MODELING MIGMATITIC METAPELITES FROM DOMES IN THE BUSHVELD COMPLEX, SOUTH AFRICA: EVIDENCE FOR MELT-ASSISTED DIAPIRISM

Domal structures formed of high-grade metasedimentary rocks of the Pretoria Group immediately underlie ultramafic rocks of the Rustenberg Layered Suite (RLS), Bushveld Complex. The domes, which have wavelengths of several km, have disturbed the footwall contact and penetrate the RLS to varying structural levels. Previous workers have proposed that the structures are diapiric, formed by magma loading and amplification of ductile floor rocks between fingers of RLS magma (Uken & Watkeys Geology 1997). Recent work has emphasized the importance of the migmatitic nature of the rocks (Gerya et al. Geology 2003), since the presence of melt and the potential for melt migration will significantly affect the rheological and thermal evolution of the footwall in these evolving structures. Away from and at the margins of diapirs, metapelites are characterised by assemblages rich in Crd (+ Bt, Kfs, Qtz ± Pl); Fe-richer rocks contain porphyroblasts of chiastolitic And >30 mm in length. Peak metamorphic conditions were around 3 kbar, and up to 750 ºC close to the contact (Waters & Lovegrove JMG 2002; Johnson et al. J Pet 2003).

Within the center of diapirs, And porphyroblasts are replaced around the margins by symplectitic intergrowths of Crd + Spl that were in equilibrium with melt (preserved as leucosome), but not matrix quartz; restricted equilibration volumes and domainal effective bulk compositions are implied. In the Marble Hall area, symplectites have been interpreted to have resulted from a second phase of prograde near-isobaric heating related to emplacement of the adjacent (overlying) Nebo Granite (Pitra & de Waal JMG 2001). However, symplectites are a common feature of many diapiric cores that are distal to the major granite bodies. Using P-T-X pseudosections constructed in the MnNCKFMASHT system and subsystems, contoured for model proportions of H₂O and melt, we suggest the symplectites record decompression, consistent with previous interpretations of these microstructures (e.g. Bucher-Nurminen & Droop, 1983). Restricted equilibration volumes along the prograde path were the result of the large size of And in combination with sluggish near-solidus reaction. Replacement of And by Crd + Spl occurred during suprasolidus decompression (± heating) on diapiric ascent of the partially molten footwall rocks into the overlying RLS.