Post-peak re-equilibration in a mafic gneiss from the KTB main hole: implications for the metamorphic evolution

Abstract

Detailed microprobe investigations were carried out on mineral phases of a mafic garnet–biotite–hornblende and garnet–biotite gneiss from the KTB main hole. The results indicate that the complex zonation patterns of garnet porphyroblasts are mainly due to post-growth diffusional exchange, as constrained by theoretical modelling. Moreover, the chemical composition of biotite and hornblende enclosed in garnet were also changed by later diffusion and retrograde net-transfer reactions. Therefore, the garnet zonation in combination with mineral inclusions cannot be used to reconstruct the prograde metamorphic evolution or to estimate the maximum temperatures attained. However, by applying the GRISP geobarometer, minimum pressures of 8.5 kbar (at 700 °C) can be derived for the metamorphic peak, well conforming to previous estimates on the amphibolite-facies metamorphic stage of the KTB metamorphics. Mineral assemblages formed during the retrograde evolution allow to estimate P–T–X conditions of approximately 3 kbar, 450 °C and X(H2O)=0.9.