Equilibration and reaction in Archaean quartz‐sapphirine granulite xenoliths from the Lace kimberlite pipe, South Africa

• Published in: Journal of Metamorphic Geology
• Main Authors: DAWSON, J. B., HARLEY, S. L., RUDNICK, R. L., IREL, T. R.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1997
• Subjects: Napier; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.1997.00017.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1997.00017.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1997.00017.x

Ultrahigh‐temperature quartz‐sapphirine granulite xenoliths in the post‐Karoo Lace kimberlite, South Africa, comprise mainly quartz, sapphirine, garnet and sillimanite, with rarer orthopyroxene, antiperthite, corundum and zinc‐bearing spinel; constant accessories are rutile, graphite and sulphides. Comparison with assemblages in the experimentally determined FMAS and KFMASH grids indicates initial equilibration at >1040 °C and 9–11 kbar. Corona assemblages involving garnet, sillimanite and minor cordierite developed on a near‐isobaric cooling P–T path as both temperature and, to a lesser extent, pressures decreased. Garnet‐orthopyroxene Fe‐Mg exchange thermometers record temperatures of only 830–916 °C. These estimates do not indicate the peak metamorphic conditions but instead reflect the importance of post‐peak Fe‐Mg exchange during cooling. Correction of mineral Fe‐Mg compositions for this exhange using a convergence approach of Fitzsimons & Harley (1994 ) leads to retrieved P–T estimates from garnet‐orthopyroxene thermobarometry ( c . 1000 °C and 10.5±0.7 kbar) that are consistent with the petrogenetic grid constraints. U‐Pb dating of a single zircon grain gives an age of 2590±83 Ma, interpreted as the age of the metamorphic event. Protolith major and trace element chemistries of the xenoliths differ from sapphirine‐quartzites typical of the Napier Complex (Antarctica) but are comparable to less siliceous, high Cr and Ni, sapphirine granulites reported from several ultrahigh temperature granulite terranes.