strike-slip fault zone juxtaposes intensely deformed, metamorphosed, and attenuated lower Transvaal formations against upper formations, which are undeformed and overlain by basal Rooiberg meltrock (~130 m, top eroded). The Magaliesberg quartzite (~500 m), a regional marker between continuous upper and lower Transvaal units, is conspicuously absent.

The basal Rooiberg (different from any known volcanite or impactite) and the extraordinary inliers are interpreted as evidence for an initial BIC catastrophe, which triggered later events. Except in the undeformed E (Stavoren) Inlier, the basal Rooiberg was destroyed at RLS contacts. It is a complex breccia with quartzite clasts (<1 mm - >100 m) in a matrix of altered glass. In the samples studied, quench crystals and an absence of phenocrysts indicate that glass quenched from a high-T melt. Three quartz morphologies suggest paramorphs of high-T SiO₂ polymorphs: 1) quench needles (cf., needles in basal Onaping, Sudbury; Stevenson, 1963), 2) laths by reaction with corroded ≤1 cm quartzite clasts, and 3) grains within the clasts, inverted into skeletal crystals. If micron-size zircon/baddeleyite indicates T>1680°C (Tegtmeier et al., this session), the temperature was in range of impacts, but not siliceous volcanism.

Hydrocode impact simulations (Collins et al., 2002) can explain the inliers as part of a peak ring in a multi-ring crater: blocks of relatively undeformed upper Transvaal units from the wall of a collapsing transient cavity (Stavoren segment) juxtaposed against deformed lower Transvaal, detached from a collapsing central uplift. In this model, basal Rooiberg would be impact ejecta of quenched superheated melt with entrained clasts of “absent” Magaliesberg.