P-T path constraints from calc-silicate metapelitic rocks east of the Venetia kimberlite pipes, Central Zone, Limpopo Belt, South Africa: Have these rocks seen granulite-facies conditions?

Abstract

The youngest, high-grade metamorphic event (M3) in the Central Zone of the Limpopo Belt of southern Africa is considered to have occurred at ~2.0 Ga. Granulite-facies mineralogies indicate peak M3 metamorphic conditions of T >800°C and P = 10 to 12 kbar and a clockwise P-T evolution. New data, while confirming the clockwise P-T evolution, indicate that calc-silicate-metapelitic rocks and amphibolites of the Beit Bridge Complex near the Venetia kimberlite pipes only experienced upper amphibolite-facies metamorphism. This is supported by the mineral chemistry of garnet that shows a prograde zonation. Detailed textural observation of minerals enabled us to evaluate the equilibrium composition during different stages of the P-T evolution. Univariant reaction curves and geothermobarometric calculations constrain these stages. Maximum pressure conditions are represented by the mineral assemblage garnet-kyanite-biotite-phengite (Si = 6.8 p.t.u.)-quartz. Estimated pressures range between 12 and 13 kbar at ~600°C. Decompressional heating was accompanied by the prograde dehydration reaction Bt + Ky = Crd + Kfs + H 2 O at 680° to 720°C and 7 to 8 kbar. Further, based on the sillimanite-producing reaction Ms + Qtz = Kfs + Sill + H 2 O and conventional geothermobarometry (Fe/Mg distribution between garnet/clinopyroxene and garnet/biotite, Si-content (6.4 p.f.u.) of phengite, GASP-barometry, Hbl-Pl-thermometry), peak metamorphism was established at ~700°C and 7 kbar in the sillimanite stability field. Furthermore, fluid inclusion investigations and theoretical considerations revealed the peak metamorphic fluid phase in the calc-silicate-metapelitic rocks to be CO 2 -H 2 O±CH 4 -rich. A variation of fluid composition with lithological differences argues for a rock buffered fluid during upper amphibolite-facies metamorphism. The lower P-T conditions when compared with granulite-facies conditions in other areas in the Central Zone are interpreted to result from the P-T evolution of a distinct crustal block within the Central Zone.