Dear all
Apologize for multiple postings.
We are currently working on some HP carbonate-rich rocks that appear to have intruded UHP eclogite at still high pressure. The mineral composition of these rocks is Fe-Mg calcite ± Fe-dolomite + ternary garnet + omphacitic pyroxene + Ti-rich phlogopite + apatite + rutile + ilmenite ± allanite ± zircon ± monazite. Large garnet contains inclusions of all the other phases, except from allanite and monazite. Exsolution of Fe-dolomite in calcite and calcite in Fe-dolomite is common. Large grains of clinopyroxene have lamellae of ilmenite in one direction and lamellae/rods of phlogopite in apparently in two directions (see attached BSE images a-d). The lamellae of ilmenite also show an internal exsolution with lamellae of magnetite (BSE image c). Of special interest are multiphase inclusions, the presence of acicular crystals of apatite in garnet (BSE images e & f), and ilmenite with exsolved magnetite (BSE image g). Matrix apatite (up to 5 modal %) frequently have needles of pure SiO2 and Fe1-xS oriented parallel to the c-axis (BSE image h and microphoto i).
The peak conditions of the UHP metamorphism has been calculated to 700-750 °C/3.35 GPa. Some of the observed features in the carbonate-rich rock indicate, in my opinion, exsolution from higher T and possibly also P. The multiphase inclusions in garnet and especially the appearance of apatite are intriguing to me. I have never seen anything like this and don’t have any clues of how such apatite is formed. I will be happy to receive feedback on all the features shown in the attached images, as this could help us to make some progress in understanding these peculiarities.
As an additional information I will mention that the rocks are strongly enriched in LREE (La 1000x chondrite) and Ba (up to 4000 ppm).
Best regards
Erling Ravna