Heterogeneous, modally banded kyanite-bearing and bimineralic eclogites from the lithospheric mantle, are common xenoliths in the Roberts Victor kimberlite (South Africa), but don’t generally occur elsewhere. They show reaction textures in which kyanite is consumed and a bimineralic grt+cpx lithology is formed.
Differences in garnet trace element signatures between differing lithologies in the eclogites are significant. The kyanite-out reaction was most likely triggered by a heating event in the subcratonic lithosphere. As kyanite contains around 100 ppm of H2O it is suggested that the kyanite-out reaction, once initiated by heating and restricted metasomatic influx, was promoted by the release of water contained in the kyanite.
Geothermobarometric calculations using measured mineral compositions in Perple_X allowed the construction of a P-T path showing a steep, cool prograde metamorphic gradient of 2˚ C/km to reach peak conditions of 5.8 GPa and 890 C for the kyanite eclogite. The kyanite-out reaction occured at close to peak pressure (5.3 GPa) and was associated with a rise in temperature to 1380˚ C. Mass balance calculations show that upon breakdown, the kyanite component is fully accommodated in garnet and omphacite via a reaction system with low water fugacity that required restricted fluid influx from metasomatic sources. It is suggested that the kyanite-out reaction is probably an integral part of the mineralogical evolution of most archetypal bimineralic eclogites at Roberts Victor and potentially elsewhere.
The steep (high-P low-T) prograde P-T path defining rapid compression at low heating rates is atypical for subduction transport of eclogites into the lithospheric mantle. Such a trajectory is perhaps best explained in a model where strong lateral compression forces eclogites downward to higher pressures, supporting models of cratonic lithosphere formation by lateral collision and compression.