Retrogressed Eclogite
“High-pressure rock falling apart on the way up”
Retrogressed eclogite is an eclogite that has partially reverted to lower-pressure mineral assemblages during its journey back to the surface. The original omphacite and garnet break down into symplectites of amphibole and plagioclase, recording both the peak burial conditions and the exhumation path.
garnet (partially resorbed), amphibole (replacing omphacite), plagioclase symplectites, quartz, rutile cores with ilmenite rims
Western Gneiss Region (Norway), Dabie-Sulu (China), Rhodope Massif (Greece/Bulgaria), Münchberg Massif (Germany)
5.5-7
Retrogressed eclogites are far more common than pristine eclogites because the high-pressure minerals are unstable at the surface — finding a perfectly preserved eclogite means the rock somehow avoided reacting during its entire ascent.
Test Your Knowledge
What does a corona of amphibole and plagioclase around garnet in retrogressed eclogite indicate about the rock's pressure-temperature history?
Related Specimens
Want to identify rocks yourself?