Petrology of ultrahigh-pressure eclogites from the ZK703 drillhole in the Donghai, eastern China

The 558-m-deep ZK703 drillhole located near Donghai in the southern part of the Sulu ultrahigh-pressure metamorphic belt, eastern China, penetrates al ternating layers of eclogites, gneisses, jadeite quartzites, garnet peridot ites, phengite-quartz schists, and kyanite quartzites. The preservation of ultrahigh-pressure metamorphic minerals and their relies, together with the contact relationship and protolith types of the various rocks indicates th at these are metamorphic supracrustal rocks and mafic-ultramafic rock assem blages that have experienced in-situ ultrahigh-pressure metamorphism. The e clogites can be divided into five types based on accessory minerals: rutile eclogite, phengite eclogite, kyanite-phengite eclogite, quartz eclogite, a nd common eclogite with rare minor minerals. Rutile eclogite forms a thick layer in the drillhole that contains sufficient rutile for potential mining . Two retrograde assemblages are observed in the eclogites: the first stage is characterized by the formation of sodic plagioclase + amphibole symplec tite or symplectitic coronas after omphacite and garnet, plagioclase + biot ite after garnet or phengite, and plagioclase coronas after kyanite; the se cond stage involved total replacement of omphacite and garnet by amphibole + albite + epidote + quartz. Peak metamorphic P-T conditions of the eclogit es were around 32 to 40 kbar and 720 degrees C to 880 degrees C. The retrog rade P-T path of the eclogites is characterized by rapidly decreasing press ure with slightly decreasing temperature. Micro-textures and compositional variations in symplectitic minerals suggest that the decompression breakdow n of ultrahigh-pressure minerals is a domainal equilibrium reaction or dise quilibrium reaction. The composition of the original minerals and the diffu sion rate of elements involved in these reactions controlled the symplectit ic mineral compositions. (C) 2000 Published by Elsevier Science B.V. All ri ghts reserved.