Metamorphic veins with kyanite, zoisite and quartz in the Zhu-Jia-Chong eclogite, Dabie Shan, China

The Zhu-Jia-Chong quartz eclogites occur within two mica–epidote–garnet–plagioclase gneiss as lenses up to 200 m thick and more than 1 km long. The eclogite is medium-grained and consists of omphacite, garnet, clinozoisite, minor kyanite and quartz. No evidence for the former presence of coesite was found. Locally, poikiloblastic amphibole occurs, which is zoned from glaucophane in the core to Mg–cummingtonite in the rim. In some localities, clinozoisite occurs as polycrystalline aggregates, interpreted as pseudomorphs after former porphyroblasts of lawsonite. The eclogite is cut by irregular metamorphic veins up to 20 cm thick and several meters long, consisting of kyanite ± zoisite ± omphacite ± rutile ± apatite ± clinozoisite in a quartz matrix. Locally, vein minerals attain pegmatite-like grain size. Geothermobarometric calculations and mineral compatibilities of peak and retrograde minerals of both eclogite and veins indicate a clockwise P–T path, characterized by initial eclogitic conditions prograding from the lawsonite to the epidote stability field. The veins are considered to have formed before peak metamorphic conditions by prograde dehydration of lawsonite to kyanite + zoisite/clinozoisite + quartz + fluid; the fluid released from the reaction favored the growth of coarse-grained vein minerals. The eclogitic peak, estimated at ∼ 24 kbar and 700 °C, is followed by a two-stage retrograde evolution: the earlier stage, characterized by a quasi-adiabatic decompression from eclogite- to the amphibolite-facies conditions, implies a rapid exhumation from depths of ∼ 90 km to mid-crustal levels; the later stage, characterized by slower exhumation rates, led to the final exposure of eclogites and country rocks. The role of lawsonite as storage of fluid in subducting plates is therefore discussed.