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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